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I am building a simplified version of this circuit: circuit

As far as I can tell, it is a pretty standard design. The load will be a dumb resistive heater (this is a sous vide project).

For now, the optoisolator labelled 3042 is connected to the controller, and I have code which flips the LED on and off when I manually send a command to the controller. The triac and a load are also connected. The second optoisolator and the thermometer are not yet connected. There is a mistake somewhere, because the load does not turn on.

I measured the circuit, and found that the optoisolator has 230 volts on its output, no matter what signal it receives, i.e. both pins 4 and 6 are always 230 volts. I think that the low-voltage side works, because the LED labelled "cooking" goes on and off when it is supposed to. I wonder if the constant high voltage means that the optoisolator is broken, or is it more likely that my problem comes from some wiring mistake?

So, my question: Is there a situation in which a correctly working optoisolator can be always turned on, no matter what signal arrives at its LED?

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  • \$\begingroup\$ Just asking for the obvious: You did attach pin 2 to GND? \$\endgroup\$
    – jippie
    Apr 3 '13 at 20:50
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    \$\begingroup\$ Isn't the "cooking" LED the wrong way round in the schematic? \$\endgroup\$
    – pjc50
    Apr 3 '13 at 21:05
  • \$\begingroup\$ @jippie yes, it is attached. I wiggled all solder points and so on. \$\endgroup\$
    – rumtscho
    Apr 4 '13 at 16:14
  • \$\begingroup\$ @pjc50 yes, it is. I am not following the schematic 100%, it is just the best sous-vide schematic I found online, but it has a few problems with the details. My LED is attached the right way. \$\endgroup\$
    – rumtscho
    Apr 4 '13 at 16:16
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Gate voltage on the BT136 triac is 5V and you are feeding it through a 1k resistor from the live side of the supply. Have you checked the power rating of the 1k resistor?

You shouldn't have to "dispence" much more than 30mA (typically) for the triac to trigger and this would imply, from a 230V supply, a resistor value of about 7.5k. Even so... it would have to be rated at nearly 7W. A 1k resistor would be a 50W.

OK... it only supplies this power when switching on but I come back to the 1k resistor possibly being broken or blown.

As for the 230V on both sides of the opto -if the BT136 triac is bust this could easily happen.

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  • \$\begingroup\$ I am using the schematic more as a guide. I used 330 and 390 Ohm resistors for R4 and R5, because these numbers are given in the datasheet of my optoisolator (which is one model later than the one in the schematic). \$\endgroup\$
    – rumtscho
    Apr 4 '13 at 16:17
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    \$\begingroup\$ @rumtscho, it will help us give you better answers if you share a schematic of what you actually built...even if that means redrawing or editing the schematic. \$\endgroup\$
    – The Photon
    Jun 3 '13 at 1:26

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