TRIAC triggering issues?

[George]

I'm trying to trigger a sensitive gate (designed specifically for direct connection to microcontrollers) BT131-600 TRIAC. MT1 is connected to neutral line, so is Vss on the PIC, The triac tho seems to only want to fire on the opposite side of the waveform, ie. when a port = high but with the AC voltage running negative, will not trigger when AC wave form goes positive.but will trigger when going negative like this it doesnt matter where the timing is - or seemingly how long the pulse is - I've tried timing at 90 deg and trigger pulse 100us to 2ms but still behaves the same. Most datasheets for sensitive gate triacs show bi polarity triggering - this datasheet doesnt specify that but perhaps implies. The voltage the circuit is running at is 4V and the resistor to gate is 220R the gate voltage on trigger is about 1 volt so that gives a drive current of about 12mA.

Any clues anyone? Would appreciate any input as I think I've tried most things. Next on the list is to try different model TRIACS

Thx

[Jerson]

The triac switches off at each zero crossing. Going by your trigger pulse rate, it matches the cycle rate. So, you get just 1 half cycle. You need a trigger pulse(yellow) that is double the line frequency(blue) if you need both positive and negative side control.

[George]

Thanks Jerson,

Tho I think you slightly misunderstood what I was getting at. When I fire the triac on only a half cycle it will work on only one side of the phase. If I fire at 10ms intervals to match each side of the phase i t will still only trigger on one side giving a half wave.

My pictures show only a half wave trigger, one way will trigger and the other wont.

[Ioannis]

I suppose you give a negative trigger pulse after the positive one, right?

Better, give us the circuit you built.

Ioannis

[George]

I have the issue solved thanks, i'm still not sure why - but I have it working. Both trigger pulses are only in one direction, gate of triac just hooked upto resistor to PIC pin. It will work if I go negative then positive - or just negative, but just cant go straight into positive. Who knows why - but its working now so I'm happy, thanks guys for looking and yr input

[definitionofis]

The BT131 needs slightly more current for triggering with reversed polarity on the gate, and with reversed polarity on the switching main terminals.
The specification says 20mA typical and 25mA might be needed for some circumstance.
You are only sending 12mA, you said. 5mA to 10mA works for positive gate polarity, says the specification sheet.

(I came here hunting for gate resistor values for BT131 before I turn my circuit on. I am at 100 ohms and thought I might be too low.)