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How does this work?

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DBB

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I'm trying to diagnose a non functioning treadmill with my limited experience. I've traced out part of the circuit that I believe is the culprit. But there's one area (at least for this discussion!) that I don't understand how it works. It's the circled mosfet transistor in the attached circuit diagram.

How does the transistor turn on/off when the gate is connected to ground through a resistor? I believe what should happen when the motor is called for is that the coil supplies 120VAC to the SCR Diode module which in turn supplies a VDC to the motor when G1 & G2 are energized. G1 & G2 are energized when 4.9VDC on pin 1 of the optoiosolator gets through it, which happens when the gate of the transistor turns on. That then in turn supplies the 55VDC from pin 4 of the isolator to pin 6 and turns on G1 and G2 which turns on the SCR diode module, giving power to the motor. Currently, there is 4.9VDC at pin 1 and 4.1VDC at pin 2 of the isolator and that never changes regardless of the call for the motor. As a point of interest, if I ground pin 2 through a 10k resistor when the motor is called for, the motor works.....but only for a second until my arc-fault circuit breaker kicks off! I haven't wanted to try it on another regular circuit breaker for fear that it's not the arc(which I can not see) that is causing it to kick off but high current!

I've replaced the SCR diode module (at a cost of $50!) and the isolator to no avail. Maybe my problem is the transistor, but I would like to understand how it works.

Thanks for any help you can give me.

Doug
 
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With the gate and source of the FET connected via the resistor the FET is almost turned completely off and acts as a high value resistor (much higher than 10k, so wouldn't activate your arc-fault trip). This allows a limited current to flow through the opto diode and so cause the SCRs to turn on alternately. If the 10k resistor allowed the motor to run then that points to the FET being dead. Try replacing it.
 
With the gate and source of the FET connected via the resistor the FET is almost turned completely off and acts as a high value resistor (much higher than 10k, so wouldn't activate your arc-fault trip). This allows a limited current to flow through the opto diode and so cause the SCRs to turn on alternately. If the 10k resistor allowed the motor to run then that points to the FET being dead. Try replacing it.
Do you think any design engineer would count on using IDss for anything? The max is 10uA at 25C. It has no minimum spec. It could be nanoamps.
I think DBB missed a connection to the gate of the MOSFET (which I assume is actually VN2222). It is the the switch for the optoisolator. There must be a current limiting resistor left out of the schematic.
 
Hi,

Ron:
I couldnt have said it better myself :)

There's something wrong with this schematic in more than one place. He'll have to trace the circuit out better and repost.
 
Thanks for your replies. I did modify the diagram to include a 180 ohm resistor upstream of the opto isolator. Does it make sense now?

What test can I perform on it without taking the transistor out. Should I be able to just ground pin 2 of the isolator and it should work or add a resistor to ground as I did or can't I do anything without replacing the MOSFET?

Thanks again for your time.

Doug
 
Do you think any design engineer would count on using IDss for anything? The max is 10uA at 25C. It has no minimum spec. It could be nanoamps.
It's certainly not how I'd design it, but that seems the only explanation for operation of the circuit as originally posted if there's no other gate connection. Mind you, there are some seriously weird designs out there; so it's not entirely impossible that someone got away with that design.

As the others have pointed out it's doubtful that the FET could allow a useful current to flow through the opto diode if the gate and source are connected as shown and there is no other connection to the gate.. Can you double-check that nothing else drives the gate?

What test can I perform on it without taking the transistor out. Should I be able to just ground pin 2 of the isolator and it should work or add a resistor to ground as I did
That should work. If the trip operates then there's a fault downstream of the opto-isolator. Could be a partial or intermittent short somewhere in the motor. Unlikely to be the SCR bridge rectifier if you replaced that.
 
Thanks guys for pushing me to look harder - there indeed is another connection to the gate that I missed. It comes from the control board, but I haven't been able to track it down more specifically yet. It looks like it may come from the micro computer chip, so I may be in trouble. I haven't got up the gumption to ground the gate again for fear I'll burn out something, but I guess that's what I'll have to do.

Thanks again. I'll open this back up once I have more info and/or questions.

Doug
 
Thanks guys for pushing me to look harder - there indeed is another connection to the gate that I missed. It comes from the control board, but I haven't been able to track it down more specifically yet. It looks like it may come from the micro computer chip, so I may be in trouble. I haven't got up the gumption to ground the gate again for fear I'll burn out something, but I guess that's what I'll have to do.

Thanks again. I'll open this back up once I have more info and/or questions.

Doug
Don't ground the gate. Ground the drain.
 
Are you able to monitor the gate voltage? If it changes (low to high) then the micro driving it is probably ok.
 
I think Roff meant ground the drain rather than the gate (which is already pulled to ground by 10k). However, if a micro is driving the gate at 4.1V then don't ground either without first disconnecting the micro from the gate, or you could be shorting the micro output to ground. The micro might not be happy!
 
I think Roff meant ground the drain rather than the gate (which is already pulled to ground by 10k). However, if a micro is driving the gate at 4.1V then don't ground either without first disconnecting the micro from the gate, or you could be shorting the micro output to ground. The micro might not be happy!
How could grounding the drain damage the micro? Or are you just concerned that he might accidentally ground the wrong pin?
 
How could grounding the drain damage the micro?
There could be a diode lurking somewhere between gate and drain, which would then pass current.
 
Ok, so scrub the hypothetical diode. I still think it would be safer to disconnect the micro if the circuit is causing the trip to operate.
 
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