About Triac driving

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#1
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Hello i just made a mcu driver dimmer with a tiny13 and to avoid the transformer and MOC optocoupler i designed the power supply with a capacitor and zener.
The problem is about the triac firing pulse,and when

1)The load is connected to A2 the triac responds only to positive gate voltage,if 5V applied the lamp then lights up but it flickers.
2)When load is connected to A1 giving positive 5v or ground to gate the lamp always lights up without flickering but mcu does nothing since gate is biased through clamp diodes.
Without triac mcu works fine reading the potensiometer value and varies the firing pulse length.
I got a mess,i made PCB also and if i knew this at first i would put the MOC and who cares about the cost.

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Hi Geo,

Triacs are a finicky device.

hey have a number of quadrants in which they will trigger but are more sensitive in some of them than others.

By quadrant I mean relative polarity of gate trigger signal with respect to cathode potential.

I shall need to dig up the actual information if You o not already posses it.

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Is the lamp big enough? A TRIAC needs a minimum current to stay conductive. Do you pulse it for a really short time or do you keep the gate triggered for the remainder of the period?

Wikipedia's article on triacs discusses the quadrants.

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The gate pulse is held until the next zero crossing and then resets.

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Besides the quadrant issue, if you are using a direct off the line capacitive ballast type power supply, it may not be sustaining your Vcc during a long gate pulse. All you need to trigger is a short one.

Tom Pappano
Tulsa, Oklahoma

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Finally i got it,the triac i use is a BT137 and has a different driving compared to L4004F51 from the initial schematic which is four quadrant driving.So changing the supply polarity from 0v +5v to 0v -5v and issuing a sbi DDRB,0 the triac is firing receiving a negative level in respect to A1 and the lamp lights,then issuing a
cbi DDRB,0 the avr pin is in high impedance and the triac goes in off state.
Many thanks to all of you pointing me the quandrants something totally out of my mind just thinking as a simple drop in replacement the one with the other compare only the low current gate.