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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?
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.
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.
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.
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).
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.
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.
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).
