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I'm wondering if someone could explain why a solid state relay has a minimum voltage that it can switch on or off. For example I'm hoping to use an SSR-10DD (http://www.fotek.com.hk/solid/SSR-3.htm) solid state relay, however the spec sheet mentions a minimum of 5V on the output side. I was hoping to use it to switch around 3V. Why would it require a minimum of 5V?

Update:

  • V_input range = 5V from an Arduino output pin
  • I_Output current range = Signal only, so less than 10ma
  • Target load type = Signal
  • Reason for isolation = want to avoid all possibility of damaging existing circuit

More details about what I'm trying to achive can be found in this question: Connect Arduino to existing circuit with seperate power supply

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  • \$\begingroup\$ what current???? \$\endgroup\$
    – Tony Stewart EE75
    Oct 19, 2016 at 2:48
  • \$\begingroup\$ I've updated my question. The current would be negligible since I'm switching a signal only. \$\endgroup\$
    – Harry Muscle
    Oct 19, 2016 at 15:35
  • \$\begingroup\$ I understand the rack mounted modules limits options for minuscule optoisolated switching, but in this case all one needs is a board to mount the connectors for this chip as a low side switch for 5V for $0.40 with 3kV isolation digikey.ca/product-detail/en/lite-on-inc/6N137S-TA1/…. or buy similar on boards online for $5 with many ports \$\endgroup\$
    – Tony Stewart EE75
    Oct 19, 2016 at 16:40
  • \$\begingroup\$ created 2nd answer better for you \$\endgroup\$
    – Tony Stewart EE75
    Oct 19, 2016 at 17:15

4 Answers 4

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Harry I don't know if you know anything about transistors but I think you have chosen the wrong part for your application.

enter image description here

This active Opto circuit needs enough voltage to drive the transistor base current to saturate the switch.

Try again.

PLEASE Specify the following;

  • V_input range,
    • Control V,I range options
  • I_Output current range
  • target load type. ( reactive , resistive/ motor/ micro etc)
  • reason for isolation ( kV? EMI? both?
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  • \$\begingroup\$ I've updated up original question with the additional details you asked for. \$\endgroup\$
    – Harry Muscle
    Oct 19, 2016 at 15:32
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... however the spec sheet mentions a minimum of 5V on the output side. I was hoping to use it to switch around 3V. Why would it require a minimum of 5V?

enter image description here

Figure 1. Block diagram.

The 5 V minimum output voltage requirement is probably required to power the output circuit driver. The datasheet says that 1.6 V will be dropped between 1 and 2 so with a 5 V supply there will only be 3.6 V available for the load.

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SSRs use SCRs or TRIACs on their switch outputs. These components have a dropout spec that gives the ON voltage supplied to a load whereby the switch component will no longer stay on.

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  • \$\begingroup\$ AC SSR's use Triac's DC SSR's use single transistor's ( open collector or open drain) \$\endgroup\$
    – Tony Stewart EE75
    Oct 19, 2016 at 3:22
  • \$\begingroup\$ @TonyStewart.EEsince'75 Does using a pair of SCRs count as a triac? (It's what I'm doing here as part of my AC 40 A split phase H1/H2 hybrid switch -- no physical triacs present, just SCRs.) \$\endgroup\$
    – jonk
    Oct 19, 2016 at 4:22
  • \$\begingroup\$ natch 4 Q's in a Triac or 2 SCR's with equal quadrant sensitivity hopefully. \$\endgroup\$
    – Tony Stewart EE75
    Oct 19, 2016 at 4:24
  • \$\begingroup\$ @TonyStewart.EEsince'75 It's a hybrid system. The TYN640's are ON only long enough to allow the mechanical relays to engage. Then they are no longer triggered by the MOC3083s until it's time to release the mechanical relay. There are other components of course which allow some adjustment. My first question here shows an earlier schematic of part of it. \$\endgroup\$
    – jonk
    Oct 19, 2016 at 7:56
  • \$\begingroup\$ Just to confirm, the dropout spec you mention is not the same thing as the voltage drop, correct? In other words, if I put 3V across the outputs of the relay I mentioned which has a voltage drop of 1.6V I would get 1.4V across it, however, because the 3V is lower than the dropout voltage I would actually get no voltage/current flowing. Just want to make sure I understand correctly. \$\endgroup\$
    – Harry Muscle
    Oct 19, 2016 at 15:35
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Harry, I know you would like to buy a rack mount or other std mount for this simple application, but all you need is a tiny shielded Reed relay with 3 to 5V control input from logic. SEEED Co. has over 500 products with gas sensors, indicators, interconnect cables and guys like Mouser may carry them. $5 or less. Then use twisted pair output to reduce noise ingress on high impedance load. Reed Relay board with screw terminals out and input jack, very reliable

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