555 timer TRIAC flasher

[Athosworld]

I made this circuit that is just an astable 555 powered by a capacitive dropper with the output connected to the gate of a MAC97A6 triac. The load (a dimmable LED lamp) does flash, but it never fully turns off, how can I solve this? The triac gate voltage and current seem to be fine.

 

[rjenkinsgb]

You are mixing up voltages within the bridge rectifier DC output, and the AC neutral.

The negative from the bridge is either near (0.6V) the neutral voltage - or near the reduced live voltage - on alternate half cycles.

You generally cannot mix connections between AC and DC sides when using a non-isolated supply.

You could use an opto-triac trigger (eg. MOC 3000 series) to trigger a triac in the AC side from the DC side without cross connecting the two.

ps. It looks like there is no current limit resistor feeding the gate?

 

[Galgso]

You can get a neutral-referenced (or live-referenced) capacitive dropper if you are willing to use a half-wave rectifier (i.e. a single diode) rather than a full bridge. That way you don't need an opto-isolator or an optically-triggered triac. You will however need a larger filter capacitor.

Also I've attached an application note that I've used before, you may find it useful.

 

[Athosworld]

You can get a neutral-referenced (or live-referenced) capacitive dropper if you are willing to use a half-wave rectifier (i.e. a single diode) rather than a full bridge. That way you don't need an opto-isolator or an optically-triggered triac. You will however need a larger filter capacitor.

View attachment 150395

Also I've attached an application note that I've used before, you may find it useful.

Does it need to be neutral or live referenced to trigger the triac correctly? Would a non-polarized cause any issues with the circuit?

 

[Galgso]

Does it need to be neutral or live referenced to trigger the triac correctly?

Yes, the supply powering the gate must share a common terminal with the main power source which is AC mains in this case.

 

[Athosworld]

Yes, the supply powering the gate must share a common terminal with the main power source which is AC mains in this case.

I meant, if it matters whether it is live-referenced or neutral-referenced

 

[Galgso]

I don't think it matters because the TRIAC should trigger on either polarity although the triggering may not be exactly symetrical for both polarities.

 

[Athosworld]

I don't think it matters because the TRIAC should trigger on either polarity although the triggering may not be exactly symetrical for both polarities.

The zener can also be placed after the diode, right?

 

[Galgso]

If you're talking about the schematic I posted earlier, no the zener must be before the rectifier diode. The way the circuit works is when the zener is reverse biased it drops a certain voltage and that voltage charges the large 1mF capacitor through the diode. On the other cycle when the zener is forward biased, it drops 0.7v and the rectifier diode prevents the capacitor from discharging when this happens so the zener must be before the rectifier diode.

 

[Athosworld]

If you're talking about the schematic I posted earlier, no the zener must be before the rectifier diode. The way the circuit works is when the zener is reverse biased it drops a certain voltage and that voltage charges the large 1mF capacitor through the diode. On the other cycle when the zener is forward biased, it drops 0.7v and the rectifier diode prevents the capacitor from discharging when this happens so the zener must be before the rectifier diode.

Thanks a lot, it works now.