TRIAC switching problem

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raf23

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I have a speed control circuit for a 230v ac motor. It has got BTA 16 Triac for pahse control and it is driven by an optocoupler moc3021 and the trigger signal is coming from a pic16f676 mc. Trigger signal is given according to an input event to the micro controller. The problem is Traic is getting switched on automatically sometimes when I turn on mains. I found that it is because somehow the triac driver gets the drive voltage from the micro controller. But it is not happening always. I cannot change anything in hardware . I also tried in software not to trigger the triac for 2 seconds from the power on but still the problem persists. Is there anything that I should try to get around this problem?
 

check the snubber ckt. there is a snubber for TRIAC and MOC3021 and combiled with gate protection. ref MOC3021 datasheet.

Microcontroller initialization should happern without any initial delay.
Microcontrller may get reset during mains ON/OFF. Due to EMI/EMC. u can chaeck that.

Trigger signal I/P event may get triggered during mains ON/OFF. chk trigger ckt.
 

Hi raf
Can you please show me the circuit that you're referring to ?
Besides these issues : when you want use a triac as a switch for alternative current , and when your command signal is supplied from a DC source , you can't simply connect it via a usual opto coupler like MOC3021 ! you need to use an opto coupler with a zero cross detection circuit . there are many of the opto couplers for such a purpose which have a zero cross detection circuit , internally . you should go through them .

Best Wishes
Goldsmith
 


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