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

Discussion in 'Electronic Basics' started by David Nugent, Jul 19, 2003.

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  1. David Nugent

    David Nugent Guest

    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.

  2. 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.
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