Means VAC Vrms

Relay operation at 230 VAC, also with 48VDC relay



A few definitions in advance, as I use them here and in the other electronics mini-courses for voltage specifications: VAC or VACeff means effective alternating voltage. VDC or VDCeff ripple-free direct voltage (e.g. battery, power supply unit) or the effective value of a pulsating direct voltage (e.g. rectification without charging capacitor). Vp means the peak value voltage of a pulsating direct voltage or an alternating voltage. Vpp is the voltage between the positive and negative maximum of an alternating voltage or the maximum difference in a ripple voltage that is superimposed on a direct voltage, as is known, for example, from the voltage on a charging capacitor after rectification. Pulsating DC voltage is usually, and also here, a half-wave or full-wave rectified sinusoidal voltage (Image: Wiki).

Instead of DC voltage, one usually reads DC voltage and AC voltage for AC voltage. The same applies to the current, with the relay current values ​​always only showing mA. The true RMS value (TRMS) is always to be understood here as the RMS value. Strictly speaking, TRMS is the same as RMS. You can read the following in the mentioned wiki page: For measuring devices that actually determine the rms value according to its definition, for the sake of clarity it is occasionally claimed that they measure the "real rms value" (English TRMS, T for true); however, an effective value cannot be real or fake or true.

According to this fact, one reads RMS instead of TRMS in the further course. Measuring the RMS voltage of a sinusoidal AC voltage is quite possible with a cheap multimeter because such instruments are calibrated for it. The measured values ​​are incorrect for non-sinusoidal AC voltages and this is all the more true for pulsating DC voltages, which is often mentioned here. But there are multimeters that correctly measure the RMS values ​​of non-sinusoidal AC voltages, and among them are those that are no longer as expensive as they used to be.

If you have a digital oscilloscope with additional numerical measured value displays on the display, you can usually also measure RMS-AC and RMS-DC voltages. So it is with mine PM3394A from Philips (Fluke) at the age of 26 (February 2019). The nice thing about this old oscillator is that you can switch between analog and digital measurement with the push of a button during the measurement.

Business: Philips (Fluke) means that Fluke bought Test & Measurement (T&M) from Philips around 1993. The oszi PM3394A is written on with Philips, but has an adhesive label with the words FLUKE on the top of the housing. Why Philips had given up the production of oscilloscopes could be due to the fact that you often had annoying trigger problems with the older Philips oszi. In fact, this is common knowledge. If someone of the older ELKO readers also had this experience, I would be interested in a short email to me.

Practical relevance: The question arises whether this electronics mini-course is of any use to the hobby electronics technician who cannot or does not want to afford expensive or new measuring instruments. Definitely yes! It is explained how the relay circuit can be optimized using empirical methods, even without a DC-RMS measurement option, and without this method, with the greatest possible practical relevance, it does not work anyway, as we will see later.

Control transformer and isolating transformer: A variac is required to perform the experiments shown here. If this term is not known, alternative terms are control transformer and variable transformer. A luxury variac has a digital voltage display. Those with imprecise voltage scaling on the round disc below the rotary knob are cheaper. The exact AC voltage has to be measured with a multimeter. Photo of such a variac:

Most inexpensive Variacs are autotransformers, i.e. there is only one primary and no secondary winding. Such transformers are also called auto transformers. The primary winding is also the secondary winding. Therefore there is no galvanic isolation! You definitely need an isolating transformer for experimentation, which is connected between the 230 VAC mains voltage and the variac. You can do without an isolating transformer if the Variac itself is designed as an isolating transformer, i.e. contains a primary and a secondary winding. Such variacs are, however, very expensive. It is therefore better to buy an isolating transformer and an inexpensive Variac than a car transformer. This is also more flexible because you don't need a Variac for many test applications and there are also test applications that are safe to touch and a Variac designed as an autotransformer is sufficient on its own. However, this does not apply to the experiments shown here! Here you need an isolating transformer and a variac, as Figure 1 illustrates schematically:

ATTENTION: Mains voltage !!! Risk of death!!! Not for beginners!

The circuits in this electronics mini-course work with 230 VAC mains voltage. Extreme caution is required! All manipulations with these circuits must always be carried out with a ISOLATING TRANSFORMER be performed! The circuit must be safe to touch according to SEV or VDE standards and built into a housing!

The reproduction of such circuits is unsuitable for beginners or hobbyists without the necessary knowledge in dealing with the 115/230 VAC mains voltage! Replication, tests, manipulations and use are always at your own risk!

Yesterday Today Tomorrow

The title of a famous three-part series in the Star Trek series "The next generation"with Captain Jean Luc Picard and the cynical god Q as the main actors. Excellent played and the content fascinating for those who are interested in such things, although you don't have to be a Trekkie for a long time. Fascinating, the often pronounced expression of the Vulcan Mister Spock technology not only in the 23rd or 24th century, in the age of the WARP drive (video with astrophysicist Harald Lesch). Electrical engineering (electronics) has always been fascinating, it is today and it will (hopefully) also be tomorrow be.

Why these introductory words to an electronics mini-course that deals with relay technology? Quite simply, the relay was already in practical use in the early era of electrical engineering. This electromagnetic switch has been continuously developed. A small, modern print relay in dual-inline format, consumes a coil power of 0.75 VA (AC relay) and there were also such relays with only 0.2 W (DC relay). Gave because when I started this electronics mini-course as an update (February 2019), I discovered that these 0.2 W DC relay types no longer exist. Only those with a minimum power of 0.4 W. Such relays in both power classes switch currents of up to 10 A and powers of more than 1000 VA. Since the electronics mini-courses are not designed for electronics beginners, I didn't change the circuits with 0.2W DC relays. In the case of a replica, the reader can easily adapt the components in the area of ​​the relay himself. And as sure as it is, whether you won't find a source with 0.2W DC relays after all. In this case, please send me a short email.

Despite how sophisticated the technology of modern semiconductor switches (e.g. solid-state relay) is and evolves, the electromechanical relay will also secure its place in the (near) future. Especially when the switching frequency is relatively low and / or because there is generally no need for overvoltage protection with regard to the contacts where it is absolutely necessary for electronic switches.

This electronics mini-course is about the use of relay circuits in 230 VAC and 115 AC mains operation (USA), one method with a DC relay (relay with direct current coil) and another with an AC relay ( Relay with alternating current coil) is explained and presented in practical circuits. There are electronics mini-courses with hands-on applications that build on the fundamentals of this electronics mini-course. More on this in the last chapter "Link list".

By the way, if you still have the impression that I started this electronics mini-course with a fairy tale hour, I would like to give you the book "The physics of Star Trek"Recommend by Lawrence M. Krauss. The foreword comes from the most prominent Star Trek fan of all: Stephen Hawking, who even made a guest appearance in a TV episode of Strar-Trek: The Next Generation ...


In terms of electronics, the 1950s and 1960s were almost entirely dominated by tube technology. While the vacuum electron tube was almost entirely there for analog technology, there was also the cold cathode relay tube in use with relays for control technology. Alternatively, there was the cold cathode thyratron, which is not discussed in my history-electronics mini-courses. Pictured here is the Cerberus GT21, which was very popular at the time.

Before we continue, something about the history of the relay, whose inventor is (allegedly) Joseph Henry, born on December 17th, 1797 in Albany, New York, USA. In 1835, Henry invented the electromagnetic relay. More details here:

My history-electronics-mini-course cold cathode tubes I deals with this historical epoch a little with practical circuits. The difference between these two tube technologies is that in the vacuum electron tube electrons, voltage-controlled by a grid, flow from the negatively charged hot cathode to the positively charged anode and this generates the variable anode current, while in the cold cathode relay tube a positive ion current from the Anode flows to the cathode, which is ignited by means of a control electrode or grid (thyratron) and switched off by briefly interrupting the ion current. During these two decades, cold cathode tubes were used for the most versatile relay controls, often directly on the 220 VAC mains voltage. At that time the mains voltage was not yet 230 VAC. Relays with coils for DC and AC currents were used for this high voltage. As it is simpler, one reads cold cathode tubes instead of cold cathode relay tubes.

On this subject, an excerpt from the company magazine from January 1959 from the company CERBERUS, which at that time manufactured not only fire alarms (smoke alarms with redioactive americium source), cold cathode relay tubes, cold cathode thyratrons and cold cathode stabilizer tubes. The following nostalgic content describes something that will have its meaning in the remainder of this electronics mini-course. It's about the rectified half-wave control of relays:

    Relay for AC operation

    Cold cathode tubes are operated with AC voltage in such a way that they act as a rectifier, i.e. only ignite and conduct current with a positive anode and negative cathode. DC relays can therefore be used. However, since a current only flows during part of the positive half-wave, a drop-out delay must be used to ensure that the relay does not drop out during the subsequent power break. So that this succeeds, very high instantaneous values ​​of the current are necessary every time. The dimensioning of the relay coil is therefore related to the dimensioning of the delay used. Instead of calculating, it is better to determine the coil data empirically, the reference point being the mean current flowing through the tube. It is measured with a normal DC instrument and should correspond to the values ​​for AC operation given for each tube on the data sheet. A good drop-out delay results automatically with thermal (bimetal) relays; However, you then have to accept a corresponding, usually undesirable, pick-up delay. The following methods are generally known for delaying the dropout in electromagnetic relays:

    a) Additional short-circuit winding.
    b) Diode connected parallel to the coil in reverse direction. (Freewheeling diode)
    c) Capacitor connected in parallel to the coil.

So much, including picture 2, from the CERBERUS in-house magazine number 9 from 1959 (CH / Männedorf). Figure 2 shows a nostalgic light control in light mode, by igniting the cold cathode tube when the photo resistor FW is illuminated. The control electrode S no longer ignites below a critical illuminance. The ion current between the anode and cathode is extinguished during the voltage zero crossing and does not ignite again with the following positive half-wave until it is bright enough again.

The ignition for the ion current flow from the anode A to the cathode K takes place secondarily through the ignition between the control electrode S and cathode K. If the luminosity increases with the FW, the FW resistance value decreases. FW, R2 and P form a voltage divider and generate the voltage Us. Us rises to the S / K ignition voltage and charges C1 through R1. When the S / K ignition voltage is reached, C1 discharges very quickly from S to K because the S / K transition is momentarily very low-resistance. This ignites the ion current from A to K through this ionization of the gas and this current flows in the 10 mA range through the relay coil until the current positive sine value of the mains voltage falls below a critical minimum value. The whole thing starts all over again when the positive half-wave of the mains voltage rises again and Us reaches the ignition voltage.

It is important that the relay remains steadily attracted during the power failure, which means that it does not flutter or hum. The short-circuit winding is symbolically indicated with a thick vertical line next to the contact on the relay coil. It is not an additional winding with many turns on the same bobbin. It consists of a thick copper ring, which is pressed into a so-called split pole in the soft iron core, and thus ensures a sufficiently high short-circuit current so that sufficient magnetic force still acts on the relay armature even when the voltage is zero crossing and it therefore remains attracted. This is why this shaded pole with the short-circuit winding is on the armature side. This trick effectively prevents the armature and the contacts from fusing in the event of AC current or pulsating DC current.

Important NOTE: Older readers of such articles, who have experienced this electronics era themselves, often want information from back then. Unfortunately, I have nothing to distribute and the little that I have to copy and send is too time-consuming. It is also useless to ask CERBERUS. The people there no longer have a clue and don't understand what you actually want. There are too many generations of electrical engineering in between. I tried this myself ...

Today and tomorrow...

The DC relay in action

Instead of a rectifying cold cathode tube, we are dealing here with rectifying semiconductor diodes. It comes out the same in the sense that the relay receives a half-wave rectified DC voltage. We will first deal with a low-power DC relay with the highest possible coil DC nominal voltage, which is relatively often available on the market (different manufacturers and products). This ensures that there is as little power loss as possible when used directly with 230 VAC. Then we experiment with an AC relay with a coil AC nominal voltage of 230 VAC and find that this relay can also work very well with a DC voltage rectified with half-waves if suitable measures are taken without great effort.

In Figure 3, we first deal with the dimensioning of a circuit with a DC relay, in which, as described in the nostalgic Cerberus article, the empirical has its justified, even necessary significance:

The voltage specifications in the circuit in Figure 3, but also for all other circuits in the following figures, always refer to the reference level, which is marked REF, unless otherwise noted. REF is identical to the neutral conductor N of the 230 VAC or another mains voltage, such as 115 VAC (UNITED STATES). The circuit also works when P and N are interchanged. This is even easily possible if a plug with only two contact pins is used. See connector diagram on the right "without earth wire".

In Figure 3, a small dual inline DC relay with a nominal voltage of 48 VDC is used, which manages with 0.2 W coil power.
DANGER! This type of relay no longer exists since January 2019. There are still relays with a coil voltage of 48 VDC, but with a power of 0.4 W. I cannot find any relays from other manufacturers of the 0.2W types. The effort to redesign is too great. Therefore, the description and schemes continue to apply to the 0.2W type in the event that such relays should still exist at some point. The reader has to carry out a re-dimensioning for the 0.4W type by converting and experimenting. It affects Rx and Cx in Figure 3. Rx consists of Rx1 and Rx2. Because the coil current is twice as large, it can be tested with half the values ​​of Rx1 and Rx2 and with twice the value of Cx. However, there can also be a certain deviation due to the relay mechanism. 0.4W instead of 0.2W doubles the current through Rx (Cx adjustment only approximately considered!). The power loss of Rx1 and Rx2 increases to about 0.9 W. This makes this circuit unattractive, which means that a 48 VDC relay is no longer suitable for 230VAC operation.

So it continues with the 0.2W relay type. The rated current of the relay is 4.2 mA. The full AC voltage of 230 VAC to reference REF (N) is in front of diode D1. The rms DC voltage of exactly the same size would result in full-wave rectification from this AC voltage. We ignore the low voltage drop at the bridge rectifier here.With half-wave rectification, as shown in Figure 3 with D1, half of all positive half-sine waves are lost and this reduces the effective DC voltage to a value that results from the effective DC voltage of full-wave rectification divided by the square root of 2. The effective DC voltage of the half-wave rectified voltage from 230 VACeff is therefore 163 VDCeff. The exact relationship between these voltages is illustrated below in Figure 4.

There should be an effective DC voltage of 48 VDC across the relay coil. Since we switch a highly sensitive low-current LED_ (scroll down!) In series to the relay coil for the purpose of operating status display, the voltage across the coil and LED increases to around 50 VDC. Rx is calculated according to Ohm's simple law, in which the effective residual voltage of 113 VDC is divided by the current of 4.2 mA. Rx is calculated to a value of 28.3 k-Ohm.

But now STOP! It is only that easy if we were dealing with a pure DC voltage. But that is far from the case. Because of the half-wave rectification, the relay receives a pulsating DC current and this generates a non-negligible self-induction voltage in the coil when the half-wave voltage decreases again after the peak value. We are dealing with the relay coil and its soft iron core not simply with an ohmic resistance. It is an impedance, more precisely inductance, and it is higher than the ohmic coil resistance. And that means that with the intended relay type, the residual voltage is not 113 VDC but only 107 VDC, because the effective DC current is lower than this 4 mA. In addition, the relay armature and the contacts flutter massively in a rhythm of 50 Hz (without Cx).

And now, as already indicated above, it becomes empirical! A capacitor is connected in parallel to LED and REL, the capacity of which is selected so large that this half-wave sinusoidal voltage is partially smoothed to a ripple voltage via the relay coil and LED, so that it has an amount of approximately the nominal coil voltage of the relay of 48 VDC as shown in Figure 3 in the diagram "with Cx = 1µF". The ripple voltage across Cx has about the same peak-to-peak value, namely about 48 Vpp.

This partially smoothed ripple voltage does not reach the lower value of 0 V at the relay coil, which is also correct. The lower ripple voltage should be slightly higher than the dropout voltage of the relay armature so that the relay does not tend to flutter. The voltage between maximum and zero at the relay coil and LED and thus at Cx is around 62 Vp. A nominal voltage of 100 V should be provided for Cx. As an electrolytic capacitor, Cx is mechanically very small, with values ​​as specified. With the specified relay type from FINDER, the coil voltage just mentioned can be achieved with a fairly precise Cx = 1 µF, provided that the previously calculated value for Rx empirically reduced from 28 k-ohms to 20k-ohms. This example shows how one can proceed empirically differently with a relay with different voltage and current data. For safety reasons, you should definitely work with a 230VAC isolating transformer when experimenting, as explained in the introduction with Figure 1. The Variac is needed to vary the AC voltage while experimenting.

How big is the power loss via Rx? The use of Cx reduces the impedance, given by the relay coil with the armature tightened and Cx, to such an extent that the residual voltage across Rx increases from 113 VDCeff to around 130 VDCeff. While the effective current through the relay coil is just 4 mA, the current through Rx is 6.5 mA. The smoothing current through Cx contributes to this. The power loss via Rx is calculated from these 130 VDCeff and the 6.5 mA. The power loss is 0.85 W. Since 1/2-watt resistors are easily available cheap goods, it is advisable to connect two resistors Rv1 and Rx2 of 10 k-ohms and 0.5 watts in series for Rx.

The current through the relay coil is out of phase with the coil voltage (inductive). The same applies to the current through Cx in relation to the voltage across Cx (capacitive). The current through the purely ohmic resistor Rx is not out of phase with the voltage across Rx. This simplifies the power calculation of Rx.

Why this empirical circuit development? Quite simply because it is absolutely impossible to calculate the impedance behavior of any relay with the information on the data sheet of a relay manufacturer. The whole thing is complex in the sense that the impedance (inductance) is not constant. It has different values, depending on whether the relay armature is open, at the moment it is attracted, dropped, or closed. You hardly have any other choice than to first calculate the circuit as if you were using it with a pure DC voltage. Kind of a rough approximation. Then you empirically determine how large the capacity of Cx and Rx should be with a test setup. This method is by no means time-consuming in this application. With another DC relay with different data, the values ​​of Cx and Rx will also differ. Although not exactly cheap, so-called resistance and capacitance decades are suitable for such experiments. Nevertheless, this is a one-time purchase that is definitely worthwhile for the electronics practitioner!

The question still arises, why is a variac needed? This can be used to determine the undervoltage at which the relay picks up safely. For a mains voltage of 230 VAC, this minimum voltage should be around 190 VAC. That is almost 20% undervoltage. It can easily be less. In the chapter "The AC relay in action"The minimum pick-up voltage is less than half the nominal voltage. If this means that the relay also has the nominal power at nominal voltage, that's perfectly fine.

The recommended MOSFET T BSS125 for controlling the relay REL is a so-called SIPMOS from Siemens, an N-channel type with an open drain-source voltage of 600 V, a maximum drain current of 0.1 A and a drain-source resistance, when switched on, from 45 ohms.

Figure 4 illustrates the voltage relationships between the AC sinusoidal voltage, the full wave and one-way rectified sinusoidal voltage, taking into account the effective voltage Urms (RMS) and the peak voltage Up. Part 4.1 shows the full wave rectification and part 4.2 shows the one-way rectification. We neglect the losses caused by the bridge rectifier BG and the individual rectifier diode D. This is also quite realistic here because the sinusoidal voltage values ​​are greater by a factor of more than 100 than the flow voltage of two simultaneously conducting diodes connected in series in the case of BG.

How large is the effective DC voltage if an effective AC voltage is fully rectified? It is exactly the same size because two half-sine waves with the same peak voltage and the same frequency feed a load per period. An incandescent lamp shines brightly whether it is connected to 230 VACeff or 230 VDCeff. This also shows that the performance remains unchanged. It can't be otherwise, because where should a performance difference be found? In part 4.1 the voltage and current frequency for the DC voltage is twice as high as for the AC voltage before rectification. Let's call this the power frequency, which remains unchanged. In both AC and DC applications, an incandescent lamp shines equally brightly and does not flicker visibly because the lighting frequency of 100 Hz is too high for this.

Let us consider the situation of half-wave rectification (part 4.2), which only generates half waves. Because every second half sine wave is missing, there is only half a sinusoidal voltage area per period instead of two and that means that the power is halved. Since a half-sine wave only generates half the power per period, this means that the effective DC voltage of the half-wave rectified voltage has only one value, which is the effective value voltage of the full-wave DC voltage or the AC voltage divided by the square root of 2 results, according to equation 2.

Because the ratio of the peak voltage (325 Vp) to the effective value of the AC voltage (230 VACeff) or full-wave DC voltage (230 VDCeff) is sqrt (2) and between the effective full-wave DC voltage (230 VCDeff) or the effective AC voltage (230 VACeff) and the effective one-way DC voltage (163 VDCeff) is also sqrt (2), this effective voltage of the one-way DC voltage is exactly half the peak voltage (325 Vp).

The AC relay in action

Figure 5 shows a very specific 230 VAC relay from Schrack. The exact data can be found in Figure 6. Please also note an important note!

As can be seen in Figure 6, this 230VAC relay is no longer manufactured by SCHRACK. There are alternatives with the same coil values ​​from FINDER. For this 230VAC replacement relay, refer to the data sheet FINDER PCB relays. FINDER has relays with a coil voltage of 230 VAC, also with a coil power of 0.75 VA, corresponding to an AC current of 3.2 mA and a coil resistance of 32.5 k-Ohm, to be read in the table "AC version"on page 3 of the data sheet. You have to evaluate the suitable distributor yourself. If necessary, ask the manufacturer. This alternative does not change anything in the original content of the circuits with this relay type.

The point is to show a simple, practical way of dimensioning and operating such an AC relay circuit with a DC voltage rectified with half-waves. We start with part 5.1, which shows us how much current the relay consumes during normal operation at 230 VAC. It's 3.2 mA. We always have to know at what minimum voltage the relay picks up (not drops out!). The mains voltage is reduced with the variac until the relay safely drops out. Then you increase the mains voltage and you note it when the relay picks up safely. In the relay used here, this voltage is 105 VAC. I only carried out the test with one relay because I didn't have another, and that means there are certain specimen variations. However, this is negligible if the tolerances are as large as this measurement shows and are completely normal. Because for a 230VAC application, the relay should switch on safely at at least 5% undervoltage, i.e. at 218 VAC. In rural areas you should consider a tolerance of -10%, which corresponds to a voltage of 207 VAC.

We come to part 5.2 with the test with a clean DC voltage, i.e. DC voltage (almost) without ripple voltage. You can see that the correct current of 3.2 mA is already set at 115VDC. This is exactly what matters, because the effective current through the coil, multiplied by the number of turns, determines the effective magnetic field strength or the attraction force on the relay armature.

Measuring RMS currents or RMS voltages is not easy, unless you have a suitable measuring instrument. However, this is not critical for the measurement in section 5.1 with sinusoidal AC voltage and section 5.2 with pure DC voltage. The experiment shows that the relay armature picks up at a voltage of 45 VDC. If you were to operate the relay at 230 VDC, the coil would be clearly overloaded. Where does it come from? Quite simply, only the ohmic resistance of the relay coil has a current-limiting effect. There is no self-induction with DC current. 230 VDC at the coil resistance of 32.5 k-Ohm results in a power of 1.6 W at a current of 7 mA.

Still picture 5.2: How do you generate a variable, clean DC voltage for this experiment if you don't have a suitable power supply unit with such a high variable DC voltage? The bridge rectifier circuit from part 5.3 is used and an electrolytic capacitor with a sufficiently high capacity (and sufficiently high nominal voltage!) Is connected in parallel to the relay coil, so that practically a clean DC voltage is obtained. A ripple voltage of a few Vpp does not matter. A capacity of around 10 µF is correct and a nominal voltage of 250 VDC is sufficient. If such a capacitor is missing in the handicraft box, you can easily help yourself by connecting several capacitors in series with lower nominal voltages, whereby all the capacitors must have the same higher capacitance values ​​according to the number of capacitors. The DC voltage is measured at the relay coil. The Variac with an upstream isolating transformer serves as a variable AC voltage source for your own safety.

Part 5.3 basically shows the same experiment again, but this time the relay coil receives the fully rectified DC voltage without smoothing by means of an electrolytic capacitor and we measure the AC voltage at the input of the bridge rectifier. It is interesting that at 115 VAC the same nominal current of about 3.2 mA is set as with the smoothed DC voltage of 115 VDC. The effective value of a full-wave rectified DC voltage is of course exactly as large as the effective value of the AC voltage in front of the full-wave rectifier (bridge rectifier), if one disregards the voltage loss of the rectifier diodes. This is realistically permissible at this high voltage. The fact that the nominal current of 3.2 mA occurs with the smoothed and unsmoothed DC voltage will have to do with the fact that the short-circuit winding in the shaded pole largely takes over the smoothing effect for the unsmoothed DC voltage. The short-circuit winding has no effect on pure DC voltage. The values ​​deviate for the minimum pick-up voltage because it is 75 VAC in section 5.3 and higher than the 45 VDC in section 5.2.

Part 5.4 shows the experiment with one-way rectification. It is interesting that with an effective AC voltage of 230 VAC, almost the same nominal current is set as with the AC application in section 5.1. Interesting because instead of two half-waves per period, only one half-wave is effective. The explanation for this is that the inductive component of a pulsating one-way DC voltage is lower than that of the AC voltage, in which the polarity changes with each amplitude zero crossing. Likewise, the minimum pull-in voltage of part 5.4 compared to part 5.1 is almost the same in both experiments.

Facit: It is not mandatory to measure the exact RMS current for this empirical test. An approximation using a commercially available multimeter is also sufficient, because the measured value does not have to be that precise. The circuit is functional when the ratio between the minimum pick-up voltage and the nominal coil voltage is roughly within the range, as shown by the circuits in Figure 5.

Now, for the sake of completeness, the short-circuit ring topic. What applies to the AC contactor also applies to the AC relay. You can read about this in Wikipedia on the subject of contactors (switches). the chapter "AC and DC voltage contactor"To get an impression of what such a short-circuit ring looks like in a contactor, the picture of a pull magnet, also from Wikipedia. The dimensions of the short-circuit ring made of copper are of course much smaller in a relay, especially a DIL relay.

This experiment was carried out with the SCHRACK AC relay mentioned in Figure 6. Since this AC relay no longer exists and FINDER recommends an equivalent one with the same key data instead, it can still be the case that, within certain limits, deviating values ​​are to be expected in the experiment according to Figure 5.

As can be seen in Figure 6, this 230VAC relay is no longer manufactured by SCHRACK. There are alternatives with the same coil values ​​from FINDER. Read the data sheet FINDER PCB relays. These relays are also available with a coil voltage of 230 VAC, also with a coil power of 0.75 VA, corresponding to an AC current of 3.2 mA. You have to evaluate the distributor yourself. This alternative does not change anything in the original content of the circuits.

With Figure 6 we focus again on partial image 5.4, which is reproduced as partial image 6.1 and in partial image 6.2 goes somewhat towards the application. If the relay coil is switched off, e.g. with a bipolar transistor or, as shown here, with a MOSFET, the rapid reduction in the magnetic field of the coil creates a high self-induction voltage in the form of a pulse, which can easily destroy an electronic switch. However, because this relay has a short-circuit winding that converts this induction voltage to a certain extent into an induction current, the remaining induction voltage may be limited. To be on the safe side, it is still worth using the freewheeling diode D2. Since D2 also enables an induction current during the switch-off process, this also slightly increases the effective current value of the coil in the switch-off phase and thus the magnetic force of the core. How much or how little the diode contributes can be easily tested by driving the operating voltage down to the area where the armature drops.

Something else is new in drawing file 6.2. An LED is inserted in series with the relay coil. This lights up when the relay is switched on, as in Figure 3. Here, too, with a current of only 3.2 mA it is necessary to use a so-called low-current LED so that it lights up brightly enough. The LED is included in the circuit with the freewheeling diode D2. The low self-induction current contributes very little to the fact that the LED flickers a little less. You only notice the flickering anyway if you look at the LED from the side, because 50 Hz is not yet that critical. With a full wave rectification the LED "flashes" with 100 Hz (not with 50 Hz) and this is not noticed at all.

The MOSFET application: Since every FET is voltage-controlled and its input resistance is extremely high, it logically does not need a resistor in front of the gate. That is why no such resistance is drawn. Nevertheless, a resistor is recommended if you want to be absolutely sure that no oscillation can occur during the switching process.The high-frequency oscillation can occur as a result of parasitic capacitances and (conductor track) inductances. Such a series resistor usually has a value of a few tens or about 100 ohms directly following the gate connection. For very fast switching applications, a small choke with a very low inductance is sometimes used instead of a resistor. Such gate resistors are used in Figure 7.

Part 7.1 repeats part 6.2 again. If we compare part 7.2 with part 7.1, we can see two differences: The operating voltage of 115 VAC (USA) is only half as high and the free-wheeling diode has been replaced by a capacitor with a capacity of 470 nF and a nominal voltage of 250 VDC. Although the maximum voltage that occurs can only be 163 Vp, you should generally be generous when choosing capacitors and not choose the nominal voltage too short. The relay is the same type with a nominal voltage of 230 VAC, and yet we operate it with a voltage of only 115 VAC, which is one-way rectified with D1. This works because the capacitor C has an additional effective smoothing function in addition to the short-circuit winding in the relay. The best way to determine the capacity here is empirical. It is not important whether the nominal current of 3.2 mA is adhered to exactly. The main thing is that the minimum pull-in voltage of the relay is reasonably lower than the voltage at rated current.

In the circuit used here in section 7.2, the minimum voltage is 75 VAC as in section 5.3, where a full-wave rectification works with 115 VAC and therefore does not require an additional capacitor. To mention it again here, a freewheeling diode instead of the capacitor is not enough. At least with the AC relay used here. With a different make with different data, you have to test whether a freewheeling diode is sufficient.

Now to the question of why the additional series resistor Rv (series resistor in front of the relay coil) is needed in part 7.2. If the mains voltage has a high positive value at the moment of switching on, the drain current of the MOSFET T is very high for a short time and could endanger it. It is the current through the capacitor C. Resistor Rv, with a value of 820 ohms, limits this peak current to a maximum of 200 mA when the positive sine half-wave just has the peak value of 163 VDCp, whereby the RC time constant is only 0.4 ms. The resistance of the relay coil is not taken into account because it is orders of magnitude higher than Rv.

The safe operating diagram of the BSS125 allows a current of 200 mA with a pulse duration of 1 ms (parameter) up to a maximum drain-source voltage of around 160 V (Up = 163 VDCp). However, the Rv * C time constant is only 0.4 ms and, in addition, the current is not constant during this 0.4 ms because C is charged exponentially by Rv. The blue parameter line is an estimate for the parameter of 0.4 ms. It can be seen that with a switch-on pulse of 163 VDCp of 0.4 ms, a current of almost 300 mA would be permissible.

The fact is, however, that this SOA diagram is not challenged at all, because the switching on and off process is much faster, provided, however, that the switching source is low enough, e.g. from a logic gate. The switching process is at least two orders of magnitude faster than the 100µs parameter shows. So the diagram does not matter, because only the maximum permissible continuous current (drain current) of the BSS125 of 100 mA and its maximum open drain-source voltage of 600 V apply. Assuming the switching edge is, for whatever reason, relatively slow at 100 µs, a drain current of 200 mA with a drain-source voltage of 400 V would still be permissible.

A coil current of 3.2 mA flows in the operating state. That means a voltage drop of 2.7 VDC over Rv with 820 ohms. This is very little compared to the operating voltage. It still has to be considered how the one-time charging of C during the switch-on process affects Rv. At the moment T (BSS125) is switched on, the maximum positive peak voltage of the pulsating half-wave DC voltage is above Rv. For a very short moment this means an output of 32 W over Rv. This performance seems very high. Seems because it lasts extremely short, because after the 5th time constant C is practically charged and that's only 2 ms. The charging time constant from C over Rv is 0.4 ms. A circuit that I implemented for an automatic mains voltage switchover for transformers (2) proves that this does not cause a problem.

Part 7.3: In the test phase I tried to destroy a 1/4 watt resistor by switching it on and off. I couldn't even do it with a capacitor nearly five times the capacity. I toggled the S switch quickly back and forth. Even after a minute, there was no noticeable warming with a simple finger test on Rv with the operating voltage switched off. I generated this 120 VDC with the series connection of four ± 15VDC power supply units.

Fig. 8 shows two circuits with the same 230 VAC relay as in Fig. 7. In parts 7.2 and 8.2, the charging time constants of Rv and C, or R7 and C are identical with 0.4 ms. The same text above applies to test circuit 7.3, starting with the words: "In the test phase I tried ..."Here, too, it does not mean anything else that a 1/4 watt resistor can be used for the R7.

The circuit on the left, part 8.1, is in operation in the 230VAC network. The circuit on the right in section 8.2 in the 115VAC network (USA). Instead of a MOSFET, you can also use two NPN transistors in a cascade connection. It is quite difficult to find bipolar transistors for low maximum collector currents in the small TO92 housing if the open collector-emitter voltage is to be more than 300 VDC, which is necessary with a mains voltage of 230 VAC. The cascade connection with two transistors is particularly suitable. The MPSA42, which is widely used, is very suitable for as little collector current as is required in the present relay circuits. It has a maximum open collector-emitter voltage of 300 V and allows a maximum collector current of 500 mA (MPSA44 = 300 mA) in the switched (saturated) state.

Instead of the MPSA42, the MPSA44 from ON-Semiconductor is also suitable. It is worth using the MPSA44 with an open collector-emitter voltage of 400 V. This also offers a slightly higher level of protection against overvoltages. In addition, the data sheet of the MPSA44 offers more information, e.g. with a safe operating area diagram (SOA). MPSA42 and MPSA44 are available from Distrelec and Farnell (February 2019).

In part 8.1 we want to examine how the transistor cascade works. The voltage is divided by the voltage divider consisting of R1, R2 and R3, the half-voltage of which is at the node R2 / R3 at the base of T2. As a result, the T2 emitter and T1 collector voltages are also at half the maximum voltage of 325 Vp at 163 Vp, apart from the base-emitter threshold voltage of T2. The two open collector-emitter paths share the value of 325 Vp.

When the transistor cascade is switched on by means of a HIGH level at Ue and the relay is picked up, the T2 base is practically at 0 Vp, based on REF. In fact, this voltage is less than 1 Vp. This voltage is made up of the base-emitter threshold voltage of T2 and the saturated collector-emitter voltage of T1. This means that the full half-wave rectified mains voltage of 325 Vp is across R1 and R2. Two resistors connected in series are used so that the high voltage is distributed approximately evenly between two resistors. The point is that you can use small 1/4 watt resistors, because a voltage of 325 Vp is too high for just one resistor.

Remember, if any overvoltages are even higher (rural regions), you have to limit the mains voltage range with zinc oxide varistors and even with other measures. The operating power loss is very low, it is less than 50 mW for R1 or R2. When switched on, T2 is also in the saturated state. This means that the T2 collector voltage against REF is only a few 100 mV.

The circuit in section 8.2 works like the circuit in section 7.2 with 115 VAC, but with the bipolar transistors MPSA42 or MPSA44. This circuit can be used in this electronics mini-course (3) instead of the same with MOSFETs. It starts with the chapter "The circuit". There the relay is in action when the circuit is in use, e.g. in the USA with 115VAC.

The following applies to both circuits in sections 8.1 and 8.2: R5 is based on the HIGH level input voltage and the T1 base current of around 0.2 mA. If the LOW level voltage of less than 0.5 V cannot be guaranteed, a voltage divider from R5 and R6 must be created with an additional resistor R6 between T1 base and REF (T1 emitter). The cross-current through these two resistors should then be about three times as high as the T1 base current, i.e. about 0.6 mA or equal to 1 mA. C1 is only indicated at this point. You can find out why C1 in connection with R5 and another resistor at the base of T1 is needed in the following chapter "Overvoltage despite transistor cascade".

Overvoltage despite transistor cascade?

Yes, that is quite possible, as I once read in a discussion in the ELKO forum. It was pointed out that individual transistors in a cascade react slightly at different speeds and therefore, if the switching voltage on the input side has a high edge steepness, an excessively high voltage briefly occurs between collector and emitter. I checked this with a small test circuit. For this experiment I used the same high-voltage transistors of the type MPSA42, but only an operating voltage of 24 VDC. It was found with a voltage variation of + Ub between 10 VDC and 30 VDC that nothing significant changes in the signal conditions. The effect I want to show is the same. This brings us to Figure 9:

Part 9.1 shows the test circuit. The operating voltage of +24 VDC is connected to + Ub. R3 is chosen so that the current through R3 is about 3.5 mA, the same as the coil current in the relay in part 8.1. Correspondingly, R4 and R5 are adapted to the 1/2 voltage ratio with approximately the same cross current Iq. The time-symmetrical square-wave signal from a clock generator with an output voltage of 5 Vp or a typical TTL voltage is applied to Ue. The frequency is in the lower kHz range.

The first experiment with part 9.2 shows the diagram without the use of C1. The full edge steepness of the square-wave voltage at Ue reaches the base of T1 via R1 and R2. Shortly before the rising edge of the voltage Ue, the two collector-emitter voltages Uce1 and Uce2 are equal. Uce1 and Uce2 are each about half the operating voltage + Ub / 2. The voltage divider from R4 and R5 generates + Ub / 2 at the T2 base. Ub / 2-Ube (T2) is at Ua1, i.e. about 0.7 V lower than + Ub / 2. No current flows through R3.

With the rising flank of Ue, the flanks of Uce1 (Ua1) and Uce2 (Ua2-Ua1) fall at the same speed. This is the joint switch-on process for T1 and T2. This means that during this dynamic process the condition Uce1 = Uce2 is always met. The situation is completely different with the falling edge of Ue, when the two transistors T1 and T2 open. This two-way cascade shows that T1 opens faster than T2. The voltage Uce1 (Ua1) rises faster than Uce2 (Ua2) and this has the consequence that for a short time of about 0.1 ms Uce1 is significantly greater than Uce2. Only in the steady state does Uce1 = Uce2 apply again. In this brief moment, T1 can get too high a voltage in the state of the circuit in section 8.1 (230VAC) if the operating voltage is high enough for it. A worst case reason to use the MPSA44 instead of the MPSA42.

Regarding the diagrams, it should be mentioned that the sketch is idealized for the purpose of simpler labeling. It is also not true to scale. The switch-on edges are also not as steep as it gives the impression here. The point here is to show how the overvoltage of T1 comes about at the moment of switching off. The numbers are correct. They correspond to the measurements.

The second experiment in part 9.3 shows the diagram with the use of C1. The low-pass filtering with R1 and C1 reduces the edge steepness of the voltage between the base and GND of T1 below the base-emitter threshold voltage at which T1 begins to conduct. From the moment of the base current and its increase, the base-emitter voltage remains largely constant. From there, there is a reduced edge steepness of the base current, which ensures that T1 is not overtaxed in terms of its speed. The time constant for the T1 base current change results from the value of the parallel resistance of R1 and R2 with C1. The slower change in the T1 collector current also puts too little demand on T2 because it behaves just as more slowly, adapted to T1. A simultaneous rise and fall of Ua1 (Uce1) and Ua2 (Uce2) occurs when the time constant R1 * C1 is about 1 ms.

This experiment shows that this effect will also occur in the two circuits of Figure 8 if C1 is not used. However, R1 * C1 time constant may be significantly smaller than here in the experiment. You have to check this with the oscilloscope.

Overvoltage without consequences

Diagram 10.1 shows the 230 VAC sinusoidal voltage at the input of the operating voltage of 230VAC (Uac), based on the reference voltage REF. Diagram 10.2 shows the half-wave rectified DC voltage at the output of diode D1 (Udc). Ue is due to a voltage that ensures that the relay is switched on. This is certainly the case with 5 VDC or TTL high level. The resistor R4 does not exist at the moment. What do you observe with an oscilloscope when you measure the voltage between the base of T2 and REF? This is shown in diagram 10.3. The exact same negative overvoltage is measured directly between the base and emitter of T2. When the sinus voltage falls below 0 V (REF) in Diagram 10.1, a negative voltage of up to -7 V is shown in Diagram 10.3, which is caused by the limiter effect of the base-emitter diode in the blocking range. In reality this negative voltage would be much higher. The current that flows through the base-emitter diode in the opposite direction, however, is so low, determined by the network of R1, R2 and R3, that T2 cannot possibly be damaged. Nevertheless, it is worth adding R4, which drastically reduces this -7 V, as shown in Diagram 10.4.

Link list

This link list contains three applications in which high-voltage transistors are used to control relays on the 230 VAC network, which also have very low power consumption. Link 1 contains the method of the transistor cascade with two MPSA42, as described here. This method is particularly suitable when only a very low control voltage is available. Then a bipolar transistor has to be controlled by means of base current. Small high-voltage MOSFETs are used in Link 2 and 3 because the control circuit supplies enough voltage to generate the gate / source voltage. A cascade formation is not necessary. Link 4 supplements the historical part mentioned at the beginning on circuit technology with cold cathode tubes and Link 5 explains in a chapter the safe working range of bipolar transistors. Also a topic that is relevant here.

Thomas Schaerer, February 1, 2007; 08/23/2009; 03/09/2010; May 16, 2010; 06/25/2014; 02/12/2019