Electronics > Beginners

Trying to determine what voltages should be in a circuit

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fourfathom:

--- Quote from: edavid on September 26, 2019, 11:48:17 pm ---The voltages shown are not possible.  There must be a wiring problem.  Maybe the zener cathode is wired to the wrong place.

--- End quote ---

The OP had substituted an N-channel fet for the P-channel.  That's the reason for the strange voltages.  "Audioguru again" spotted it.

edavid:

--- Quote from: fourfathom on September 27, 2019, 12:14:39 am ---
--- Quote from: edavid on September 26, 2019, 11:48:17 pm ---The voltages shown are not possible.  There must be a wiring problem.  Maybe the zener cathode is wired to the wrong place.

--- End quote ---

The OP had substituted an N-channel fet for the P-channel.  That's the reason for the strange voltages.  "Audioguru again" spotted it.

--- End quote ---

The MOSFET type is irrelevant (well, not to the circuit actually working, but to the gate resistor heating up).

The only conduction path from the gate resistor to ground would be through the MOSFET gate and the FR107 (both would have to be blown or miswired).

However, that's not possible, because the MOSFET drain is at 23.7V.

Therefore we know that either the schematic or the voltages are wrong (or maybe both).

fourfathom:

--- Quote from: edavid on September 27, 2019, 12:50:56 am ---The MOSFET type is irrelevant (well, not to the circuit actually working, but to the gate resistor heating up).
--- End quote ---

Oops, I was looking at the drain-source diode but you're right, that's not going to conduct to the gate.  What *does* happen if you exceed the gate-source breakdown voltage?  Does the gate start conducting to the channel?  Is this permanent, or potentially recoverable?

So I finally looked at the voltages noted on the schematic, and yes, there is no current path that would bring the resistor/gate connection down to 12.8V.  Given that, we can't trust that the schematic, and/or the voltages are correct.  The 100 Ohm resistor is getting hot so we know there is indeed current flowing through it, but where is it going?  We can't tell from what we have in front of us.

gmc:
Thanks for all the feedback. I've ordered the correct mosfet and will replace and re-measure all the voltages and re-check all the wiring.

I see a few people have build this circuit without issues so I'm hoping the circuit is correct. Msybe my dodgy wiring. I'll triple check everything again.

gmc:
Problem solved.  :-[

Traced everything again. Lesson learned. Trace where things should be going, also where they  should not be going. I built the circuit on the stripboard and missed in cutting one of the tracks so there was a connection on the fet going where it wasn't supposed to. 

Thanks again for the pointers. When I get the proper fet I'm sure it will all work.  I'll retest all the components as well, hopefully it didn't blow anything.

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