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Electronics => Beginners => Topic started by: Marmotta on January 01, 2020, 06:42:27 pm

Title: Do I need any extra circuitry for a basic power relay?
Post by: Marmotta on January 01, 2020, 06:42:27 pm

I'm building a low powered PC with an external graphics card and noticed while testing everything for the first time that the graphics card would be constantly powered on, regardless of the state of the PC motherboard. As the motherboard and graphics card both run off separate 12V inputs, I'd thought I'd put a power relay on the input of the graphics card and connect the coil to a low-current 5V signal from the motherboard which would only be active when the PC was powered on.

Since the 5V signal will only ever be present when I want the power passing through the relay to the graphics card, is there any reason to add any further circuitry to the relay for practical or safety purposes?

Title: Re: Do I need any extra circuitry for a basic power relay?
Post by: David Hess on January 01, 2020, 11:49:51 pm

That should work fine.

Title: Re: Do I need any extra circuitry for a basic power relay?
Post by: JustMeHere on January 02, 2020, 12:03:46 am

You need a fly back diode on the coil. 

Title: Re: Do I need any extra circuitry for a basic power relay?
Post by: David Hess on January 02, 2020, 12:57:50 am

Quote from: JustMeHere on January 02, 2020, 12:03:46 am

You need a fly back diode on the coil.

The flyback diode will not hurt but the bulk output capacitance on the 5 volt supply will absorb the back EMF.