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I am trying to connect an SSR to a DC voltage source that is either at 2.9v or 12.3v. I want to have the SSR switch on when the DC source is @ 12.3v and off when the source is at 2.9v.

I can't seem to find an SSR that has an OFF voltage in the proper range.

The DC source is computer controlled and monitored (voltage and current), so simple voltage reduction methods like a resistor may not work.

Any thoughts on how to solve the problem? The solution needs to be simple an robust as it is an automotive application, so there are vibration, heat and moisture issues to contend with.

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  • \$\begingroup\$ "The DC source is computer controlled and monitored (voltage and current) ..." What do you mean by this? Is the output constant voltage or constant current? What was the load when you measured this? \$\endgroup\$ – Transistor Sep 17 '16 at 15:29
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Adding a Zener diode will raise the turn-on voltage of the SSR to its turn on voltage + the Zener reverse breakdown voltage.

For those not familiar with SSRs, the inputs have a current-limiting circuit internally to allow operation over a wide range - typically 3 to 32 V.

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You are thinking this problem is far more complicated than it actually is. Just set yourself up a transistor switch circuit that changes state at some point between the 2.9 V and 12.3 V levels. A good value could be say ~7 V. The transistor can then simply switch a normal SSR input on an off. In this example select the resistor that feeds the SSR input to source the current needed to operate the Opto Coupler inside the SSR.

enter image description here

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  • \$\begingroup\$ Hand drawn because it is the fastest way to communicate the idea!! \$\endgroup\$ – Michael Karas Sep 17 '16 at 15:18

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