Triac and resistor (needed)

[Jony130]

If we have this circuit with triac and MOC3041.



Do we really need R1 and R1 if MOC3041 is a zero-cross operated triac?

 

[Dvinchy]

R2 R3 Instead of these can be used single r=300Ω

 

[Hero999]

The resistors limit the surge into the gate when the TRIAC is triggered.


Surely a single 330R resistor could be used rather than two.

 

[Jony130]

But MOC3041 is a zero-cross so TRIAC will be triggered when the voltage is close to zero.
And after the main TRIAC is triggered then he short MOC3041 triac.
So when this surge current is gonna to flow?

 

[Hero999]

Fair point.

I don't know, I'd just use a resistor, it's not like it's expensive.

 

[crutschow]

But MOC3041 is a zero-cross so TRIAC will be triggered when the voltage is close to zero.
And after the main TRIAC is triggered then he short MOC3041 triac.
So when this surge current is gonna to flow?

Yes, but the TRIAC takes a finite amount of time to turn on (typically about 2µs). During that time excessive gate current could flow and zap the TRIAC or the transistor in the optical coupler. The resistor is insurance against that happening.

 

[MrAl]

Hi there,


With 300 ohms in series with the gate with a triac that takes 50ma to turn on the line voltage will have reached roughly 15v and that is about a 2.6 degrees firing angle. Now 55us later say the triac turns on (a slow triac). That's roughly another 1 degree which means the line voltage will have reached about 20v, and that means the gate will now see about 67ma, and then the main triac turns on and shunts the voltage so the current drops to zero. That sounds pretty good to me.

On the other hand, without a resistor it is very hard to determine just what the current will be because we dont really know the dynamic resistance of the gate. Yes, it may work or it may work for a while and then blow out. That means we would be taking a risk.

If you read the second part of my signature line, "One expert specification is worth a thousand tests", that makes a lot of sense when we look at the data sheet for the device, where the experts who have already taken the time to specify this part have determined that a gate resistor is necessary. Thus, that's the best reason to use one and not take the chance of not using one. Additionally, they specify not a 300 ohm resistor but a 360 ohm resistor, which BTW handles the possible peak voltage better than a 300 ohm resistor which keeps the peak current in the device below the specified value of 1 amp at all times.

 

[saeedali]

hello all experts.

all zero cross optotriacs have a parameter with name of "inhibit voltage". this means that if
voltage on phototriac became higher than 20v and lower than 5v it doesn't conduct

Since the voltage is limited to 20 V or less, the series gate resistor
that limits the gate drive current has to be much lower with a
zero crossing opto-driver. With typical inhibit voltage of 5 V, an
alternistor triac gate could require a 160 mA at -30 °C (5 V/
0.16 A = 31
gate resistor). If the load has a high inrush current,
then drive the gate of the triac with as much current as reliably
possible but stay under the ITSM rating of the opto-driver. By using
22
for the gate resistor, a current of at least 227 mA is supplied
with only 5 V, but limited to 909 mA if the voltage goes to 20 V.

with 330 ohm resistor in 20v the gate current will be 60ma and it is possible
that the triac isn't turn on at all in low temperature or use a triac that need more gate current.

do you confirm my understanding?

best regards.

 

[rumpfy]

can I suggest that two resistors are used because of 'Voltage rating of a resistor'.
According to the Philips data book for resistors, the CR16 series is a 0.25 watt resistor with a diameter of 1.6 mm and a length of 4.5 mm. The limiting voltage for this is 150 V rms. That's what the data book says.
In the past I have seen high power carbon resistors used in HV circuits (vertical deflection circuits) of television receivers (valve types). The use of a high power resistor is to get a higher voltage withstand capability.
In this case, the power dissipation of the resistor is not great, so a physically small unit could be used; but this would probably have insufficient voltage withstand capability, so two resistors are are used in series.