moc3023 triac optocoupler

Thread Starter

denison

Joined Oct 13, 2018
328
I am using the moc3023 for phase control of a triac. see attachment for a circuit Fairchild semiconductor has sent me. the Fairchild circuit actually shows a 360ohm between the 240vac line and the moc3023. I used a 1000 in a ltspice simulation which showed 90w power over the resistor.

when I built the circuit with a 360 ohm 1/4 watt resistor this blew the resistor immediately. this is not hard to see as you are dropping 240v over the resistor. when I contacted Fairchild they said to use at least a 1watt resistor.I now have some rox3s 3w metal oxide film resistors. will these withstand the 240v?

The rox3s are rated at 350v which doesn't seem to mean much when they are only 3w. the length of pulse shown on the spice simulation is a max of 80us.

can anybody suggest the power of the resistor I should use? and is their a calculation to show the power dissipation over a very short period of time?
thanks.

sorry I couldn't do the attachment as is was the wrong file type for this application.
 

Thread Starter

denison

Joined Oct 13, 2018
328
The resistor is the one you show as 180 ohms not the load. The Fairchild schematic shows 360 ohms here. Now assume a source of 240vac as in the Fairchild schematic and a triac load of 24 ohms. Add to this the resistance over the gate drive circuit in the triac I have used of 67 ohms.
From this you can work out that the 360 ohm resistor has to withstand approx. 100 watts of power. 240v source and 3 resistors in series; 24,360 and 67.
The resistors I have bought now are rox3s, 560 ohms and 3watt. They also give a figure of 350vac which I presume is the ac voltage they can withstand for a small time period which they don't give. I will need to find out what they mean by this.
Anybody on the forum want to try this circuit with a 1 watt resistor as suggested by Fairchild and risk blowing it. The trouble is my circuit has other components that might be damaged also.
 

eetech00

Joined Jun 8, 2013
3,859
Hi

I have built the circuit your describing but using the MOC3033 with 120v load. The 220v version your describing should use a 360 ohm resistor instead of 180. There shouldn't be a lot of current flowing thru this resistor. On my circuit it is 0.5 watt. There is another resistor from gate to neutral side of load (330), on my circuit this is a 1 watt resistor. The load should be connected on the neutral side of AC line. It sounds like either the opto or the Triac isn't connected correctly.

See below.

Edit: Just noticed your using a different opto circuit, but it still sounds like the opto or Triac is wired incorrectly or damaged. There shouldn't be much current flowing thru the 180 ohm resistor.

upload_2019-4-5_7-4-38.png
 
Last edited:

Thread Starter

denison

Joined Oct 13, 2018
328
Hi

I have built the circuit your describing but using the MOC3033 with 120v load. The 220v version your describing should use a 360 ohm resistor instead of 180. There shouldn't be a lot of current flowing thru this resistor. On my circuit it is 0.5 watt. There is another resistor from gate to neutral side of load (330), on my circuit this is a 1 watt resistor. The load should be connected on the neutral side of AC line. It sounds like either the opto or the Triac isn't connected correctly.

See below.

Edit: Just noticed your using a different opto circuit, but it still sounds like the opto or Triac is wired incorrectly or damaged. There shouldn't be much current flowing thru the 180 ohm resistor.

View attachment 174221
Hi

I have built the circuit your describing but using the MOC3033 with 120v load. The 220v version your describing should use a 360 ohm resistor instead of 180. There shouldn't be a lot of current flowing thru this resistor. On my circuit it is 0.5 watt. There is another resistor from gate to neutral side of load (330), on my circuit this is a 1 watt resistor. The load should be connected on the neutral side of AC line. It sounds like either the opto or the Triac isn't connected correctly.

See below.

Edit: Just noticed your using a different opto circuit, but it still sounds like the opto or Triac is wired incorrectly or damaged. There shouldn't be much current flowing thru the 180 ohm resistor.

View attachment 174221
Hi


I have built the circuit your describing but using the MOC3033 with 120v load. The 220v version your describing should use a 360 ohm resistor instead of 180. There shouldn't be a lot of current flowing thru this resistor. On my circuit it is 0.5 watt. There is another resistor from gate to neutral side of load (330), on my circuit this is a 1 watt resistor. The load should be connected on the neutral side of AC line. It sounds like either the opto or the Triac isn't connected correctly.

See below.

Edit: Just noticed your using a different opto circuit, but it still sounds like the opto or Triac is wired incorrectly or damaged. There shouldn't be much current flowing thru the 180 ohm resistor.

View attachment 174221
Referring to your circuit. Suppose the load is 24 ohms and you are switching at the peak of 240v (336v).
Just before the triac switches on the load will be 360+330+24 =714 ohms. Doing a calculation for power this is 79 watts over the 360 ohm resistor which is why my 1/4 watt resistor blew.
When the triac switches the power is even larger. Assume the gate to neutral resistance of the triac is 67 ohms. The total resistance is then 360+67+24=451 ohms. This gives a power over the 360 ohm of 268 watts.
So how can an ordinary resistor withstand this large power? Is it the short firing pulse? did your 1/2 watt resistor survive? Fairchild suggest at least 1 watt for the resistor.
 
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