# Solid State Relay operation

A solid state relay specifies a trigger current of "7.5mA/12V". How much current does it draw at 5V?

This is the relay. I'm controlling it with an Arduino which has a maximum current draw of 40 mA per pin. Am I correct in calculating it multiplying 7.5 by 12 then dividing by 5 (the new voltage)? That gives me 18 mA, well within the limits of the Arduino pin - is it really that simple, or am I missing something important?

• A relay is not a constant power device, it will not draw more current at lower voltage. In fact it may not trigger at all at 5V if it's specified as a 12V relay. Commented Jul 18, 2015 at 22:49
• @JohnD: the linked datasheet shows that it is an SSR, rated for 3 - 32 volt input. Commented Jul 18, 2015 at 22:55
• @PeterBennett Thanks, Peter, I was in a hurry hence the comment vs. an answer. You're correct, so the relay should work at 5V, but it's still not a constant power device so current shouldn't be MORE than the current at 12V. What it will actually be really depends on the internal circuitry. Commented Jul 19, 2015 at 2:47
• Looks like there's internal current limiting circuitry inside the SSR. The control side works from 3-32VDC. If the Arduino pin can handle 40 mA then that is enough to drive the SSR input @ 5VDC directly without damage. The datasheet even says CMOS IC or TTL compatible. Commented Mar 17, 2021 at 21:20

I have a comparable "hockey puck" SSR handy. It also has the 3V to 32V input, although it's a different model (ESR5102401000Z). I made some quick measurements.

I was curious how these solid state relays (SSR) manage to accept a control voltage with a fairly wide range: from 3V to 32V. Of course, the datasheets for these "hockey puck" SSR don't provide the details about the input side. I can think of 3 schemes. Different models of SSRs may use different schemes.

### Current limiting resistor

The LED and resistor are such that the current is low enough at 32V not to destroy the LED, but still high enough at +3V to activate the relay. In this arrangement, the input current will increase linearly with input voltage.

### Series current limiter (active)

The input current shouldn't vary much with input voltage. The constant current source may be more or less stiff. The plot in Spehro's answer suggests a series constant current regulator in his SSR.

### Shunt current limiter (active)

The input current will increase linearly with input voltage. Excess current shunted around the LED, but still drawn from the input. Here's an example of what a shunt current limiter may look like (source).

• If there is one common factor in all these possibilities it is that the current should be <= 7.5mA at 5V. Until someone comes up with one that uses a SMPS constant power regulator... Commented Jul 19, 2015 at 2:46
• Load is missing in the triac side? Commented Jul 19, 2015 at 3:44

It will draw about the same current at 5V. The IR LED used in the SSR requires a certain amount of current to switch reliably and the typical input circuit for a 3-32V input SSR is a simple constant-current regulator made with discrete BJTs.

Here is another manufacturer's graph showing input current vs. voltage:

Of course this is just an educated guess on my part as the datasheet is characteristically inscrutable and/or mute on the point (as well as lacking safety agency approval markings), but every similar SSR I've seen works similarly to this.

• Apparently, not all SSRs regulate the input current like that. Have a look at my answer in this thread. Commented Jul 19, 2015 at 2:43
• Well, nothing beats measurements! +1 to you. That one could also meet the datasheet spec for the OP's SSR> Commented Jul 19, 2015 at 2:45

Your calculation is wrong - it should be 7.5 * (5/12), as we would expect a lower current with a lower voltage if there is just a resistor in series with the LEDs..

However, the datasheet shows an "input circuit" between the input terminals and the LEDs, so there may be something more than just a resistor in series with the LED - perhaps a constant current circuit, so the LEDs won't get excessive current with a 32 volt input.

I'd guess the current at 5 volts might be a little less than 7.5 mA, but probably not more than 7.5 mA. The only way to be sure is to measure it.

• perhaps the input circuit is some sort of current-sourcing buck regulator. infrared LEDs need less than 1V to operate. Commented Jul 18, 2015 at 23:34

Adding to Spehro Pefhany's answer. The datasheet shows an equivalent circuit model for the input side of the of the SSR. Have a look at the diagram on the right for DC control. You will see a current limiter in series with the input pin (+DC Pin 3).