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Author Topic: Need Help! Using an electromechanical relay to switch high power inductive load.  (Read 827 times)
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Hi there.

I'm currently building a project which requires me to switch the mains (230V) input to a 300W (36V 8A, 5V 1A) linear power supply using a relay, controlled by an arduino board. I have an optocoupled 5V electromechanical relay circuit which can be powered by the arduinos 5V supply however i'm not sure if the protection diode will take the back emf generated when the power supply is switched off. Is this suitable if the diode is rated to take the voltage kick back, or does anyone know whether or not I'd have to use a solid state relay for this? I'm fairly new to all this but do understand the physics a bit, any advice on how to work out the back emf would also be greatly valued.
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Hi there.

I'm currently building a project which requires me to switch the mains (230V) input to a 300W (36V 8A, 5V 1A) linear power supply using a relay, controlled by an arduino board. I have an optocoupled 5V electromechanical relay circuit which can be powered by the arduinos 5V supply however i'm not sure if the protection diode will take the back emf generated when the power supply is switched off. Is this suitable if the diode is rated to take the voltage kick back, or does anyone know whether or not I'd have to use a solid state relay for this? I'm fairly new to all this but do understand the physics a bit, any advice on how to work out the back emf would also be greatly valued.

The protection diode across the relay coil terminals is to protect the arduino from the transition spikes caused when you turn off the relay coil. Any AC load wired to the relay's output contacts should have no effect on the arduino as the electrical isolation between the relay's coil and the relay's contact will/should prevent any harm or effect on the arduino.

Lefty
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cool, thanks for that. Sorry for sounding ignorant but my head feels a little bit like jelly from reading. I was under that impression but got worried when I read things about the terminals sparking due to the back emf. Is this just a problem with damaging the relay contacts then? or is there any chance a current could be induced in the relay coil (larger than the diodes rating)?
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cool, thanks for that. Sorry for sounding ignorant but my head feels a little bit like jelly from reading. I was under that impression but got worried when I read things about the terminals sparking due to the back emf. Is this just a problem with damaging the relay contacts then?

Yes

 or is there any chance a current could be induced in the relay coil (larger than the diodes rating)?

No

Lefty

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