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I have made up a basic output circuit and it seems to work, but I'm not clear how to size the resistor. I'd like to make sure this works for the long term and I am not overlooking any details. How would I go about calculating this resistor value? Is the diode required for solid-state relays? Also, am I missing anything else for this to work reliably?

enter image description here

SSR Relay: Idec RSSDN-25A: datasheets don't seem to be the best? Looks like 5V @ 10mA coil

The ESP32 we are using has a max GPIO amps of 40mA, recommended 20mA. So I guess it would best to size it for 20mA.

2k resistor, 1/4W [how do I size this?]

Currently using the PN2222ABU transistor

Diode, is this required for a SSR relay, or just for electromechanical relay? 1N4007G

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  • \$\begingroup\$ The circuit diagram shows a electromechanical relay with a coil, and therefore D1 is needed as a flyback diode. Yet the RSSDN-25A in the linked datasheet is solid state and don't think a flyback diode is necessary. Consider editing the question to clarify this. \$\endgroup\$ Dec 29, 2023 at 17:05

2 Answers 2

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According to the datasheet, the input current is internally (in the SSR) regulated to 10 mA. It can be connected directly to the ESP32. No other components are necessary.

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    \$\begingroup\$ The ESP32 datasheet gives Voh (High-level output voltage) as 0.8×VDD, or 2.64 V for a 3.3 V VDD supply. The RSSDN-25A Pick Up Voltage is 3 V. Thus, connecting the ESP32 output directly to the SSR input might not provide reliable operation. \$\endgroup\$ Dec 29, 2023 at 17:12
  • \$\begingroup\$ Yes, I agree. While this may work, it could cause some problems. I'd like to run it through the transistor, just would like to confirm the resistor sizing and diode requirement. Thank you \$\endgroup\$
    – Spy
    Dec 30, 2023 at 4:25
  • \$\begingroup\$ Hello, is anyone able to help out with my question. I want to make sure this will last long term. Thank you. Connecting directly I'm not sure will work 100%, so I'd like to confirm the resistor and diode sizing/requirements properly. \$\endgroup\$
    – Spy
    Jan 2 at 5:24
  • \$\begingroup\$ The circuit shown in your schematic should work correctly with the current values. As mentioned, the diode is unnecessary. I would confirm that the relay reliably switches off when the transistor is turned off, if it "sticks" on for whatever reason, try putting a ~10k ohm bleeder resistor across it. \$\endgroup\$
    – vir
    Jan 2 at 21:58
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I would suggest choosing the resistor to get 0.5mA to 1mA of base current so 3k ohm should be fine.

The diode is unnecessary for an SSR but it won’t hurt anything.

It is a little disconcerting that the photo shows an input range starting at 4V, meaning that it would be unsuitable for 3.3V operation. Suggest you clarify the discrepancy or choose a different SSR.

The concern is not so much that it might not work (which is easy and relatively inexpensive to deal with), but more that it might not work reliably- failing after some time or at temperature extremes.

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