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Simple SCR based Application Circuits

In this article we are going to learn a few interesting SCR based application circuits and also learn the main features and properties of a thyristor or SCR device

What's an SCR or Thyristor

SCR is the acronym of Silicon Controlled Rectifier, as the name suggests it's a kind of diode or a rectifying agent whose conduction or operation can be controlled through an external trigger.

It means that this device will switch ON or OFF in response to an external small signal or voltage, quite similar to a transistor but yet hugely different with technical characteristics.

Looking at the figure we can see that a SCR has three leads which mat be identified as follows:

Keeping the printed side of the device facing us,

The right end lead is called the "gate".

The center lead is the "Anode", and

The left end lead is the "Cathode"

SCR C106 pinout datasheet, specifications

How SCRs should be connected:

The gate is the trigger input of an SCR and requires a DC trigger with a voltage of around 2 volts, the DC should be ideally more than 10mA. This trigger is applied across the gate and the ground of the circuit, meaning the positive of the DC goes to the gate and the negative to the ground.

The conduction of voltage across the anode and the cathode is switched ON when the gate trigger is applied and vice versa.

The extreme left lead or the cathode of an SCR should always be connected to the ground of the triggering circuit, meaning the ground of the triggering circuit should be made common by connecting to the SCR cathode or else the SCR will never respond to the applied triggers.

The load is always connected across the anode and an AC supply voltage which may be required for activating the load.

SCRs are specifically suited for switching AC loads, DC loads will not work with SCRs because with a DC load since the SCR is triggered, it will latch and will not switch OFF even after the gate trigger is removed.

The following examples shows a few application circuits which can be built and also modified for more useful purposes:

Simple Rain Alarm:

SCR based rain sensor

The above circuit of a rain alarm can be used for activating a AC load, like a lamp or an automatic folding cover or shade.

The sensor is made by placing to metallic pegs, or screws or similar metal over a plastic body. The wires from these metals are connected across the base of a triggering transistor stage.

The sensor is the only part of the circuit which is placed outdoors, for sensing a rain fall.

When a rain fall begins, water droplets bridge the metals of the sensor.

water triggered AC load through SCR

Small voltage start leaking across the sensor metals and reach the base of the transistor, the transistor immediately conducts and supplies the required gate current to the SCR.

The SCR also responds and switches ON the connected AC load for pulling an automatic cover or simply an alarm for correcting the situation as desired by the user.

Simple Burglar Alarm

SCR burglar alarm circuit

We discussed in the previous section regarding a special property of SCR where it latches in response to DC loads.

The circuit described below exploits the above property of the SCR effectively for triggering an alarm in response to a possible theft.

Here, initially the SCR is held in a switched OFF position as long as its gate stays rigged or screwed with the ground potential which happens to be the body of the asset which is required to be protected.

If an attempt to steal the asset is made by unscrewing the relevant bolt, the ground potential to the SCR is removed and the transistor gets activated through the associated resistor connected across its base and positive.

The SCR also instantly triggers because now it gets its gate voltage from the transistor emitter, and latches sounding the connected DC alarm.

The alarm remains switched ON until its switched OFF manually, hopefully by the actual owner.

Simple Fence Charger, Energizer Circuit

SCRs becomes ideally suited for making fence charger circuits. Fence chargers primarily require a high voltage generator stage, where a high switching device like an SCR becomes highly imperative. SCRs thus become specifically suitable for such applications where they are used for generating the required high arcing voltages.

CDI Circuit for Automobiles:

As explained in the above application, SCRs are also widely used in automobiles, in their ignition systems. Capacitive discharge ignition circuits or CDI systems employ SCRs for generating high voltage switching required for the ignition process or for starting a vehicle ignition.

Need Help? Please leave a comment, I'll get back soon with a reply!


  1. Hi Swagatam

    How can we use an SCR as a latch to have the supply voltage at the output when triggered?

  2. Hi Abu-Hafss,

    The load should be connected in between the positive and the anode of the SCR, meaning the positive to the scr anode has to pass through the load before reaching the ground (cathode).

  3. Hi Swagatam

    You proposed me that SCRs could be used as latch in my E-C-B identifier circuit. The latches I used there were to pass on and lock the signals detected from the opto-couplers. So, what I understand from your above post is that SCRs could not be used there, only CMOS would do the job. What do you say?

  4. Hi Abu-Hafss,

    an scr could be used for your application. the scr gate could be connected to the optocoupler output, anode to positive supply, while the cathode could be used as the latching 12v output.
    A 1k resistor could be used for terminating the scr cathode to ground for helping proper gate conduction.

  5. Hi Swagatam

    If I am not wrong you proposed the connections as below:

    However, in this way the output we get is about 5V when triggering voltage is HIGH. And then it drops to zero when trigger is LOW.

  6. Hi Abu-Hafss,

    Yes that's right, there's no reason why this will not work.
    The anode 9V would also reach the cathode just like a diode would do I assume.

  7. Hi Swagatam

    Here is the working configuration of an SCR as a latch.

    Now, as you can see the SCR Latch involves 1 diode, 3 resistors and 1 SCR. In my transistor analyzer circuit, I have use 3 x 3 latches. I am wondering if SCR latch would be worthwhile against CMOS Latch chip CD4043?

    Moreover, since I haven't used that chip I am little confused over the function of ENABLE pin. Does it mean that when the chip is powered on, if the input is LOW the chip would lock that status to output as LOW? And we have to send signal at ENABLE pin to reset it for a fresh input?

    Or the chip waits for the input after being powered on?

  8. Hi Abu-Hafss,

    The scr will get latched with the 9v supply, not with the 5v gate supply, so it's strange how the voltage across the 200ohm resislor is showing as 4.5V, and 200ohm is too small you should have used a 1k resistor.

    Unless you do practical tests, confirming them will be impossible.

    for 4043 IC, if the enable is held low, the ouputs will show a high impedance and will become completely open, that is neither low nor high.

    I have explained it in this article:

  9. Hi Swagatam

    The voltage across 200 Ohms resistor is not 4.5V, it is 8.3V (9V - 0.7 of internal diode of SCR).

    The value of R1 should be below 800Ohms for EC103A and below 2.7k for BT151.

    R3 is necessary when using BT151 to avoid gate-cathode leakage.

  10. Hi Abu-Hafss,
    I mistakenly saw the gate voltage waveform and interpreted as the cathode wave form...ok got it.
    By the way you can try BT169 instead of's a low power scr, will suit well for your application

  11. Good Day Mr Swagatam. I would like to ask if you have a circuit which utilizes the aforementioned SCR with Thermistor as a sensor? would like to have one to drive a cooling fan. THANKS!

  12. Good day jolt, yes it's possible, you may refer to the following design for more info:

  13. If I may, just wanna ask what are the important parameters to look at when checking Op-amps? For DC purposes only. We're having a project, just a simple temperature sensor and im using LM741. the circuit seems to be working perfect in multisim and all, im using the schmitt trigger but dont know the important specs in the 741, datasheets have so many specs in them. we have to make a report nonetheless. THANKS A LOT!

  14. you may refer to the following post for more info:

  15. Good Day! Please help, I've been simulating in multisim, and im using 2n1599 SCR. it says it needs 10mA, 3v (Igt, Vgt) respectively, yet even if i provide only 3volts without enough current, just in uA's, the SCR still triggers. Does it really work like that?

  16. I am sorry, I have never used simulators for checking my circuits, so cant say much about these softwares.

  17. Sir Swagatam Majumdar, can i ask if the scr 106m is same with the scr T106M1, ?

  18. not sure xerxesbrake, you'll have to consult the shopkeeper himself.

  19. in case of a Dc load across scr, its gate requires to be triggered once to switch on the load. But will the scr respond to the On or Off trigger when AC load is connected

  20. for AC, the load will stay activated only as long as the gate voltage is present and switch OFF immediately when it's removed...

  21. In case of AC load across an scr, its performance would be almost at 50% as only the positive half cycle is fed to the load. Now using an scr with phase control, only this positive half cycle can be controlled mean that the load efficiency can be lowered than 50%.
    Phase control with a Triac, the load efficiency can be controlled more than that of an scr. Am I correct sir?

  22. The voltage response or the range would be 50% since it's designed in that way, but the efficiency would be 100%, as the SCR would be doing it's job fully.

    so yes for triac the range of control is 0 to 100% or full 360 deg and for scr it's 0 to 50% or 180 deg.

  23. a Triac does not switch symmetricaly
    what does it mean?

  24. the two halves of the triac might not conduct at the same power rate with a given gate voltage...

  25. If I replace the scr with a Triac to drive the AC load in your designed Rain Detecting circuit will it work?
    I mean to know whether the load will get full wave voltage?

  26. yes definitely it will work, in the second design

  27. Sir I want to drive only one relay with a single transistor stage on pin 3,4,and 7 of ic 4017.
    these three pins will have their own resistor?
    I just short these three pins and have a single resistor for transistor base?

  28. Mujahid, please specify your application need for the correct answer...three relays will click sequentially but if one relay is used it will stay switched ON permanently while the sequence is traveling from pin3 to pin7

    shorting will not do, you'll have to connect the 3 pinouts through diodes and then short the cathodes of the diodes...this point can be then connected with the relay driver transistor stage via a resistor.

  29. Actually I want to drive only one relay with 3 pinout to gain the required time
    Sir why should I use diode to short these pins? Why not without diode?

  30. if you don't use diodes then the 4017 output will get short circuited across each other making the IC hot...

  31. In a washing machine circuit configured around ic4017 the pinout were shorted via resistors
    can we use resistor in place of diode

  32. If an scr holding current is 20mA and the load across its anode and cathode is 10mA. Will the scr latch or not?

  33. resistors can be used but diode is right way to go, please show me the link which you are referring to.

  34. Washing machine circuit :
    supply 12v dc, 4017 pinouts divided into two group 1st group: 3,2,4,and 7
    2nd group: 1,5,6 and 9
    Each pinout of 1st group is shorted via resistor of 4k7 and 2nd group same.
    Then connected to the transistor base c 828
    each group has its separate transistor and relay .

  35. using resistors will produce 50% voltage at the transistor base, diodes will give 100% voltage, .........depending upon the application needs both will work.

  36. What factors should be taken care of when selecting a Triac

  37. current and voltage breakdown specs

  38. timer circuit , configured around ic 555, supply 12volt dc, relay Resistance 200ohm

    With the above specs kindly sir what transistor should I use and what should be its base resistor value?

  39. you can connect the relay directly with the pin3 of the IC, if you want it through a transistor then you may use a 2N2222 or a 8050 transistor with a 10K base resistor.

  40. What is Astable Mode and monostable Mode with regard to ic 555

  41. "connecting the relay directly with ic pin3"
    Sir I remember You asked me not to connect the relay directly with ic output.

  42. astable is the flashing mode of 555 output, monostable is just s single pulse mode in response to a ground signal at pin2

  43. 200 ohm will consume 60mA so it can be used directly at pin3

    Please show me where I said the opposite, so that I can correct it?

  44. I'm sorry sir I don.t remember the post
    but you said that the ic output current is insufficient to drive the relay directly. For that you asked me to use transistor.

  45. I'll never say that, a 555 IC can always power small relays directly, all the circuits in this blog which has a relay with IC 555 is directly connected to its pin3 ...unless the case is special.

    that's why I wanted to see where I said that.

    But If the relay is heavier then it might not be advisable because the 555 might heat up and get destroyed.

  46. sir,
    is it possible to connect ac neutral and dc negative together

  47. Vipin, it doesn't matter whether it's neutral or phase, polarity is never considered for AC inputs, while integrating with DC circuits

  48. sir
    is there any problem when a load of 5A is connected to scr and scr is fired from a 12v,200ma DC supply.

    what is the effect of load current on Dc power supply?

  49. Vipin, your SCR must be rated to handle at least 6 amps, otherwise it will get damaged or burned.

  50. if the source is 200mA then the SCR will not get affected but the voltage might drop drastically due to the high consumption from the load....

  51. dear sir can i use BT136 or BT139 instead of C106?


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