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Homemade Solid State Relay (SSR) Circuit Explained

Mechanical type of relays can be pretty inefficient in applications which require highly smooth, very swift and clean switching. The proposed circuit of an SSR can be built at home and used in places which require truly sophisticated load handling.

A solid state relay circuit with in built zero crossing detector is described in this article. The circuit is very easy to understand and build yet provides with useful features like clean switching, free from RF disturbances, and able to handle loads up to 500 watts.

We have learned a lot about relays and how they function. We know that these devices are used for switching heavy electrical loads through external isolated pair of contacts, in response to a small electrical pulse received from an electronic circuit output. Normally the trigger input is in the vicinity of the relay coil voltage, which may be 6, 12 or 24 V DC, while the load and the current switched by the relay contacts are mostly at the levels of AC mains potentials.

Basically relays are useful because they are able to toggle heavy connected to their contacts without bringing the dangerous potentials in contact with the vulnerable electronic circuit through which it is being switched.

However the advantages are accompanied by a few critical drawbacks which cannot be ignored. Since the contacts involve mechanical operations, sometimes are quite inept with sophisticated circuits which require highly accurate, quick and efficient switching. Mechanical relays also have the bad reputation of generating RF interference and noise during switching which also results in its contacts degradation with time.

Triacs and SCRs are thought to be good replacements in places where the above relays prove inefficient, however these too may involve RF interference generation problems while operating. Also SCRs and Triacs when integrated directly to electronic circuits require the circuit’s ground line to be connected with its cathode, which means the circuit section is now no longer isolated from the lethal AC voltages from the device – a serious drawback as far as safety to the user is concerned.
However a triac can be very efficiently implemented if the above discussed couple of drawbacks are completely taken care of. Therefore the two things which must be removed with triacs, if they were to be efficiently replaced for relays are, RF interference while changeover, and the entry of the dangerous mains into the circuit.
Solid State relays are designed exactly with the above specifications, which eliminates RF inference and also keeps the two stages completely aloof from exh other.
Commercial SSRs can be very costly and aren’t serviceable if anything goes wrong. However making a solid state relay all by you and using it for the required application can be just what the “doctor had ordered.” Since it can be built using discrete electronic components becomes completely repairable, modifiable and moreover it provides you with a clear idea regarding the internal operations of the system.
Here we will study the making of a simple solid state relay.

Circuit Description

As discussed in the above section, in the proposed SSR or solid state relay circuit design the RF interference is checked by forcing the triac to switch only around the zero mark of the AC sine phase and the use of an opto coupler ensures that the input is kept well away from the AC mains potentials present with the triac circuit.
Let’s try to understand how the circuit functions:
As shown in the diagram the opto coupler becomes the portal between the trigger and the switching circuit. The input trigger is applied to the LED of the opto which illuminates and makes the photo-transistor conduct.
The voltage from the photo-transistor passes across the collector to the emitter and finally reaches the triac’s gate to operate it.
The above operation is pretty ordinary and is commonly associated with the trigger of all Triacs and SCRs. However this may not be enough to make the RF noise eliminate.
The section comprising the three transistors and some resistors are especially introduced with the view of checking the RF generation, by ensuring that the triac conducts only in the vicinity of the zero thresholds of the AC sine waveform.
When AC mains is applied to the circuit, a rectified DC becomes available at the collector of the opto transistor and it conducts as explained above, however the voltage at the junction of the resistors connected to the base of T1 is so adjusted that it conducts immediately after the AC waveform rises above the 7 volt mark. For so long the waveform stays above this level keeps T1 switched ON.

This grounds the collector voltage of the opto transistor, inhibiting the triac from conducting, but the moment the voltage reaches 7 volts and nears zero, the transistors stop conducting allowing the triac to switch.

The process is repeated during the negative half cycle when T2, T3 conducts in response to voltages above minus 7 volts again making sute that the triac fires only when the phase potential nears zero, effectively eliminating the induction of zero crossing RF interferences.

Parts List for the proposed solid state relay circuit

R1 = 120 K,
R2 = 680K,
R3 = 1 K,
R4 = 330 K,
R5 = 1 M,
R6 = 100 Ohms 1 W,
C1 = 220 uF / 25 V,
C2 = 474 / 400 V Metalized Polyester
Z1 = 30 volts, 1 W,
T1, T2 =BC547B,
T3 = BC557B,
TR1 = BT 36,
OP1 = MCT2E or similar.

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    1. Looks like a pretty useful circuit, planning on making a few if I get around to it. One question though, is C1 polarised (I'd guess not)? The value of C3 also isn't listed in the parts list...


    2. Thanks!

      C1 is non-polar(Metalized Polyester), C2 = 0.1uF/400V

    3. Hi! Maybe the circuit I`ve been looking for. I`m struggeling with noise when connecting a 220V, about 100W AC-motor via a mechanical relay. The noise often cause the microcontroller in the control circuit to reset or hang. A few questions:
      There is no unit on C2, and no value on C3? When the trigger is applied I understand that the triac will turn on when the voltage is inside -7 to +7 volts. But will the triac stay on as long as the input to the optocoupler?

      1. Hi, the value of C2 = 0.5uF/400V, C3 = 0.1uF/400V.
        Yes, the triac will stay ON as long as the input to the opto is present.

        However I think today there are even better and efficient circuits available using lesser number of components, please refer the following article:

    4. Hi Swagatam

      I am looking for an SSR which can handle 100A 12V load, input voltage 6 - 12V.
      I assume the above circuit is suitable for AC main loads.

      1. Hi Abu-Hafss, if the shown triac is rated adequately as per the specified load current then surely it can be applied for the same.

      2. Hi Swagatam

        I have read somewhere that DC SSR should not have zero crossing detector and should have an SCR instead of traic.

      3. Hi Abu-Hafss, the above circuit is designed for controlling AC loads at mains level, therefore a zero crossing becomes advantageous for suppressing noise and interference.

      4. That's what I am asking, can you offer me some design of DC SSR without zero crossing detector capable of handling 100A at 12V.

      5. ...if possible i'll try to design and publish soon.


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