[General] Relay controlled by a digital output

[Karipain99]

Hello everybody!
I would like to ask a question about energizing a relay from a digital output in source mode.

Is it correct to put the flywheel diode parallel to the coil just to prevent the voltage spike due to the e.m.f. of the coil through my ground (when switching of the digital output)?
Thanks in advance.

 

[Truedeal]

Yes and it is desirable. The inductive kick from a coil can exceed triple the supply voltage.

 

[Karipain99]

Digital Input + Coil of relay

Hello everybody!
Yesterday I posted a question about how to energize a coil using an output of a microcontroller.
Now, my application is to read if the coil of a relay is energized or not.

In the picture attached you could see the connection done.
please, I have one question about this:
will this connection burn the digital input pin when switching off the coil?
will the current produced from the back e.m.f pass through the flywheel diode and then return to my digital pin and burn it?
Thanks in advance.

 

[Easyrider83]

It will burn in both cases. In the first one you are exceed the maximum load current for individual pin. Usualy it is a 20mA, but loading output pins is a bad practice. In second case you need at least 100k resistor for protection the pin. God save thouse who desired to improve all inputs with protection diodes.

 

[Karipain99]

Thanks for the quick response.
Actual, I don't know how the pin is configured if there is an internal resistance in series like ou said or not.
So you said that if there is no resistance the pin get the spike of the coil through the flywheel diode?
Thanks.

 

[Karipain99]

I verified the voltage spike on that pin using oscilloscope and the voltage spike is -10Volt in 1us, so it's work.

 

[Truedeal]

I would use a transistor to Isolate the IC. Or an OPTO-Coupler.