Switch Ac loads using Mosfet's (as relay)

Posted by Ilias-Man On Saturday, February 4, 2012 8 comments
We will now see how we can switch AC loads using Mosfet's transiostors (using mosfet's as relay)
Two MOSFETs with their source pins connected together will drive AC loads.

An Opto-isolator or logic gates can be used to switch the mosfets on and off, to prevent damaging mosfet's because we want the mosfets to be full on (saturation region) or full off , if you apply an insuffisant voltage to the gate of the mosfet's it will function in the linear region and the power dissipation will be splited between the load and the mosfet, which will burn the mosfet at high currents.



Fuser said...

Just what I was looking for. Thanks !

Serge Strokov said...

Very useful description, thank you!
I have a question: I understood that the necessity of use of the opto-isolator is justified by necessity of driving MOSFETs directly into the saturation region. But why the opto-isolator (implimented as a combination of LED and photovoltaic cells, I suppose) guarantees that the saturation voltage is reached? Can't it be the case, that if LED is dimly on, the photovoltaic cells produce less voltage, thus driving the MOSFETS just into their linear region?

Serge Strokov said...

Oh, and the second question is why not to solve the problem of driving the transistor to the saturation regime simply by conditioning that the driving voltage should be either zero (in the closed state), or such that it brings the transistor into saturation?

David de Bethel said...

you might want to check the Vds of that MOSFET before trying to switch 230Vrms...

terryjmyers said...

@David De Bethel. Looks good to me!

terryjmyers said...

@David De Bethel. Looks good to me!

Tom Courtney said...

Hi, excellent Post. I'm trying to find an Opto-isolator which will generate the trigger voltage on the gates of the FETs. All the opto's that I have seen so far only switch a voltage/current but do not generate a voltage which could to trigger the gate. Do you have any suggestions (or more accurate name) for the opto part?

Lars Englund said...

Hi, I've been trying, unsuccessfully, to dim a 60W indascent lightbulb with two back to back mosfets similar as in this post:
My circuit looks like this:
(there is also a 2A fast blow fuse and a 7D471K MOV on the mains input)
Below is the output from my scope:
C1: Live_230VAC
C3: Drain of Q5
C4: Source of Q5 and Q6

Q6 gets burning hot
Q5 stays cool
The lamp (obviously) flickers as we are only getting every other half wave.
Thinking about the circuit I feel that it will be impossible to get a Vgs voltage that is higher than the voltage at the source pin since the source pin will see the peak of the 230VAC?
Is there a way (preferably simple & cheap :)) of solving this kind of double mosfet dimmer?

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