High Loads (AC) / Opto-Triacs & Photon


Yo’re correct @WebDust21, but for onesy-twosy quantities, with thick traces like this project has, it was way down on my list. I figured next time :smile:

@frlobo: I think R1, R8, R10 & R13 can all be 0.25W.

Since your loads are either predominantly resistive or low-power and inductive, I’d say you would be fine with 1W parts for R6/R11. If you want to run to 2W, no problem. Frankly, as others have pointed out for this application, with zero crossing triacs, they’re icing on the cake.


I thought it was because the electrons had a hard time making the sharp turns, and would fly off the road into the ditch? :stuck_out_tongue_closed_eyes:



You may find this article interesting regarding usage of SSR. I have a system running using some Sharp SSR S202S01F 8 amp. This controls a motor and I use asynchronous or random fire for this duty because of the inductive nature of motors. I have a snubber network using 0.5 watt 47 ohm resistor and 0.1 uF capacitor.
This has been rock solid for months of trials no random resets of the Photon / Core no apparent noise issues at all.


Best regards


Ok. So I decided to build it :smiley:

Can anyone suggest a good way to test in a safe matter?


FIx it to a piece of plexiglass - lots of people forget about the pins on the underside having 110/220/240V on them, and get into all sorts of trouble.

20W light bulbs (oven lights, if you’re in the US) make great test loads.

Run the photon from a stand-alone battery for prelim testing (like one of those little USB rechargers), that way if there’s a problem, you won’t bugger up your computer by injecting 110/220/240 via it’s USB port.

If you can find/borrow an isolation transformer, that will make things much safer as you test things out, because while there is still 110/220/240V on the high voltage side of the circuit, which can get messy if you put your finger between pins, it is no longer trying to get to earth through you. This is what isolation transformers are made for.

Above all, go slowly and keep your fingers away from those SSRs.

Oh, and that’s a nice looking PCB you’ve got there.


That is great advice!

Do you know what kind or where can I get one?

thank’s I got advice from the best! :stuck_out_tongue_winking_eye:

Pardon my drawing and for abusing your brain!!.. But hear it goes. Does this sketch looks good to you? Any particular way I should consider the VAC wiring? (neutral, hot) :



Just wanted to thank everyone on the forum for the help. Today I did the first tests and everything worked as expected.

The boiler triac is getting a tad hot. I can’t tell the temp, but after testing a mug pour I removed the AC power and touched the triac heat sink, I could keep the touch for about 10-12 seconds before feeling it to hot to keep touching.

I am trying to figure out what the maximum allowed temperature is on the data sheet:

But there are a bunch of temps. I am guessing it’s Tj (125 deg c) But I don’t know… Anyway.

Thanks for the help and the isolation transformer @AndyW !!


A TRIAC will drop around 1.2v (two PN junctions), so at 10A, that’s 12 watts of power that has to be dissipated. I had to use a heatsink on the 12A-rated MAC12M TRIAC in my elevator controller for this reason.

Looking at the datasheet, I see a “maximum power dissipation” of approximately 9 watts at 10A. Worth noting is that it’ll dissipate 50 watts at 40 amps! (That comes out to a calculated voltage drop of 1.25v.) Also somewhat important is the TRIAC’s heat dissipation without a heatsink, I.E. junction-to-ambient temperature coefficient. That indicates 50°C per watt, but I guess I don’t understand that number, as you aren’t boiling the coffee on the TRIAC. Unless I misread what you wrote :stuck_out_tongue_winking_eye:.


Glad you’re getting it working and learning more about the application.

I wonder how much current your boiler takes ? The estimate of 10A thrown around doesn’t seem out of line, but a real measurement would be good.

According to the ST data sheet for their BTA26, that would dissipate <= 10W.

As far as I can tell, the tab on these parts is electrically isolated (but you should triple check the exact part number you are using against a data sheet), so you should be able to bolt on a heatsink of some sort (use heat conducting paste when you attach it) - you’ll wish you had rotated it 180 degrees on the PCB so that the heatsink did not interfere with the phoenix connector, but hey.


There’s always version 2 :slight_smile: I have a couple of fixes already in place!

So what would the limit of temperature be to know if I need a heat sink. I will measure the temp with an iR temp sensor.

Thanks for the info.


That is a good point. I am not boiling the coffee with the TRIAC :slight_smile: