Gate of triac - Electronic Circuits Projects Diagrams Free

Fahime · 31st December 2007, 08:20 AM

Hi,
I am designing a dimmer with a triac. It's mentioned in the teriac datasheet that the max gate current is 35mA. I have designed the attached circuit but I don't know how to calculate the gate current. The circuit works properly but I'm not sure it's safe while triggering or not.
Please check the circuit and let me know your opinions.

dknguyen · 31st December 2007, 09:04 PM

What does the 22nF capacitor do and what does the 0.1uF capacitor do? (I'm not good at figuring out circuits like this). I'm not sure which one is sending the current pulse to turn on the triac. But I think you can calculte the current pulse by looking at the voltage of the capacitor just before it discharges and then use I = V/R to estimate the peak current pulse. It doesn't seem to me it would be too high, especially for the 15K resistor and the 0.1uF capacitor. I'm not sure about the 22ohm resistor and the 22nF capacitor, but I don't think it will cause a problem either. (no real reason though).

mneary · 31st December 2007, 10:13 PM

Your peak gate current will occur when the diac triggers; basically the diac's "delta-V" voltage divided by 22 ohms. The duration of this pulse will be approximately 22*22*10e-9, which works out to about half of a microsecond.

We could be more specific if you include part numbers for the diac and triac.

Hayrettin Demir · 31st December 2007, 11:14 PM

Quote:

Originally Posted by Fahime

Hi,
I am designing a dimmer with a triac. It's mentioned in the teriac datasheet that the max gate current is 35mA. I have designed the attached circuit but I don't know how to calculate the gate current. The circuit works properly but I'm not sure it's safe while triggering or not.
Please check the circuit and let me know your opinions.

Is this Fig.Test Circuit...?
But A1-A2 pins in Terminal short circuit.And Test Circuit (!) not to work.

dknguyen · 31st December 2007, 11:59 PM

HEhe, yeah. Good call. I would assume the OP would stick a light bulb in series with the triac? (hopefully anyways!)

Fahime · 1st January 2008, 09:24 AM

Quote:

Originally Posted by dknguyen

What does the 22nF capacitor do and what does the 0.1uF capacitor do? .

They're just for changing the trigger angle of the gate. they shift the input voltage of gate to a better control of lightening.

Quote:

Originally Posted by mneary

Your peak gate current will occur when the diac triggers; basically the diac's "delta-V" voltage divided by 22 ohms. We could be more specific if you include part numbers for the diac and triac.

The diac is DB3 and the teriac is BT138. the diac conducts in 33v. as I calculate the current of conducting the diac is 33/22=1.5A and it's more than 35mA.
But in the datasheet of teriac u see 2 factors: 1. the max gate current=2A
2. the max gate trigger current =25-50mA
when the circuit works it seems safe but i'm not sure that 1.5 A is in the moment of triggering or not. Pleasedescribe it for me.

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31st December 2007, 09:04 PM	(permalink)
dknguyen	What does the 22nF capacitor do and what does the 0.1uF capacitor do? (I'm not good at figuring out circuits like this). I'm not sure which one is sending the current pulse to turn on the triac. But I think you can calculte the current pulse by looking at the voltage of the capacitor just before it discharges and then use I = V/R to estimate the peak current pulse. It doesn't seem to me it would be too high, especially for the 15K resistor and the 0.1uF capacitor. I'm not sure about the 22ohm resistor and the 22nF capacitor, but I don't think it will cause a problem either. (no real reason though). Last edited by dknguyen; 31st December 2007 at 09:07 PM.

31st December 2007, 10:13 PM	(permalink)
mneary	Your peak gate current will occur when the diac triggers; basically the diac's "delta-V" voltage divided by 22 ohms. The duration of this pulse will be approximately 222210e-9, which works out to about half of a microsecond. We could be more specific if you include part numbers for the diac and triac.

31st December 2007, 11:59 PM	(permalink)
dknguyen	HEhe, yeah. Good call. I would assume the OP would stick a light bulb in series with the triac? (hopefully anyways!)