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P Channel Mosfet Resistor Protection Issues

Leggyownz

Feb 7, 2017
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So I have a machine running a heater within it that hasn't been working. I've been tracing it down, through a relay, Hex Inverter, And Gate and finally back to this Mosfet. The Mosfet wasn't turning on. It constantly had 12v at the gate and 12v at the source. The gate is connected to a resistor that is connected to the source side. The gate also goes off to a connector that leads to another board. I assumed the power was coming from the other board but after removing the connector, the 12v still remained. I found it had to be coming from the resistor. So I removed the resistor and the heater suddenly works.

Resistor is a 680 Ohm resistor. 12v on both sides of it. The resistor reads around 690 ohms. I would prefer to add protection back into the circuit but I don't actually know what to do now. What would be causing the issue? The heater was working previously and then stopped working on me. The source side is connected directly to 12v power with nothing else in the way between it and the power supply other then of course traces and wiring. Site won't let me add photos currently. Will try to add them as a reply.. Datasheet of Mosfet below.

https://www.infineon.com/dgdl?folde...41454&fileId=db3a304325afd6e001264b5840530c71
 

Bluejets

Oct 5, 2014
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5,881
Google wants membership ......P Mosfet with positive rail voltage on the gate is off which is correct.
Remove the voltage and let it float could tend to let the mosfet come on which also appears correct.
Suggest you have other issues.
Do you have any details of the machine, circuit layout, drawings etc that might help.
Best to supply these instead of your own diagnosis of what is wrong.
 

Leggyownz

Feb 7, 2017
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My home computer allowed me to upload the pictures.
I work on laboratory equipment. Most of the times the manufacturer will turn us away on support, parts or anything else we ever need. I'm lucky to find a user manual most times let alone any sort of schematics Which I can say in 2 and half years, I've seen schematics twice. Of course those were the times I didn't even need them.
Hopefully the pictures can provide some information.
I can't wrap my head around any other issue, like I mentioned, the resistor goes straight to power on one side and the gate the other side. But the gate also branches off over to the second board through a series of cables and connections. When the heater gets to temp, it sends power to the gate, turning it off. Functions just fine without the resistor.
I'm missing something here which is why I turned to the forums for help.
 

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Bluejets

Oct 5, 2014
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the resistor goes straight to power on one side and the gate the other side. But the gate also branches off over to the second board through a series of cables and connections.

The resistor is there to stop the gate floating.
I'd suggest something wrong with the gate signal.

e.g.that electrolytic cap looks to be in a bad way.
 

Leggyownz

Feb 7, 2017
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Well in that case, where would the 12v the resistor is passing go to? Seems like it will always be present.

I think the cap looks that way just because of the way the lights are. It's been tested with a capacitance tester.
 
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Bluejets

Oct 5, 2014
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Well in that case, where would the 12v the resistor is passing go to? Seems like it will always be present.

Doesn't go anywhere.
There needs to be a potential difference for current to flow.
What it does is hold the gate at source level to stop the gate floating and turning on with noise etc.
 

Leggyownz

Feb 7, 2017
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But in order to have the negative potential needed to power the gate on, the 12v would have to drop below source. With the resistor allowing power across it, aren't both sides of the resistor going to always be the same, therefore never having the ability for potential difference?

I'm just trying to fully understand the situation but it's not making complete sense to me, so bear with me.
 

Bluejets

Oct 5, 2014
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5,881
Attached is a typical P mosfet arrangement.
 

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