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Controlling an AC load with a MOSFET

  • Author Tahmid
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  • Blog entry read time 2 min read
In most cases where you want to control an AC load, a triac or SCRs will be used. However, it is not easy to drive a triac or SCR. The drive requirement for the triac or SCR makes it sometimes difficult to control it as we want. One thing is that we can not turn the triac or SCR on or off as we desire, because once we turn it on, it latches and stays on until the next zero crossing or until current stops flowing through it. Also, driving a triac with reference to MT1 (or A1) is not as straightforward as we would want.

However, a MOSFET can be controlled as we want. Set the gate high (with a sufficient voltage) and current can flow from drain to source. Set the gate low and current can no longer flow. Convenient!

However, a MOSFET can only be used to control DC loads since it is a unidirectional switch - current flow can be controlled when it is flowing from drain to source, but can not be controlled from source to drain. So, certainly it can not be used to control AC loads. Right?

Well, directly, it can not be used to control AC loads the same way you'd control DC loads. But, with some clever circuitry, it can be used to control AC loads. And here's how:


Perhaps you don't see how it works now. But consider the two diagrams below, which show the flow of current during the two AC half cycles. I'm sure you'll get it better then.



As you can see, due to the bridge rectifier, the MOSFET always "sees" a DC voltage as the drain is always positive with respect to the source. Thus, with this combination of the bridge rectifier and MOSFET, by controlling a DC switch - the MOSFET, you can control the AC load.

The MOSFET must be turned on fully by driving it high by at least 8V above source level - 8V with respect to source.

So, you can turn the MOSFET on and off at any time and accordingly turn the load on and off as required. This makes driving the AC load so easy!


Control the duty cycle (MOSFET on time) with a microcontroller and you can dim the lamp easily.

The MOSFET won't contribute to the losses. The diodes will, but it won't be too high. It will be quite efficient.
Hi Tahmid!

Excellent work there, thanks for the reference.
I have never used this circuit in practice, but a question came in mind after I read this entry.

Bridge has a voltage drop of let's say 1V. So when the AC waveform drops below 1V, then the bridge cannot conduct, and thus the MOSFET cannot conduct either. Will this dead time around 0V affect the load current in the case of 100% duty cycle? If yes, are there techniques to override this limitation?

Thanks in advance!
1V is very small with reference to the total voltage. For 220V, peak voltage is 311V. So, 1V is such a small portion of that.

V = A sin (wT)

Substituting into the formula:
V = 1, A = 311, w = 100*pi (as I took frequency=50Hz)

The equation solves out to give T = 0.000010235s.

So, MOSFET does not conduct for (2 * 0.000010235)s every half cycle, which is 0.2047% of the AC sine wave.

As you can see, that is very very small and probably not very important. Remember that the triac, which is used conventionally, also has a voltage drop (and does not usually conduct at 1V). So, the MOSFET does not cause a problem.

If lower voltage drop is still required, two MOSFETs can be used back-to-back, eliminating the requirement of the bridge rectifier. The only drop now is the MOSFET body diode. If a MOSFET is carefully selected, so that it has low body diode Vf, you can minimize voltage drop.

I hope I could make this clear. :smile:
Tahmid, Do you found any limitation or disadvantage of this technique in practically your self...?

I haven't found any limitation. One drawback is that there is a small power dissipation in the diodes and the MOSFET. So, devices should be selected accordingly and heat dissipation and cooling should be taken into consideration when and where necessary.
Very innovative & useful...
Can be used for small size load because thyristors are very rugged & MOSFET'S are so sensitive for load...
Great work...
With proper design, it can be used for large loads as well. MOSFETs are "rugged" too.
Hello sir,
Why did you choose MOSFET? Why not IGBT or BJT?

And what purpose do the R1 and R2 serve?
I have a circuit which is similar to yours. I am facing a problem. Can you please help me to solve it?

If the load is not connected, the bridge diode and mosfet will get burned, right???

I want a circuit when the power is directly connected accidentally without load, the circuit should not get damaged. How to modify the circuit? Please help.

Thanks in advance.

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