Creating a SPDT out of a SPST solid state relay

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Stanford
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Creating a SPDT out of a SPST solid state relay

Postby Stanford » Fri Jan 17, 2020 9:09 pm

Hello,

I would like to connect two SPST SSRs A & B (125AC load, 3VDC input) to create a single SPDT SSR. One terminal on each SPST SSR will be connected together for the "common". The other terminal on SSR A will be connected to Line and the terminal on SSR B will be connected to Neutral. I will use two (2) GPIOs from my micro, making sure that both outputs are never high ('1').

(note: the "common" is connected to a motor coil. I would have a second SPDT arrangement on the other terminal of the coil. This will allow me to turn the motor CW or CCW. application: washing machine motor. a separate start coil (with a centrifugal switch) is used, which I am controlling via a single SPST.)

I also thought about using a single GPIO rather than two (for each SPDT). See attached picture - bottom. I am concerned about this approach since when the GPIO is switched, the SPSTs may momentarily be "on" at the same time, thus causing a short. Should I worry about this? (switching time is 1ms for the SSR's I'm using).

Any caveats or feedback on this approach? The only issue I see is if both GPIO's are high. This would cause a short between Line and Neutral, but through two SPSTs.

What would happen if this short were to occur?

thanks

IMG_9332.jpg

Stanford
Fresh meat
Posts: 2
Joined: Fri Jan 17, 2020 8:44 pm
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Re: Creating a SPDT out of a SPST solid state relay

Postby Stanford » Sat Jan 18, 2020 7:04 am

Correction: response time is 10ms.

More info on this. The AC SSRs I am using use the zero-crossing control method, which is why the response time is 10ms (1/50Hz is 20ms period, half is 10ms, worst case is controlling immediately after the zero crossing, causing a possible delay of 10ms by the SSR until the next zero crossing).

Using zero crossing, the switch is not turned on/off by the SSR until the AC waveform is at zero. The same issue of having both GPIOs on at the same time holds (having both on would cause a short between L and N). However, using a single GPIO (bottom circuit in the photo) would probably work.

I suppose my fear would be that if the single GPIO signal controlling both SSRs does not cause both SSRs to switch at the exact same zero-crossing, then L and N could be shorted (nanosecond window of opportunity).

Comments?


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