# Variable Phase shift control circuit for PWM circuit

#### Amr Wael

##### Member level 4
Hello ,
I am designing a circuit for controlling the phase angle (Firing angle) of a TRIAC. I don't want to use a microcontroller.
I am have a circuit that generates a PWM signal. I want to design a circuit that introduces a variable phase shift to this PWM signal from 0 to 180 degrees for example
, this variable signal is going to be the control signal of the TRIAC.

I found many ways but all of them use zero cross detection and a microcontroller to adjust the firing angle.
I just want to do it in analog way. I Tried using an All-pass filter that worked perfectly for shifting Sinusoidal signals but it didn't work when phase shifting a square signal .
My AC source frequency is just 50 Hz so the firing pulses need to be the same frequency.

Thank you very much in advance.
Best Regards ,
Amr Wael

Use an all pass to shift a sine followed by a comparator to convert it to a square wave.....

Why not a micro ? Here is what it takes to do code for a pot to pulse width generator -

The code on the right is generated by the blocks you drag and drop onto a canvas
and configure. 6 year olds are using it in class to program robots.

Regards, Dana.

#### FvM

##### Super Moderator
Staff member
I Tried using an All-pass filter that worked perfectly for shifting Sinusoidal signals but it didn't work when phase shifting a square signal .
My AC source frequency is just 50 Hz so the firing pulses need to be the same frequency.
Isn't the AC source a 50 Hz sine? Why can't you use an all-pass to delay it?
The usual solution is of course a RC time delay, starting with the typical dimmer circuits (variable R, C, diac). You didn't however clearly describe your intention. What's the control source?

#### KlausST

##### Super Moderator
Staff member
Hi,

PWM and phase shift control are different things. Don´t mix them.

Klaus

A PWM edge can be moved to create a phase shift relative to its clock.

So a 50 Hz clock, x N for some resolution, can create an output whose
phase is adjustable while maintaining same period as source.

Dana.

#### barry

without seeing your PWM circuit, it’s impossible to suggest how to control its phase shift. But, as Klaus says, the mixing of phase control and PWM is a bit muddled. What are you trying to do?

OP wants to shift phase of a 50 Hz signal. So we have two signals, one
50 Hz considered reference signal, so its phase is ZERO.

Take a PWM, feed it a clock of 5000 Hz, set its duty cycle at 50 %, its now
creating a signal same as reference. Now change the comparator register
for duty cycle, freq stays the same, edge moves, eg, PHASE CHANGE.
This is used extensively for all kinds of phase related generation/clocking.
In fact one can do a strictly software PWM and preserve duty cycle while
shifting phase. I think I have seen PWMs that even have register control
over both edges to effect duty cycle preservation.

Easy to implement synchronous designs as well.

But as I originally posted, I think this meets his analog only criteria -

Use an all pass to shift a sine followed by a comparator to convert it to a square wave.....

Regards, Dana.

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