Driving a Triac Directly

[ACharnley]

Hi,

I haven't played with triacs before so want to ensure I've got this right.

Requirement

I need to apply an over-voltage shunt to collapse the input voltage until the AC cycle returns to zero. A triac seems suitable.

Current: constant 500mA.

Chip

I'm looking at https://uk.rs-online.com/web/p/triacs/7958704/ which has a low ~3mA @ 1V turn-on.

Am I right in saying;

1. the triac is latched like a transistor so a gate resistor is required, probably to let 10mA flow as a good margin
2. the heat dissipated will be the voltage drop * current, so 1.2V*0.5A = 0.6W
3. there's no possibility of voltage flowing to the gate (I can't have the mcu destroyed).

Many thanks,

Andrew

 

[ronsimpson]

I think you are OK.
You know the triac will latch on until the voltage/current crosses zero.

 

[ACharnley]

I do, thanks Ron.

 

[alec_t]

possibility of voltage flowing to the gate

There can be slight reverse leakage current through the gate, at under 1V, via gate capacitance, if there is a sudden change in the voltage applied to the main terminals.

 

[dr pepper]

Triacs can have different trigger currents depending on the mains quadrant, so you need to ensure the gate current is high enough to trigger the triac worst case.
You also need the micro to have one power leg referenced to the mains which has safety issues, isolated with an opto is better.

 

[ACharnley]

No mains here, and have chosen a triac to meet those current requirements. Thanks!

 

[MaxHeadRoom78]

If using a MCU typically a zero cross opto is used.
See Fairchild AN-3006.
Max.

 

[ACharnley]

I take it it's to disconnect the high voltage for safety? Without it there's still no chance of that voltage going to the MCU pin?

 

[MaxHeadRoom78]

I take it it's to disconnect the high voltage for safety? Without it there's still no chance of that voltage going to the MCU pin?

Something called Galvanic Isolation.
Max.

 

[ACharnley]

Something called Galvanic Isolation.
Max.

Hopefully given I'm playing with only 36V AC it won't present itself to be an issue. I realise I forgot to mention that in the initial post.

 

[MaxHeadRoom78]

You still preferably require isolation if dealing with MCU control.
Max.

 

[schmitt trigger]

By Murphy's law, if something can go wrong, it will.
So without an optoisolator, if the Triac fails for whatever the reason, your MCU could become collateral damage.

In the end, is like driving without a spare tire. As long as you don't get a flat, you can drive many thousands of kilometers and don't miss a beat.
But if you get a flat on a lonely stretch of a road and the night is coming, well, you would really wish that you had one.

 

[ACharnley]

If the triac fails many other components would fail alongside the mcu. The only benefit from using the mcu is variable voltage control, which in itself is a bit pointless, and which could be a liability if the MCU crashes for some reason (i'm prototyping the system to discover reliability). I could use the old zener/resistor route instead.

One thing I didn't verify, a positive current to the gate will turn it on for both AC cycles?