MOSFET in place of relay?

Thread Starter


Joined May 15, 2010
My home thermostat has relays to switch the heat and A/C. They are very loud. Is there any reason why this couldn't have used MOSFETs for the switching?

Analog Ground

Joined Apr 24, 2019
The relay provides electrical isolation from the AC and heat units. A MOSFET would not. A solid state relay using an optocoupler and MOSFETs provides isolation. The relay is probably switching a control signal (typically 24VAC) and not the AC power directly.

Edit: Corrected "24VDC" to "24VAC"
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Joined Mar 14, 2008
So a small SSR should work as a replacement.

What are the ratings of the mechanical relays (control voltage and contact current)?
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Joined Aug 27, 2009
You still see relay usage in modern designs so Home Thermostat design must include some pretty old school HVAC system low voltage AC requirements. Little latching relays are certainly not cheaper than mosfets so using latching relays provides something worth the extra cost.


Joined Oct 28, 2015
Are you sure it's the relays that you can hear? Maybe it's something else?
Have you ruled out Rent a Ghost?
I wish my ears were as good as yours.
Good luck.


Joined Apr 11, 2010
The mounting for the board may be serving as a sound box, amplifying the relay clicks. I’d consider replacing the mounting screws with a little longer screw and use rubber isolating washers between the control board and mounting box.


Joined Jan 27, 2015
My electric stove arc's real bad when the elements turn on, jeez I need to pull it apart and see if the cheap little relays are melting the solder, sounds horrible, but barely notice any clicking

So what about the circuits in hotair stations, that use an zero-crossing AC opto-coupler, to trigger a SCR/triac ? They use 5V logic iirc, to run huge current things like BA16 iirc, at like 15 or 25Amps.
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