Your latest schematic has a 10k resistor for the load, not a light bulb.The attached drawing is exactly what I had, the switch was a mini clip and the load was a test light which draws about 150 ma.
Where did you connect the light bulb?
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Your latest schematic has a 10k resistor for the load, not a light bulb.The attached drawing is exactly what I had, the switch was a mini clip and the load was a test light which draws about 150 ma.
Sorry, I got distracted, grand kids spent the night, I meant to change it 100 ohm and I did on an edit.Your latest schematic has a 10k resistor for the load, not a light bulb.
Where did you connect the light bulb?
I did build it on a push the lead in the hole type bread board with full length lead on everything and the FET was about 10" from the board and another 10 or so inches from the the load,( sometime the head lamp out of a car, some time a 5, 2, or 1 ohm 50watt resistor or a combination of the 3, I even tried the motor with a 100 ohm 1/4 watt .68uf snubber and even let some smoke out of the resistor, because of the high freq, I assume.)The schematic in post #28 is missing very important supply bypass capacitors. Without them the loaded Mosfets can oscillate.
If you built the circuit on a breadboard then it will certainly oscillate.
There is your oscillator. The stray capacitance and inductance are far too high. Supply bypass capacitors might help.I did build it on a push the lead in the hole type bread board with full length lead on everything and the FET was about 10" from the board and another 10 or so inches from the the load
ROFF asked the question, but I am also eager to see a published example of this mysterious phenomenon.getting back to what was happening to the FET with the switch for a moment, i think the parameter you exceeded was the max dv/dt figure for the gate. in the data sheet it's most likely listed in volts/microsecond. exceeding the max dv/dt for the gate will cause the gate insulation to break down, even if you don't exceed the Vgs of the FET.
Thanks for bumping this question. I hate it when you challenge someone and they just totally ignore it.ROFF asked the question, but I am also eager to see a published example of this mysterious phenomenon.
Thanks for coming back on this issue, unclejed613.i think you're right, i misread the data sheet. i saw the dv/dt listed on the line just above the gate charge spec.
mental note to self: a new pair of reading glasses is in order
Thanks for bumping this question. I hate it when you challenge someone and they just totally ignore it.
MOSFETs are fragile if not applied correctly. When a novice kills a MOSFET, the last failure mode I would consider is one that I cannot find documented anywhere.
Static electricity may have killed your transistor.
If you didn't have a diode across an inductive load, flyback voltage could have killed it.
If you had the source and drain swapped, that could have killed it.
Roff, mneary, please reread my post, especially the last part. Please build circuit and test and report back.The attached drawing is exactly what I had, the switch was a mini clip and the load was a test light which draws about 150 ma. If you build the circuit using a 100 ohm resistor, and use an on/off switch or just touch wires together, you will prove a point, I don't know which point, but if the FET dies, that's one point, if the FET lives and works, that's the other point.
This is my guess as to what happens, with the switch open, a reverse charge builds up on the gate as shown in the attached drawing, maybe there should be a bunch more + in other areas, I DON'T KNOW, then when the switch is closed and the gate charge is drained off and the polarity of the gate charge is reversed, the fragile insulation is damaged because it happened so fast and the FET dies.
This is purely a guess, and I have only my own reasoning to pull from.
I don't want to throw challenges out, but if you are correct. building and trying the circuit can't hurt.
Kinarfi
What is it that I need to bypass? Because my power supply is limited to 5.7 amps and I'm working toward driving a motor that can pull upwards of 50 amps and more if I could stall it, I have a really big gel cel sitting next to my breadboard with #16 wire from battery to the board and the power supply keeps the battery at 14 as if in the vehicle with the engine running. The circuit works great with a p FET, and I plan to play with it today. Expect more scope picts If I wasn't ashamed of my messy bench, I'd even throw in a photo of the bread boarded circuitYour problem is that the circuit is built on a breadboard and the supply bypass capacitors are missing.
What is it that I need to bypass? Because my power supply is limited to 5.7 amps and I'm working toward driving a motor that can pull upwards of 50 amps and more if I could stall it, I have a really big gel cel sitting next to my breadboard with #16 wire from battery to the board and the power supply keeps the battery at 14 as if in the vehicle with the engine running. The circuit works great with a p FET, and I plan to play with it today. Expect more scope picts If I wasn't ashamed of my messy bench, I'd even throw in a photo of the bread boarded circuit
Thanks
Kinarfi