PWM & Triac

[zesla]

Hello guys,

I wanna see is it possible to trigger a Triac by the PWM modulation?
I want to connect The A1 and A2 pins of the Triac to the Mains (110V/AC) through a bulb and control the bulb by the PWM modulation injected to the gate of the Triac through a PIC microcontroller.

Thanks beforehand.

 

[alec_t]

Phase-angle control of a triac is the norm. Is your PWM frequency the same as the mains frequency?

 

[ChrisP58]

Once triggered, triacs stay on until the current through them drops to below their holding current. You cannot turn them of under gate control. As such, you cannot control them with PWM like you can a mosfet.

You can control them with a PIC, but you have to decide when to turn them on, knowing that the turn off time is fixed. As alec said, the decision is phase angle related.

 

[zesla]

Phase-angle control of a triac is the norm.

Hi alec,

Do I need to consider the Phase angle control here? Why is it important?

Is your PWM frequency the same as the mains frequency?

Yea, the PWM frequency is 60Hz, the same as the Mains frequency.

 

[zesla]

Once triggered, triacs stay on until the current through them drops to below their holding current. You cannot turn them of under gate control. As such, you cannot control them with PWM like you can a mosfet.

You can control them with a PIC, but you have to decide when to turn them on, knowing that the turn off time is fixed. As alec said, the decision is phase angle related.

Thank you,

Can you explain your meaning and your solution please?

 

[zesla]

The high voltage section of what I want to design:

 

[Mike odom]

you need to know when the mains cross zero. This is when the triac turns off. For full illumination, turn back on immediately. To lower illumination, or dim, wait before turning on. This can be done with PWM, but you'd have to invert the signal and be in sync with the mains. In other words, instead of turning on for a certain amount of time before turning off, you turn off just before the zero crossing for a given amount of time, and then turn on again.

Easier to detect zero crossing and then delay turn on (up to 180°) before triggering on again, the shorter the delay, the brighter the light.

 

[Jaguarjoe]

There are IC's that do all this.

Zero crossing triggering minimizes RFI (Radio Frequency Interference)

Just because the Pic runs at 60hz and the mains is at 60hz does not mean they are in phase.

 

[Mike odom]

There are IC's that do all this.

what ICs? The guy is asking for help, not a lecture.

Zero crossing triggering minimizes RFI (Radio Frequency Interference).

while this is certainly true, and we do trigger at zero crossing (I use MOC3032), you can't trigger at zero crossing if you're trying to control power. Triggering at zero crossing will only net full power. Although, you can obtain half power by triggering at either other zero crossing.

Just because the Pic runs at 60hz and the mains is at 60hz does not mean they are in phase.

agreed, which is why I said if you're using PMW, you'd have to sync up to the mains.

 

[ChrisP58]

Here is an application note from Microchip that covers it.

https://www.electro-tech-online.com/attachments/40171a-pdf.69315/

 

[Mike odom]

I'm not too crazy about hooking the ac directly to the micro, and neutral to ground. I'd opt for more of this arrangement:

View attachment pfi_ckt.PDF

 

[Mike odom]

here's a less confusing look. Opto is LTV814. AC goes across R34 and R38.

**broken link removed**

 

[dr pepper]

I dont want to appear as a lurker mike, however you can to an extent control power and fire at mains zero, its done with large loads in industry, burst firing modules do it, you can fire for a half cycle, then a full cyclle, then a full cycle and a half and so on, it doesnt give you a lot of control, but for large loads that take a lot to change, such as a 10kw heater the system works well and doesnt needs loads of filtering.

As far as the op is concenred however with lamp dimming the way to go about that is as mentioned phase angle control, you'd need an i/p to the micro that tells it the mains just went through zero, then a timer would start creating a trigger o/p when its timed out, the timer would be the opposite of the power level, zero being max power.

Take a look at this:

**broken link removed**

 

[Mike odom]

lurk away, dude...

yes, I did mention you could cycle skip, but it is not commonly used on lamp dimmers. I suggested a opto for zero crossing because the reference design did not have a transformer. Transformer on the mains for isolation is by far the best design methodolgy. By far...

 

[dr pepper]

Yes and an opto as you pointed out in #11 makes things a lot safer.
If you dont go for isolation dont forget that all your front panel controls, pot etc need to be insulated as if they were connected to the mains.