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I'm attempting to adapt a project with the wiring diagram shown here: http://www.glacialwanderer.com/hobbyrobotics/?p=9

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But instead of the electromechanical relay used in the above link, I'm using an SSR with a 25-amp load current capacity (this one: http://www.fotek.com.hk/solid/SSR-1.htm)

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My question is, should I still include the diode and transistor as shown in the first link's wiring diagram? Would that be redundant/unnecessary when using an SSR as compared to an EMR? Just wiring up the Arduino pin to the SSR seemed to successfully be able to toggle the SSR's load circuit (with the AC disconnected of course) but I think I should be concerned about adequate safety, reliability etc...

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Driving an SSR directly by an MCU output is the correct way to use them, you don't need anything else.

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You just drive it from your uP but remember to keep the current limiting resistor (the 1k). I forgot it once and fried two SSRs (~$13)

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The shown SSR has a trigger current of 7.5mA with 12V. I found also a listing from 6-25 mA. Using a 5V GPIO you should ensure that the uP can drive 20mA, which is the case with the Arduino. So you can drive it directly. The data sheet of the manufactor doesn't state the need of a current limiting resistor. I know about comments that it does not work always this way with 3.3V GPIO with i.e. Rasperry Pi. These values seem to close to the limit. Also note, that this might work alone but starts to fail if you drive more things with other pins as sum of all pin currents is limited.

Therefore it is still recommended to use the transistor with a resistor (1k is fine) like shown in your post. This way to trigger current does not flow over the GPIO but over the 5V supply. This is the usual connection if you drive e.g. LEDs with a uP. Also this way allows to provide 5V to the SSR from a uP with 3.3V logic. You don't need the diode at all for the SSR.

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