If controlling a solid-state relay from an ESP32 GPIO pin, is a flyback diode required or recommended?
This is from the data sheet.
I don’t think it’s required because the SS relay is using an optocoupler, not a coil, but Google is giving me conflicting information. Shocker…
Usually yes, in similar circuits with optocoupler.
Thanks @srg74.
You mean that they are required, even though there is no inductive load? What I don’t get is, what would its function be?
I have used these without adding anything and have not had any problems. Now you got me wondering if I need a diode now 
Features: Optical isolation between the input loop and the output loop. So does it really need a diode?
Here is quote from tech forum:
“Looking at the “Standard” circuit for using one of the Sharp S108T02/S208T02 SSRs, they have included a diode across the input pins, but I believe this is just to avoid exceeding the maximum reverse voltage of the internal LED (which for that particular device is only 6V). The specific datasheet for those SSRs can be found on sharp website.”
Looking at the data sheets, and reading comments, I’m no smarter… I’m going to go with, an extra diode isn’t going to hurt. My reasoning is simple. If there were a surge (from wherever), the diode would “reverse conduct” essentially shorting the spike to ground. The few extra cents are probably a good idea, and best viewed as insurance…
I appreciate the discussion. Thanks!
No, it shouldn’t hurt.
On the other hand if you’re connecting via a standard ESP GPIO and are worried about a “surge from somewhere”, what makes the SSR such a special risk?
That logic would apply to anything attached to a GPIO, meaning you should always add a reverse diode?
In the end, SSR’s are specifically designed to deal with isolating their inputs from their outputs.
You can add a diode if you wish, but I wouldn’t (and haven’t).
I think the confusion is that it is good to add a diode on the load side of DC loads and it is a good idea to add a MOV, snubber or similar to an AC load. Each of which to protect the SSR from current bounce back when powering the load off. It sounds like that is more common of an issue with inductive loads, more so like motors or things with large coils.
I could still be wrong 
Nothing. There is no coil to induce a voltage.
I think the issue is if there is an inductor in the circuit. If the optocoupler is reverse biased with a high enough voltage, it will conduct, and possibly get crispy. Here’s a nice explanation, but not referring to SSRs specifically.
Here’s a quote from:
How Freewheeling Diode Work: Easy Explanation (etechsparks.com)
"Flywheel diodes are in fact clamping diodes served to provide a DC discharge path for the remnant stored energy from an inductive load such as a motor or a solenoid. In principle, this is counteracting the back emf due to Lenz’s Law for any devices built on the principle of electromagnetism such as an inductor or a soleniod. In short, it is a protection mechanism.
Usually, a transistor or a current driver is connected at one end of such inductive load. Without flywheel or clamping diode, such remnant energy can surge the transistor, hence causing it to breakdown."
$.02 for insurance, if any inductive loads are in the circuit, is well spent imho.
This is about what I’m thinking.
With all the speculations around it
ON THE END
is it is not a Fault to include a SAFE N4148 to the input DC Pns
on larger i woudt use a Fast N4001-N4007 depüending on Voltige used 12V 4001 --24V N4007
