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    Suspect a TRIAC failure at neutral rail

    Hello,

    I have a coffee grinder, works on 220V, stopped working. I opened the case and started inspecting any possible problem.


    The HOT line is going through couple limit switches: one at bean hopper lid and one at coffee container for safety that the machine has the hopper and container in place.

    So the 220V is there all the way to the motor 1st brush.

    The neutral line on the other hand is controlled by a triac which receives the pulse from pic12f508.


    There are two problems I encountered:

    1. There a 3rd limit switch, which is activated by a button to start grinding. When there's power, the switch has 5V, so when I press, it should pass the 5V from 1st pin to 2nd pin. But the 5V isn't passed even I tested the switch with continuity when the machine is off, when I press the switch, there's a contact. But when power is on, when I press the 5V isn't passed!

    2. The triac BT134 has 9V on its gate, but the motor won't start. Does the triac works with 0V?

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    Re: Suspect a TRIAC failure at neutral rail

    If the switch is closed and you don't see 5v, that would indicate a short, dont you think?


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    Re: Suspect a TRIAC failure at neutral rail

    Yeah, that's one high sign of a problem, but I think it works as a pulse for active low signal.

    But the triac isn't working! It suppose to open the neutral rail to the motor 2nd brush, but it isn't! On the the MT1 to hot rail there's 220V, but from hot rail to MT2 there's 0V. There's 9V on the gate.



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    Re: Suspect a TRIAC failure at neutral rail

    This is my analysis of the board

    Click image for larger version. 

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    Re: Suspect a TRIAC failure at neutral rail

    When you say the switch is "not passing" 5V, do you mean one side of the switch has 5V and the other side doesn't? That certainly means a bad switch, don't you think? Or, do you mean when the switch is closed the 5V supply gets pulled to ground? As said before, this indicates a short.


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    Re: Suspect a TRIAC failure at neutral rail

    Quote Originally Posted by barry View Post
    When you say the switch is "not passing" 5V, do you mean one side of the switch has 5V and the other side doesn't? That certainly means a bad switch, don't you think?
    The switch works fine, but the surprising thing is that when I press it, it doesn't pass the 5V because the other side is 0V where I guess it should be 2.3V .. I'm not sure, but guessing it should be 2.3V doesn't sound logical. Because 2.3V still a high voltage to e.g. a microcontroller.

    Or, do you mean when the switch is closed the 5V supply gets pulled to ground? As said before, this indicates a short.
    I think that's what's going on there.

    In the picture, the yellow box which has the 0V and 2.3V .. that's what I'm talking about, that's the point where I think it has a problem. Because the triac has 9V on its gate all the time!! What's going on exactly, I'm not sure.

    I don't know if the triac should receive a 9V or not, or it maybe works with 0V. But I don't think triacs work with active low. I know the are PNP transistors and n-ch MOSFETs. But I'm not sure about triacs.

    My question now is that if there's a 9V on the gate of the triac, but the triac isn't working, e.g. passing the neutral of the 220V line, so what does that mean?



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    Re: Suspect a TRIAC failure at neutral rail

    In the early 80s I worked at a company where we used thousands of TRIACs per week on many products.

    If I recall correctly every TRIAC failure that I saw was always a shorted device.

    How are you measuring the gate's 9 volts? From gate to what point? It should be to MT1.
    Be aware though, since a TRIAC is a latching device, the gate drive may be a narrow pulse with a phase shift, which you cannot really measure with a DMM.

    Lastly, TRACs are not voltage driven but current driven. You should really measure the voltage drop across the series resistor. But again, if it is a pulse, you won't see it with a DMM.
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    Re: Suspect a TRIAC failure at neutral rail

    Quote Originally Posted by schmitt trigger View Post
    In the early 80s I worked at a company where we used thousands of TRIACs per week on many products.
    wow must be beautiful years :)

    If I recall correctly every TRIAC failure that I saw was always a shorted device.
    Yes, I guess pretty much most semiconductor devices are shortened when they fail.

    How are you measuring the gate's 9 volts? From gate to what point? It should be to MT1.
    I'm measuring it from the gate to the circuit DC ground.

    Please, if you could check post #4, I included an image with squares of the important parts.

    If you check the traic signal bath, it's coming from a transistor. But the 9V actually coming from the DC power supply which regulates 5 and 9V for the control circuit, it has one little pic microcontroller.

    But you know, maybe there's something wrong with the microcontroller. I receives a signal from the switch and activate the transistor which in turn activate the triac. But there's always 9V on the triac's gate!

    Be aware though, since a TRIAC is a latching device, the gate drive may be a narrow pulse with a phase shift, which you cannot really measure with a DMM.
    So if it's a high frequency pulse, then it should be coming from the mcu, so if there's just plain 9V without pulsating, then the chip is broken, because I think the mcu could just drive the low pulse of the 9V, so if the mcu is broken then there's a 9V on the triac's gate ... that's my guess.

    Lastly, TRACs are not voltage driven but current driven. You should really measure the voltage drop across the series resistor. But again, if it is a pulse, you won't see it with a DMM.
    Ahaa, so do you mean that I can't measure the actual voltage on the gate? But if I'm getting 9V from the gate to the common ground of the circuit, then what does that mean?

    - - - Updated - - -

    I'm actually thinking to disconnect the mains rails and connect them directly to the motor, is that safe?



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    Re: Suspect a TRIAC failure at neutral rail

    Again
    Please Measure from gate to terminal 1.

    Or better still, measure the voltage drop across the gate resistor. Using ohms law, calculate the drive current.
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    Re: Suspect a TRIAC failure at neutral rail

    I took out the board and connect the wires directly to the motor, and the grinder worked like a charm, so it's obviously the triac because it's the last stage of switching and didn't work even there was a voltage on it's gate.



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