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I wanted to try the basic circuit of MOC3051 which is a random phase optoisolator triac driver.

I used this circuit from the datasheet: enter image description here

with following parameters:

  • Vcc : 3.3v
  • Transistor : BD139
  • R : 1K
  • Triac : BTA40-700b
  • Load : Resistive 9.4K
  • AC line : from a rotary voltage converter (from 0 - 300 VAC)

I've also implemented zero detection and firing angle handling by a pic microcontroller. The circuit works perfect when the AC LINE voltage is 24VAC and when I checked the signal across the load, it changes perfectly in response to firing angle.

The problem is when I increase the AC LINE voltage up to 90VAC the signal across the load started to vanished at some firing angle and above 100VAC it vanished completely. I've tried different values for R and re-assembled the moc3051 with the new one but nothing's changed.

Do you have any clue why is this happening when AC LINE voltage is increasing while the main purpose of these drivers are to work with 110 - 240vac voltage range?

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  • \$\begingroup\$ Check your (PIC input) zero detection scheme. All other things being equal, the pulse will get 1/10 as wide if you increase the supply voltage by 10:1. \$\endgroup\$ Jan 20 '20 at 12:03
  • \$\begingroup\$ Thanks, checked. Zero crossing detection pulse works fine and every 10ms triggers an interrupt. \$\endgroup\$ Jan 20 '20 at 12:45
  • \$\begingroup\$ How about the length of the trigger pulse to the opto? And is the LED current high enough with sufficient margin? \$\endgroup\$ Jan 20 '20 at 12:47
  • \$\begingroup\$ I've tried different on-time duration for trigger pulse but nothing's changed. Also the LED is driven by 30ma which is in range according to the datasheet. \$\endgroup\$ Jan 21 '20 at 6:45
  • \$\begingroup\$ Well this is odd but I replaced the MOC3051 with MOC3052 and it works... \$\endgroup\$ Jan 21 '20 at 9:34
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Your issue may be with the particular load and trigger current pulse you are using for the higher voltage.

The current has to have sufficient time to build up to the holding current for the triac or it will not stay on for the remainder of the cycle (it will just momentarily conduct for the duration of the trigger pulse, much like a transistor). The triac is deliberately made a bit insensitive to keep from false triggering (eg. dv/dt).

Inductance in the load, too much series resistance in the gate circuit (1K is a bit on the high side) or a too-short trigger pulse can all contribute to this.

If a DC trigger level turns the main triac on reliably then your problem is related to the trigger pulse.

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  • \$\begingroup\$ Thanks for your reply. The opto LED is driven by about 30ma with 100R series resistor with transistor collector. Also about the duration pulse, I've tried different on-time duration but it seems it lose control when the AC LINE voltage increases. I think you're right and there is something wrong with triggering the opto triac. Also I made another change which is put 30K resistor series with triac gate. It works and I see the desired output but the peak-to-peak voltage decreases significantly. \$\endgroup\$ Jan 21 '20 at 6:42
  • \$\begingroup\$ Well this is odd but I replaced the MOC3051 with MOC3052 and it works... \$\endgroup\$ Jan 21 '20 at 9:34
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I don't think you need an additional transistor to drive the opto. Below circuit works fine at 220 VAC:

opto ckt

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  • \$\begingroup\$ Thanks. Yes it would work without the transistor but it's not the problem. I used MOC3051 instead of MOC3021. I will check with this opto as well. \$\endgroup\$ Jan 20 '20 at 12:50
  • \$\begingroup\$ Well this is odd but I replaced the MOC3051 with MOC3052 and it works... \$\endgroup\$ Jan 21 '20 at 9:34

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