Learn to regulate an AC load using PWM Signal
Yes, the AC load can be dimmed or controlled using the PWM technique. PWM is Pulse-Width Modulation which is mostly used in DC circuits to control DC loads but if it is configured correctly, it can be used in AC load. In this article, we are going to see how we can use the PWM signal to control AC loads.
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What is PWM (Pulse Width Modulation)?
PWM or Pulse Width Modulation is a kind of digital signal where the frequency is kept in a fixed range and the duration of the on-time and off-time is varied. This digital signal is mostly used in DC loads to deliver variable power or controlled power.
The frequency of the signal is 1/T where T = total time.
Where, Total time = on time + off time.
Depending on this on time and off time, duty can be calculated.
duty = on time/ total time X 100 %
Applying PWM signal to DC load:
When PWM signal is used to drive the DC loads, it can be used directly to drive the switching device like Transistor or MOSFETs. Depending on PWM signal’s duty cycle, the load intensity is varied like this:
This way, driving a DC load in variable intensity becomes so much easy. But applying this PWM signal to drive the AC load is not complex at all. Only one technique will make it easy to drive the AC load.
Using PWM signal for AC loads:
We know the AC signal is alternating its direction 50 or 60 times per second depending on countries. On the other hand, Transistor, MOSFET, or IGBTs work in unipolar ways that mean only one way. That is why, if we want to use Transistors, MOSFETs, or IGBTs for AC signal directly, it will not work for a full cycle. It will work for only a positive cycle. Because all of these switching devices work only in one direction.
Even if we combine a PNP and an NPN transistor or a P-channel and an N-channel MOSFET, it will not work properly. That is why we can not use these switching devices for the AC signal with the PWM gate drive. But using a TRIAC needs zero crossings and a timing signal for TRIAC firing angle control. Generating a PWM signal is much easier than that. So we need to modify our circuit little bit to use a Transistor type switching device which can be driven with a PWM signal.
Circuit modification of AC loads for PWM signal:
In this circuit, the AC signal is rectified and that rectified signal is passing through the MOSFET. So for both of the positive and negative cycle of AC signal, MOSFET is getting a uni-direction current flow.
Now, if we use a PWM signal to drive the MOSFET, the AC lamp will glow. And the modulated signals for this circuit is:
A suitable PWM signal is now can be used to drive the MOSFET. Changing the duty cycle of that PWM signal, we can control our AC loads as we want.
Note: Here, the frequency of the PWM signal should be much higher than the input AC signal to get a smooth result. And as a high frequency is used to drive the load, a high-frequency bridge diode must be used. Otherwise, EMI will disturb other devices running nearby.
Now, we need to make our PWM signal for our MOSFET to drive the load.
As you can see, the output is modulated by the PWM signal. So if we select a higher frequency for the PWM signal, the output will be more smooth. But note that, this circuit will generate EMI (Electro-Magnetic Interference) to nearby devices. So you should consider using this technique before you make one. But proper filtering will reduce the EMI.
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