How many SSR relays can be driven from a RP2040 GPIO

Question

So I have tested and confirmed that I can use an G3VM-61GR2 solid state relay to switch an amplified audio signal. I would now like to understand how many of those I can safely drive from the GPIO of RP2040 / Raspberry Pi Pico. The datasheet says:

• Trigger LED forward current (I_FT): typical 0.6 mA, max 3 mA.
• Operating LED forward current (I_F): min 5 mA, typical 10 mA, max 25 mA.

Which one of these do I need to consider? Can I wire it up so that the LED never draws more than 5 mA, and thus with a conservative guess of 50 mA for the total of GPIO, I could drive ten SSRs? Or would even 2 mA suffice? Because I want to drive around twenty.

What would be a good way to ensure I can drive twenty switches? Should I just add a BJT between each GPIO pin and the SSR? Is there a simple circuit I can set up without complex calculation with respect to the BJT? Is there a "multi-channel BJT" (IC) that is recommended?

Answer 1

Which one of these do I need to consider?

It all depends on how quickly you want the device to operate: -

At 5 mA the turn-on time is 1 ms. If this is OK for you then use 5 mA.

If you need to turn on the device in 200 μs then you'll need 20 mA.

I could drive ten SSRs?

Can your IO supply 50 mA and maintain an output voltage a little above the level below: -

I say a little above because there will be a spread of at least +/- 10%.

Then, the other problem you have is that if one device is wanting to naturally produce a terminal voltage that is 10% low (circa 1.2 volts) taking 5 mA, it will hog the current from other devices and they won't activate.

So then you need to use series resistors to maybe drop 2 volts so that the currents are better regulated for each device. Then you face the extra problem of needing to supply an output voltage of around 3.3 volts whilst sustaining 50 mA. Not sure an IO will sustain this voltage when delivering 50 mA.

Of course, if you have a supply spare of maybe 15 volts you could wires all ten devices in series and activate them via a transistor. You still need a current limiter though.