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Facing problem with AC controller and relay switch

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Full Member level 2
Apr 17, 2010
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I’ve made a circuit of an AC controller (light dimmer) with 3 on/off switch to control AC load. I used PIC microcontroller to control the whole things.

Detail description -
1. Three relay (6V dc; 250V AC, 10A) to switch on/off three AC loads. PIC MCU takes input using push button switch and provides output to relay driver (base of BC547 BJT).
2. One TRIAC based light dimmer using zero cross detector and microcontroller to set desired firing angle of the TRIAC. Two push button switches have been used to increase/decrease firing angle (8 steps). MOC3021 random phase opto isolated TRIAC driver has been used in PIC output and the gate of BT134 TRIAC.

Power line details-
230V, 50Hz

Light intensity can be varied by pressing increase/decrease push button and it is found okay. But when all the three relays are switched on the intensity significantly reduced. Which is quite unusual and it is not acceptable at all.

Say, for example, all the switches are off; I increase firing angle to 4steps and observe the intensity of the bulb. Now, if I switched on all the relays the intensity of the bulb goes down. May be the firing angle somehow increases after switching on all the relays. Intensity comes back to previous state after switching off the relays. I used a USB Mobile charger (slightly higher than 5v and 500mA approx.) as the power supply for the while circuit. I measured terminal voltage for both the case and found no significant change. It stays above 5v.

I cannot find out the reason behind it.

1k ohm resistor is there in PIC output pin and the transistor base resulting 5mA current from each pin. Should this current be significant for the slowing down the timing?

If I touch the ground of 5v supply, the problem is solved. Also no problem found if I connect AC line's ground and 5v supply's ground together.
Last edited:

The problem is solved automatically. :smile: My be it was due to some loose contact.

Now, I have another query. I found that BTA08 is a snubberless logic level TRIAC. But, I didn't find any application circuit of this. How can I trigger this logic level TRIAC? It the gate is logic on then the TRIAC is on? of if gate is logic 0 then TRIAC is off. Is it ok? If yes, MT1 should be connected to neutral line and this has to be connected to the ground of digital circuit. Am I right?

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