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Help me fix this oven controller circuit please!! See schemativ


David Nugent

Jan 1, 1970
I have a controller out of a moisture oven. It is about 20 years old, and I
need to get it working. It has a digital set point (temperature you want
the oven to go to). When the oven reaches the set point, it is supposed to
turn off (stop heating). I have no schematic for the controller, but have
created a schematic of the area in where I believe the problem lies.

There is a LED that says (Heating when Lit). When the oven is heating up,
the LED stays on. When it reaches the preset temperature, the LED starts to
flicker and then goes out. So I believe the logic side of the controller is
working fine.

The oven element is controlled by a TRIAC (Which is tested ok). It heats up
ok, and if I disconnect the Gate lead, the temperature starts to drop.

There is an "Unknown device" I've posted a picture of before. Maybe in the
schematic, it will make more sense to someone. I'm guessing it's some sort
of opto-isolator, but it's quite large about 2" long, 1" high and 1/2" wide.

A few things I noted/tried:

Out of circuit, pins 3/4 on unknown device look "open" (Meter flashes
infinite resistance). However, if pins 3/4 are not connected to the circuit
(While pins 1& 2 are), the oven will not heat. This puzzled me.
I figured it it was "open" that it would make no difference if it was there.

Disconnected the gate of triac while the oven was heating. Oven started to
drop temperature (Show that triac turns on and off).

As a mention of note: The oven originally had a Q4025P triac (That was
shorted - Oven always on/heating). I was given a BCR16HM triac from a local
electronics supplier and told it would be a fine replacement (Actually looks
a little more heavy duty).

Here is a curious point. When I first discovered the original Triac was
shorted, and replaced it with the new one, the first time the oven powered
up, it worked. It reached it's temperature and then the triac turned off.
But since then, it hasn't worked properly. As I mentioned I rechecked the
new triac and it seems fine.

I would greatly appreciate any help suggestions anyone can give me. I'm
pretty stumped at this point.


Dimirtij Klingbeil

Jan 1, 1970
David Nugent wrote:

The 'unknown device' may be a relay. Check the voltage drop at the 'open'
connectors. If it changes between zero and some high voltage with no other
voltages between those 2 points, chances are high that it is one. I do not
know your circuit, so many of the following suggestions may not apply. If
the unknown part is indeed a relay (I hope you can test it, if not, ask for
details), chances are very high that the relay is in working order (because
it shows 'infinite' when disconnected and the oven works when connected).
The same applies if it is an opto-isolator. So try to figure out whether
the circuit controlling this relay/opto is working properly by checking the
voltage between pins 1 and 2. It should change when the oven heats up.
Check the transistors, especially high-power ones if any exist. Check all
electrolytic capacitors (the current through them should stabilize around
zero). As the device is assumed to operate at more than room temperature a
transistor or capacitor failure is possible. If the 'unknown device' is not
a relay, try to find out its input/output characteristics (voltage,
current, resistance, nonlinearity/switching point (if one exists)), look
out for values that seem to be 'far away from the real word'. Try testing
the conductivity of pins 3 and 4 in both directions. If one conducts, it
may be some sort of micro switching PSU isolating LV and HV circuit parts
(although those were a rarity at that time, I don't believe it is one). You
said that the whole thing is triac-controlled, but is the triac really
controlling the power to the heater or just switching it on and off? I have
seen devices using triacs for both purposes so far. In the first case the
'device' is more likely to be an opto, in the second case a relay. Further
you should check the diodes in the circuit and make sure none is shorted.
If you have any schematics, you can e-mail them to me, I will take a look
at them, but don't rely on me finding anything to solve your problem.