Hello,

I am trying to build a precision dimmer via a TRIAC and Microcontroller and am having technical difficulties. I am using an optocoupler package FOD817 to trigger the gate signal.

-I have verified that my trigger circuity is working. So, the FOD817 opto-transistor is opening and 12VDC is indeed getting to the TRIAC gate. However there is no drop across the 220 ohm resistor so current must not be flowing.

-I have verified that 120VAC wiring to the TRIAC is correct

-I do know that MT1 and MT2 are not interchangeable, and must be connected properly though I did try the other way for kicks

-I have tried to bias the TRIAC gate via a voltage divider much like you would a BJT but that did not work either.

-I have hooked up the secondary side of my transformer to a dummy load to see that enough current can indeed be drawn

I have the TRIAC wired up in series with a 60W incandescent bulb. Right when the MCU triggers and voltage is present on the TRIAC gate I an hear a faint "tink" in the light bulb but then nothing till the next cycle... and no light.

Any help would be greatly appreciated!

-Alex

 

The rectified voltage ground shall be connected to MT1 of the TRIAC.

Post the whole circuit as we can tell if it is ok to connect the ground to MT1.

 

I've attached the schematic showing how I hooked up the TRIAC. Thanks for you help Mik3

Well now that makes sense since I measured a low impedance (~75 ohms) from the Gate Pin to MT1... this makes the DC injection loop? With this connection are there worries of 120VAC signals escaping into my 12VDC circuitry?

 

I think you have to connect the floating ground to MT1. However, be careful because the 12VDC will be connected to the mains 240VAC, so it is not safe to touch the 12VDC.

 

Hello,

You can better look at the MOC30XX series.
The MOC301X and 302X are non-zerocrossing.
The MOC303X and 304X are zero crossing.

I have attached the datasheets.

Bertus

 

Cool I got it working thanks to everyone for their input....

So, the datasheet for the TRIAC I am using (Q2025L6 made by LittleFuse) states that the test voltage used when supplying the Gate Signal is "12V" thus I assumed they meant 12VDC and that is why I wanted to use a DC signal to trigger the TRIAC.

I'm sure 12VDC would have worked but as Mik3 pointed out I needed to connect my DC GND to MT1. My rectification diodes are rated at 50V so I was hesitant to try this.

I happened to have an MOC3020 laying around and connected it as seen in the data sheets supplied by Berts!!! Got rid of the transformer and rectifier bridge and used the 120 mains to trigger....it worked, yay!

Is it at all common to use a DC signal to trigger a TRIAC?

 

The rectification diodes would not see the 240VAC across them, thus it would be ok to connect the 12VDC. However, it would not be safe to touch the 12VDC line since it would be connected to the 240VAC.