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# TRIAC Peak gate voltage

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#### tellMeDaBasix

##### Member level 1
sus trigger circuit

I confused with some TRIAC entry in datasheet, specially Vgm (Peak gate voltage) that is written 5V.

Does that mean I should not put more than 5V between gate and MT2 of TRIAC?

In not, then most circuit, say like : **broken link removed** or http://www.epanorama.net/usercircuits/dimmer1/regulador.pdf

Here you see diac connected to GATE of TRIAC and DIAC trigger voltage ~30V.

That mean, when DIAC trigger, voltage across DIAC is > 30V

Since diac connected to GATE of TRIAC, so voltage across GATE of TRIAC is ALSO > 30V

This more than the given 5V.

So what is problem?

Where my understanding incorrect please tell me and understand?

triac trigger

bump

triac gate voltage

:-(

bhishmar

### bhishmar

Points: 2
triac vgm

Your link doesn't work for me, so I cannot comment on what you indicated. More importantly one should not exceed the maximum gate current. If the 30 volts is current limited below the maximum gate current, the 5 volt value will never be exceeded.

### tellMeDaBasix

Points: 2
triac gate trigg

Tralfaz said:
Your link doesn't work for me, so I cannot comment on what you indicated. More importantly one should not exceed the maximum gate current. If the 30 volts is current limited below the maximum gate current, the 5 volt value will never be exceeded.

Hi, Image here : **broken link removed**

Please now help explain why 5V gate current not exceed

triac gate trigger

I looked up a datasheet for the T0609M triac. As I interpret what it says, it does not indicate to me any gate voltage limitation, 5-volts or not. It only indicates a threshold voltage of 1.5 volts or 10 ma. to trigger. It indicates an average gate input power of 1-watt with 10-watts as the absolute maximum. and gate current at an absolute maximum of 4-amps in a 20 microsecond trigger pulse. You've got to be driving a really big load to need to hammer the gate that hard.

As I look at your circuit, I can see that the gate is triggered with the 30 volts as you can see. Considering that the gate resistance is probably in the area of 60 ohms or so, that makes the trigger spike current about 500 ma. well under the 4 A maximum.

Is there a problem with the operation of your circuit?

### tellMeDaBasix

Points: 2
triac gate trigger voltage

What if I change the TRIAC to BT139?

Then will TRIAC be ok?

diac trigger capacitor voltage

I looked up two datasheets for the BT139 part. One a Phillips which indicates the 5-volt limitation that you mentioned. The other a Semi-Well brand that indicated a 10-volt limit. Both indicated a 2-amp gate current as a maximum which tells me that the Phillips part must have a lower gate resistance. So, in order to use that type triac, you will need to use a 3-4 volt SUS, or add a current-limiting resistor in series with your 30-volt SUS, I would recommend something to the tune of 150 ohms. That would limit the gate current (and thereby the voltage) and should still be enough gate current to fire the load.

Having the 30-volt SUS in your circuit does limit the output to around 170 VAC. Using the BT139 and changing to a 3-volt SUS would result in greater maximum output.

Hope this helps.

diac 30 volt

In my opinion, your calculation doesn't apply to usual DIAC trigger circuits (as shown above in this thread). You are ignoring two points:
- the pulse that can be sourced from the 0.1 uF trigger capacitor at maximum allowed gate current would be shorter than the DIAC current rise time. The real trigger pulse is longer but has a considerably lower peak current.
- the DIAC has finite on-state voltage further reducing the peak current

Practically, it's rather unlikely to damage even a low gate current TRIAC as BT139 by connecting a 30V DIAC with a 0.1 uF trigger capacitor.

Having the 30-volt SUS in your circuit does limit the output to around 170 VAC.
No idea, how you calculated this? The Vrms output reduces only by a few % due to the residual phase angle with 30 V DIAC.

P.S.: I measured the trigger pulse at a standard light dimmer circuit with a 30V Diac and a 68n trigger capacitor and found peak Ig = 1.2 A and peak Vg = 6 V.

vgm en un triac

Having the 30-volt SUS in your circuit does limit the output to around 170 VAC.

It was late here.

So the answer to the initial question is that the Diac cannot deliver an overvoltage to the gate due to the time it takes to fully conduct relatiive to the discharge time of the capacitor.

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