I know that you have to place a diode in parallel with a relay to use with i/o on microcontroller. Would you have to do the same with a relay board such as......

http://www.sainsmart.com/module/acce...-pro-mini.html http://www.sainsmart.com/module/acce...p-arduino.html

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    \$\begingroup\$ Your links somehow got abbreviated (the "...") \$\endgroup\$
    – stevenvh
    Commented Jul 10, 2012 at 4:45

2 Answers 2


If you're still talking about the relay module from your other question, the answer is no, the diode is not required. This uses solid state relays which don't have a coil. It's that coil in an electromechanical relay that causes a voltage peak when switching off, against which the transistor has to be protected.

If you would be referring to one of the other modules using electromechanical relays, like this one, those have a protection diode on board.


The links don't work in your question - if you are talking of this product, then yes, they have diodes in parallel with the coils - I couldn't find an "official" schematic, but on the 2-channel version I found this picture:


You can see a 4148 (D3) across the coil. Also the relay is driven by Q1, not your Arduino pin, so it requires much less current and adds "layer" of protection.

If it's this: http://www.sainsmart.com/module/acceleromete/arduino-pro-mini.html then it has a similar arrangement, but adds an optocoupler for isolation (note this will need a higher drive current than the others, 15-20mA according to the page):


If it's one of the solid state versions, then as Steven notes the diode is not needed - these use a drive transistor also:


So basically, no, you don't need a diode for any of them, just drive directly from your Arduino pin.


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