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Mosfet Based SPDT Solid State DC Relay (SSR) Circuit

Mosfet Based SPDT Solid State DC Relay (SSR) Circuit

In this post we'll study a simple high current mosfet based solid state relay, which can be used in place conventional bulky SPDT mechanical relays. The idea was requested by Mr. Abu-Hafss.

The Design

A simple high current SPDT solid state relay or an SSR can be constructed using a couple of mosfets and an optocoupler, as shown in the digaram above.

The idea looks self explanatory.

In an absence of an external trigger, the lower mosfet stays switched OFF allowing the upper mosfet to conduct through the 10k resistor connected across the positive and the gate of the mosfet.

This enables the N/C contact to get active, and a DC load connected across the supply positive and the N/C gets activated in this situation and vice versa.

Conversely in the presence of an input trigger, the mosfet connected with the opto emitter gets an opportunity to switch ON, switching OFF the upper mosfet.

In this situation a load connected between positive and N/O points gets activated or vice versa.

The Circuit Diagram

Mosfet SPDT SSR Switch Circuit

About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

41 thoughts on “Mosfet Based SPDT Solid State DC Relay (SSR) Circuit”

  1. Howdy, Friend! Interested to Learn Circuit Designing? Let's Start Discussing below!
  2. Hmmm this is a part by part explanation of the schematic. The schematic looks easier to understand now:) Actually I wanted a simplified explanation, the same way as you explained in your 1st and the 2nd comments.

    Anyway, since we do not want to disturb the wiring layout in any manner, the best way to solve this would be by providing directly replaceable mosfet relay modules which you can exchange with the existing mechanical relays. The AC relays could be replaced with AC SSRs. I’ll work on it and let you know soon.

  3. I checked it, it looks too lengthy, so it can take many hours to get it sorted. A block diagram instead would be more suitable and easy to understand, or a written description step wise would also do.

  4. Could you upload it on any free image hosting site and give me the link? then I can check it out.

    Or you may simply explain it verbally so that configure the schematic myself

  5. Thanks so much for help. I now need to do some testing to see how this works out.

    My next problem is eliminate 2 relays.
    1. When 12v switches on I need to direct 12v load (up to 10amp) to the rest of the devices.
    2. When 12v switches on I need to switch on the AC mains (250v) at 2amp
    Because this system includes a pa amplifier I need to be careful about interference
    So looking at various transistors and triacs has left me floundering.
    Please help!

    • Just to be sure, is this 12V from the battery and solar panel? For all these arrangements we might need a slightly more complex circuitry.
      The interference from the diodes can be avoided by adding 1000uF capacitor across the common cathodes and ground.

      If you can provide the complete scenario point-wise I may try to figure it out through a schematic.

  6. Really? That’s fantastic. Is there a danger of the two sources oscillating between the two if the voltages are close (say 0.3v difference) because I don’t think the TV will work at 18.5v

    • Yes it may oscillate if both the sources are at close levels. You can add a 19V volt regulator between the TV and diodes, this will ensure that the TV always gets 19V regardless of the source

  7. Wow! This sounds too easy. The 12v solar battery also runs a sound console and some led lights so the buck converter is on all the time. If this converter and the mains power supply (smps) are both on and connected in parallel (with protective diodes) which one will the TV draw current from?
    Thanks Paul

    • It will depend which source has a slightly more voltage. If the SMPS has 1V higher than the buck output then the TV will draw from the SMPS and vice versa. So basically the diodes work like silent switches and allows only the source which has higher voltage for the load.

  8. Hi Swagatam
    I scratching my head to see if your spdt circuit will work for me.
    I have a TV which requires 19.5v DC.
    If AC power is on then it gets 19.5v from a mains power supply.
    When AC fails then it needs to switch over to a solar battery backup with a 12v to 19.5v step-up converter.
    When AC returns then it needs to switch back after a short delay to allow the power supply to come up to speed.

    Help appreciated, thanks

    • Hi Paul,

      No need of using this complex circuit. You can feed the positives from the solar controller and the AC adapter through separate diodes and connect the common cathode ends with the TV positive terminal. The negatives can be all joined into common. However for getting a delayed response from your adapter you may have to add an additional delay timer at the cathode side of the adapter diode. Hope this helps!

  9. Hi Swagatam,
    I would like to do something similar, but maybe you can help me how to do it.
    I have many 4.2V ION cells in series and would like to either couple the cell in or bypass the cell.
    I would like to control each cell by a MCU (Arduino Mega) and select if the cell needs to be i serial with the other cells, or needs to be bypassed.
    Thanks for your work, really appreciate it.

    • Thank you, however I am not sure regarding the specs of the shown mosfets, please refer to the datasheet for the answer, or you can replace the same with any other known n-channel mofet.

      • Thank you for your quick answer. I’m admiring your work.
        I have a doubt maybe you could tell me about MOSFET. how transistor BJT have a drop voltage fixe or nearly 0,7V, i would like know if mosfet, have a drop voltage fixe how transistor BJT of 0,7V? Or only drop voltage MOSFET is calculed on your RdsOn?
        I am createing a system of backup energy with battery for my circuit, when supply power main is cut, my battery begin work, I am use battery lipo 3.7V and doing a step-up for 5V just when power main is cut. when main power is cut, i have MOSFET P that begin work, flowing current for my circuit main again. I cant to lost nothing voltage when this mosfet switch. In my simulator everything work normaly, but still do not a real circuit for test, maybe your experience can anwser my doubt about mosfet have some lost voltage how transistor

  10. Great circuit! But, why is the optocoupler needed? Why can’t control the switches directly from the trigger input? thx!

    • yes opto coupler can be avoided and the input directly connected with the trigger point, an opto may be used only if the trigger source needs to be isolated from the relay

  11. Very nicely written. Thanks for your precious work. One question though. In absence of an external trigger, what is the input at gate for the lower MOSFET? SHould it be connected to ground or to a supply voltage?

  12. Hello sir thank you for this circuit now I can implement the over load protection circuit,relay have being my problem. Please may I ask.Q1 to achieve high amps I just need to use high amp mosfet.eg to handle 200amps I use like irfp2907 right? Q2 will the mosfets need heat sink as I plan to use it for up to 150amps inverter.Q3 how do I modify it to handle higher volts , say 24-48 volts. Q4 if I use 4 mosfet to make it DPDT will it work. Can the optocoupler handle it? Q5 what will be the recommended mosfet gate volts .Lastly Q6 what will be the trigger volts. Also is it ac or dc? I look forward to your reply is very important to me . Am Shedrach proudly your Student.

    • Hello Sherach, here are the answers:

      1) yes you just need to upgrade the mosfet current (or voltage) specs for achieving higher wattage at the output
      2) heatsink will be strictly required, may be with fan cooling also.
      3)For higher voltage make sure the mosfets are rated above that limit…for 48V it should 60V and so on.
      4) 4 mosfets can be used for getting DPDT operations.
      5) 10V minimum and 15V max
      6) the trigger voltage can eb anywhere from 2V to 15V but make sure to add a roughly calculated limiting resistor in series accordingly for safeguarding the opto LED.

  13. I am currently using an automotive SPDT relay to simply switch a 0 ohm ground signal that my cruise control unit is looking for. This is the trigger that shuts the cruise control off when you tap the brake pedal.

    I am currently using a relay and it works fine but that the relay is loud and click every time I tap the brake pedal drives me crazy. I see in your above diagram and write-up you use the word "load" for both the N/C and N/O terminals so that tells me this design would work for situations where the relay is being used to switch circuits drawing current. My question is whether or not the same circuit can be used for a near-zero load (ground signal wire switching) or if I'd need to stick with something like the mechanical relay to accomplish this?

    • You can effectively use the above solid state relay as long as the "load" current rating does not exceed the current specs of the mosfets, as long as this is maintained the circuit would serve the purpose well….however the above SSR will require a positive signal for activation, a ground signal will not work with the above design.

  14. Hi Swagatam, nice to see that there are people like you in this world. And, more, thanks for your precious work.

    It should be interesting to publish an DPDT SSR…it shouldn't be so different from an SPDT: in the end a DPDT is a twin-SPDT!

    In addition, it could be interesting to evaluate the possibility to add the optional "stepper" characteristic (at first pulse, the switch takes place and remains unchanged until a next pulse will be issued, see "stepping relay" on Wikipedia).

    What I'm talking about is a full conversion of a mechanical "Form C" contact (break-before-make) DPDT into a perfect Solid-State equivalent, with 6 (3+3 independent each other) switching contacts (of which power & voltage can be easily adjusted by changing the components's values) plus the 2 low-voltage terminals to which apply the voltage needed to operate the switch.

    There are already, on the market, devices like the one proposed but these are very expensive and often unavailable for tailor-made needs so it will be very useful for many people to learn how to build one by ourselves.

    Many thanks in advance.

    Fabrizio, Rome – Italy (fabrizioricciarelli@gmail.com)

    • Thank you so much Fabrizio,

      The first two specifications that you have mentioned looks feasible to me and could be designed.

      However I am not so sure about the "form C" counterpart specs, I'll try to investigate it though, and see whether or not I am able to transform it into the solid state version.

      I'll try to include the idea in my blog, possibly soon.

  15. I want to use relay to on & off motor as per level of water tank.current 10-12 amp.
    10amp 120vac or 10 230 vac which relay use?
    bc547 or sl100 ehich transistr use

  16. Hi swagatam
    i want to use relay for on & off motor as per water level in tank .
    current approx.10-12 amp.
    so 10amp 230 vac,10 amp120 vac,which relay use ?
    transistor bc547 or sl 100 which is suitable?
    plaese reply me.

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