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I'd like to use some solid state relays to turn on and off a high voltage, low current DC inductive circuit (120-150VDC, 20-60ma). The SSR would only be switched on and off at a maximum of once every few minutes-- it would not be used for signaling.

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(EDIT: Details about the SSR added below)

The specific SSR itself is a Fotek SSR-25 DD-- the DC 25A model sold here, rated between 5-220VDC and up to 25 amps. It's definitely a counterfeit, however, so it can only be trusted at up to around 10 amps according to my research (AC counterfeit versions were reviewed well here and here; I'd expect the DC version to have similar results, but I haven't found reviews for the DC version specifically). I can't seem to find a datasheet even for the "real" component, perhaps because Fotek never manufactured one named SSR-25 DD. The Fotek DC to DC SSR datasheet is here, rated up to 10 amps load for the 220VDC version.

Since I have such a low current requirement (60ma max), I felt comfortable enough to test these out for how inexpensive they are and spent $10 on a few after I originally asked this question. I will post results of my tests for posterity later.

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The load is inductive. I've read about inductive AC loads potentially having issues with SSRs depending on the type of SSR, but I don't see anything about inductive DC loads thus far. This leads me to believe the issue must only be with AC circuits due to AC modulation, but I'd like to be sure.

1. The first question is simple-- would an inductive DC load have any issues running through an SSR?

The second and third questions are in regards to the signaling that will pass through the load side of the SSR after it is switched on. The circuit will pulse off and on at a speed of up to 45 modulations per second (45 baud, or up to about 1 modulation per 22ms; also remember that this signaling is being reliably generated by another component on the circuit, not the SSR).

2. If an SSR is rated to operate a load between certain voltages (5-220VDC in this case), when the circuit modulates between 0 and n volts, would the SSR effectively turn off and on even though the control circuit has never turned off, thus introducing a delay in each signal modulation?

3. Would an SSR having an inductive DC load modulating at a frequency like this cause any problems, particularly a delay or distortion in signaling?

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  • \$\begingroup\$ most AC SSRS use triacs to switch the load, most DC SSRS use mosfets. \$\endgroup\$ – Jasen Mar 17 '18 at 6:40
  • \$\begingroup\$ I suppose 150VDC isn't officially considered "high voltage", but having usually worked with 3-5VDC, it certainly seems high to me. \$\endgroup\$ – weildish Mar 17 '18 at 17:23
  • \$\begingroup\$ @weildish Varies from one country’s legislation to another, but 1000 Vdc seems to be a round and magic number. \$\endgroup\$ – winny Mar 17 '18 at 17:56
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1. The first question is simple-- would an inductive DC load have any issues running through an SSR?

Main issue is you didn't specify what you mean by "high"

  • DC SSR's can be expensive for high voltage if you mean 1kV
  1. If an SSR is rated to operate a load between certain voltages (5-220VDC in this case), when the circuit modulates between 0 and n volts, would the SSR effectively turn off and on even though the control circuit has never turned off, and on even though the control circuit has never turned off, thus introducing a delay in each signal modulation?

No

  • DC SSR's are MOSFETs so if Opto input is active, output RdsOn is low as long as within normal voltage and current limits.
  • unlike AC SSR's which use Triacs
  1. Would an SSR having an inductive DC load modulating at a frequency like this cause any problems, particularly a delay or distortion in signaling?

Possibly, if something was overlooked. You really need to specify full details. Schematic, test results etc. Specs..

DC SSR's >=230V

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  • \$\begingroup\$ Tony, you've got your answers formatted as quotations. \$\endgroup\$ – Transistor Mar 17 '18 at 9:45
  • \$\begingroup\$ Tony, thank you for your input. See details on the SSR itself and the voltage/current requirements added to the question. \$\endgroup\$ – weildish Mar 17 '18 at 17:25
  • \$\begingroup\$ @weildish Show schematic including body diode if any or external one. If you know how a FET switch works. test it for the linear transfer function V ac in vs Out. \$\endgroup\$ – Sunnyskyguy EE75 Mar 17 '18 at 18:30

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