Re: PMOS in parallel with NMOS



Jim Granville schrieb:

Excessive current destroys it merely because of the excessive heat
that the current flow produces. It's that simple.

WRONG.
A little knowledge is a dangerous thing.

Google surge and resistor, and you might learn something.
Failure modes are anything but simple.

A little story about that:

I've fixed my guitar amp a year ago. There was a huge chasis mount 10W 10K resistor running at a constant voltage of roughly 370V.

My first thought was just a big WTF? Why would someone puts such a overrated resistor into a commercial product. The resistor was expensive and I bet the labour work for mouting the resistor and hand-soldering the cables wasn't cheap either.

It turned out that this very resistor burned out. More than 10 times overrated but still it turned into a piece of ash. I traced it back to some arcing at the tube-sockets. That - together with the inductivity of the power-transformer - caused spikes that burned away the resistive wire inside the heatsink.

I know that this is a completely different world for those who deal with 3.3 or 5V supplies. Arcing is more common in these devices than you may think. It happeds often if you cut-off the power supply or do other nasty things. Overall it tought me a lesson:

Your job is not done if you put voltage, resistance and current into the equation and find out that you don't exceed the power-rating in the *usual* case. You have to take the corner-cases into account as well.

In my tube-amp I measured thousands of voltages comming out from the transformers in the switch-off moments. I did some math and research (inductance is crazy!), found out that this had to be expected in the case and then understood why the original designer used a 10W resistor at that place.

Crazy stuff, these tubes. So simple and elegant on the one side, but so difficult to make a reliable product on the other.

Nils

Btw, comment to the OT: Every minute you torture an innocent tranny, microcontroller output-pin or LED god kills a kitten :-)
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