Hello,

I am trying to build a circuit to dim 6 LED light bulbs individually, (in as small a space as possible to fit in the fixture) without using Galvanic isolation.
The problem is I cant see how to trigger a logic level triac from a MCU pin, when a Full bridge rectifier circuit is used for a power supply.
Triac driver, Figure 3a


I'm trying to use a power supply similar to the one found here (note c3 should be connected to negative rail)


Any advice would be greatly appreciated (all tests are done with an isolation transformer)

 

I take it the inputs from D1 and D2 are for zero crossing detection to the microcontroller, and you're feeding the triac gate with pulses?

Mt1 should be connected to ground and Mt2 the load.

 

That's right, the BM1Z002FJ uses them for zero crossing, and the output at 'acout' is a pulse going to the mcu (ESP8266)

The problem i think, is if i connect Mt1 to neutral(ground) it is then referenced to a voltage other than the rectified power, and only triggers on half the sine wave

 

What will cause the triac to turn off?

 

Sorry, I'm not sure of how to answer that question? I was hoping to have the triac turn off as it passed the zero crossing point (Connected in the path Live, Triac, Load, Negative)
Ive just given LTspice a try (seems like a nice bit of software) to mock up a basic circuit, to hopefully show the issue.


As the simulation hopefully shows, im not sure how to supply a logic level signal (~6v) to trigger a triac from a non isolated power supply

 

Why don't you use a power MOSFET instead of a triac. That will eliminate the turn-off problems.

 

wouldn't that only work on dc? Im hoping to keep ac passing through the 240v led bulb as im not sure how it will handle dc

 

OK. I mis-understood. I thought the rectified AC was powering the LED.

 

I had the same misunderstanding as @KeithWalker. The zero crossings should turn the Triac off.

You don't need what appears to be a zener diode and that 10 uf capacitor in the gate circuit. A simple resistor would be enough, or is that just for the simulation?

Are you going to be able to get enough gate current into the gate with that 10k resistor?


Are aware that LEDs have very limited ability to withstand reverse voltage and that some manufacturers tell "Don't do that!" Apparently it can cause degradation and eventual failure. I have not done that since reading some datasheets.

Not able to comment on the bmz1002fj because I can't find the datasheet

 

You can connect the circuit as shown below, to get a rectified full-wave signal to the LED and SCR:

 

Thank you all for your help. Sorry I think I didn't describe the project spec as I should have done to start with (I was trying to prevent convolution of the issue).

I'm trying to run a 230v light fixture that uses 6x e14 6w led (non dimmable, but that may change in the future) bulbs.

The schematic for the first iteration that worked is attached, but i would now like to remove the need for the MOC3041's by losing the galvanic isolation.

I was hoping while i had compromised the isolation I would also be able to use a transformerless powersupply to save some extra space on the board

 

The datasheet for the AC Voltage Zero Cross Detection IC can be found here
and the datasheet for the transformerless powersupply chip can be found here

 

I had the same misunderstanding as @KeithWalker. The zero crossings should turn the Triac off.

You don't need what appears to be a zener diode and that 10 uf capacitor in the gate circuit. A simple resistor would be enough, or is that just for the simulation?

Are you going to be able to get enough gate current into the gate with that 10k resistor?
View attachment 266473

Are aware that LEDs have very limited ability to withstand reverse voltage and that some manufacturers tell "Don't do that!" Apparently it can cause degradation and eventual failure. I have not done that since reading some datasheets.

Not able to comment on the bmz1002fj because I can't find the datasheet

R2, C2 and D5 are just a stand in for the 3.3v signal from a MCU, powered from a voltage rail derived from a full bridge rectifier. Though you are correct a 10K isnt a good choice to give enough current, I will have a try again with a lower value ^_^ thank you

 

Maybe you've already resolved this, but I don't really understand the question. Is the MCU output not triggering the triac?

I've always used an optoisolator between the MCU out and the triac. If you use an optoisolated triac like Q4025, that will handle the zero crossing turn-off and you can use a MOSFET (if you wish) or triac for the power switching.

 

It appears the micro is talking WiFi. I think the micro can sit on the Neutral wire and not be isolated.
Use Logic Level MOSFETs, the micro can drive 8 FETs directly.
Why zero crossing switching? Why not switch any time with the MOSFETs?
For dimming, forget phase shift, just PWM at 600hz.
For powering the micro, use a "transformer less, Capacitor" supply.

Sorry I don't have time to make a schematic.

 

See HERE how triac can be controlled.

 

That's right, the BM1Z002FJ uses them for zero crossing, and the output at 'acout' is a pulse going to the mcu (ESP8266)

The problem i think, is if i connect Mt1 to neutral(ground) it is then referenced to a voltage other than the rectified power, and only triggers on half the sine wave

Connect Mt1 to the "negative" side of the rectifier. I don't really understand what the problem is here. I'm currently using a zero-crossing trigger with a triac and it works fine.