Mosfet Issue - full auto on power up

[venture]

I am not really experienced with MOSFETS. I installed one in a gun and it shoots full auto as soon as I plug in the battery. Does anyone recognize this symptom?

[Kos-Mos]

Yes. The actual FET is burnt.

If you can replace the FET chip, the rest should be fine.

[venture]

So it is a defective FET?
It is just a picossr so the whole thing should be replaced if it is bad.

[ThunderCactus]

There could also be a short in the trigger wires

[HKGhost]

I'd say burnt mosfet. I know because back when I use to make custom ones, if it goes full auto when the battery is plugged in, chip is bad. Could have happened when you were soldering it.

[Kos-Mos]

Well the PICO is way under-rated for the use intended. I personally don't think it's powerful enough. SO8 is too small of a package to drive anything over 10A, cannot dissipate the heat.
The custom ones I make use a D2PAK with a 5W zener to protect it. It's the size of a quarter, but bullet-proof.

[venture]

Originally Posted by ThunderCactus

There could also be a short in the trigger wires

I checked trigger functionality with a ohmeter before soldering the fet onto the wiring.

Can the heat of soldering damage the fet?

My wiring is replacement wiring method:

Positive goes straight to motor from battery. one Fet lead goes from trigger contact to positive battery terminal. Other Fet lead goes from other trigger contact to the fet. Negative wire goes straight from battery to motor but is cut and soldered to the underside of each end of the fet.

[Kos-Mos]

If you mixed the two blakc/negatives, yet it will fry it.
And heat can cause damage, but the culprit might be ESD if it was a new FET. Before being installed to something, the FETs are really sensitive to magnetic fields and static discharge, so if you put it close/against a strong motor, it might kill it.

[HKGhost]

Yes. The soldering heat can damage the mosfet. If you leave it on for more then 10-15 second, that could be enough to fry the unit. If you followed the instruction perfectly, then the unit is bad if you're getting auto firing when battery is plugged in.

[venture]

Originally Posted by HKGhost

Yes. The soldering heat can damage the mosfet. If you leave it on for more then 10-15 second, that could be enough to fry the unit. If you followed the instruction perfectly, then the unit is bad if you're getting auto firing when battery is plugged in.

Thanks for the input. I think I did the install correctly (as described above). It must be a bad fet.

[ManateeMatt]

Or its in backwards. IE you have the motor side connected to the battery side. I did the same thing with a Pico. Make sure the the side with the trigger wire is connected to the battery, and the other side is connected to motor. I spent a long time yelling at one. It didn't occur to me that it made a difference, but then I remembered my schooling from many years ago.

My wonderful paint image

[venture]

Originally Posted by ManateeMatt

Or its in backwards. IE you have the motor side connected to the battery side. I did the same thing with a Pico. Make sure the the side with the trigger wire is connected to the battery, and the other side is connected to motor. I spent a long time yelling at one.

Thanks, Matt. I will check that. Was that the same symptom as your problem - full auto on power up?

[ManateeMatt]

Yup. Exact symptoms.

[venture]

Just checked it and the negative wire is, indeed, the opposite as to what the diagram in the manual shows. I did not think it would matter, but there you have it. THANK YOU, Matt!

I won't know for sure until I desolder and resolder it correctly, but you called the backwards part.

[venture]

Desoldered and resoldered the negative wire on the correct terminals. Works fine now.
Thanks again, Matt. Good call.