Summary: Continuing my search for a solution to my problem, I have landed on normally closed solid state relays (NC 1 FORM B SSR). Could you tell me if I'm doing this right?

What I need: What I need, is a switch, that is closed when the power to my MCU is off, and that is open when the power to my MCU is on. I have a 12V PWM coming in and when power is on, I substitute it with my own 12V PWM. (Basically a normally closed switch, which opens completely when power is on.)

In the schematic below, I have tried to illustrate my situation.

Why I need it: Since PNP Transistors (due to high Emitter-Base Voltage), P-MOSFETs (due to at times negative Source-Drain Voltage) and JFETs (due to maximally 12V available as control signal) don't work, I have landed on SSRs.

I have no experience with PCB mounted SSRs, so I need your help. I found this relay which seems to suit my application: G3VM-353G Do you think this will work as intended?

Schematic 1: My "challenge"/problem


simulate this circuit – Schematic created using CircuitLab

Schematic 2: Will this work?


simulate this circuit

Thank you!

Edit specifiying the signal: As per request: The PWM signal to be passed through or interrupted is a 1kHZ, normally 0-12V, 0-100% duty cycle signal. To pass through states, the amplitude is sometimes modulated, changing it to 0-9V, 0-6V or 0-3V. Current is minimal (except for measuring, no current is drawn from the signal). Voltage drop tolerance is ±3%, duty cycle tolerance is ±0.1%.

  • \$\begingroup\$ Does R2 need to be 100kohm or can it be (say) 1kohm or lower? \$\endgroup\$ – Andy aka May 30 '15 at 22:01
  • \$\begingroup\$ R2 represents my external "load" (specifically it's another system entirely, measuring the PWM signal I output). I can't control its size. \$\endgroup\$ – Johis May 30 '15 at 22:03
  • \$\begingroup\$ But, can you load the output of the relay with 100 ohms as a permanent fixture in your design? If so then the 65pF that Spehro mentions becomes fairly insignificant. \$\endgroup\$ – Andy aka May 30 '15 at 22:09
  • \$\begingroup\$ The PWM signal going to the load also has a varying amplitude which is important, so I cannot add any load to it, since that would yield a voltage drop. I need to pass through the signal as "clean" as possible. The relay datasheet already states 15 Ohm ON resistance and the C[IO] is only 0.8pF. Current leakage is indicated as 1uA maximum. That would mean the effect shouldn't be as strong as in Spehro's example, right? \$\endgroup\$ – Johis May 31 '15 at 9:54
  • \$\begingroup\$ It's not 0.8pF, it's 68pF - use the OMRON site omron.com/ecb/products/pdf/en-g3vm_353g.pdf. I think it might be a good idea to divulge the exact nature of the signal because at the moment nothing is looking promising except a mechanical relay (as mentioned in your previous question). Without compromising any potential solution state what the signal is and what it shoul;d connect to. State permissible losses and degredations to the signal also. \$\endgroup\$ – Andy aka May 31 '15 at 9:57

It will sort-of work. The capacitance between terminals when the relay is off is typically about 65pF, so you'd see something like the green trace below (5kHz 50% square wave input shown) across the 100K resistor. Red trace is the 'PWM' input.

enter image description here

  • \$\begingroup\$ If I understand your answer correctly, you're saying that even when the relay is off, some of the signal can still have an influence across its terminals? Do you have any suggestions on how to achieve what I want? \$\endgroup\$ – Johis May 30 '15 at 21:47
  • \$\begingroup\$ @Johis Not really since I have no idea what you actually want or whether the 100K resistor is realistic or not. Sometimes series-parallel switch combinations are used, but that seems like overkill. \$\endgroup\$ – Spehro Pefhany May 30 '15 at 21:50
  • \$\begingroup\$ I have a more detailed description of my case here: electronics.stackexchange.com/questions/172835/… I have an external 12V PWM signal going through my board. When the power to my MCU comes on, I want to interrupt this signal and replace it with my own. Edit: The Datasheet for this SSR states C[IO] to be 0.8pF. \$\endgroup\$ – Johis May 30 '15 at 21:59
  • \$\begingroup\$ Am I simulating this correctly? circuitlab.com/circuit/965aa6/ssrcoutsimulation \$\endgroup\$ – Johis May 31 '15 at 11:52

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