In my experiments with mosfets and high voltages, i need some kind of control over the current that mosfet takes from the battery during switching. I usally put a needle amber-meter in series between load inductor and battery. What i 'd like to confirm is that in this way i measure the mosfet's pulsed current. For example, IRFP460 can handle up tp 80A pulsed current. Is it safe to read an indication of i.e 50A? Is there any other way of measuring this without affecting the current passage?
Tnks
Jeg
Quote from: Jeg on November 21, 2013, 06:05:08 AM
In my experiments with mosfets and high voltages, i need some kind of control over the current that mosfet takes from the battery during switching. I usally put a needle amber-meter in series between load inductor and battery. What i 'd like to confirm is that in this way i measure the mosfet's pulsed current. For example, IRFP460 can handle up tp 80A pulsed current. Is it safe to read an indication of i.e 50A? Is there any other way of measuring this without affecting the current passage?
Tnks
Jeg
Have you measured the series resistance of your needle ampmeter? Mine have DC resistances that are not negligible, even as high as 10 ohms on some scales.
Also, the response time of these meters is slow, so if your pulses are short you may not be getting an accurate indication of peak values but rather may be reading an average value.
The proper way is to use a small current-viewing resistor in series with your mosfet circuit, and to look at the voltage drop across that resistor with an oscilloscope. If your pulses are long enough you may be able to do this with a DMM instead: put a 0.1 ohm resistor between the mosfet source and the negative supply rail, and use your DMM on the voltmeter scale to look at the voltage across this resistor, then calculate the current based on Ohm's Law. Current in Amperes = Voltage across resistor / resistor value in Ohms.
The mosfet data sheet will give the maximum time of the high current pulse it is rated for. All the mosfet ratings assume that you have it mounted correctly on an appropriate heatsink.
Tnks Tins.
I have to say that i couldn't imagine meter's resistance is so high. 418 OHM!!! It was a load itself for my mosfet. I made an other resistance of 10.45 milli Ohms with one meter of AWG15 wire and i saw for a first time how current waveforms look like, by connecting it as you said. I 'll try to find a 0.1 Ohm, because 10.45mOhm is very small and near to the value of solder's paste resistance. This gives me an inaccuracy of many amber.
Jeg
That's my problem too >:( It's not easy to predict what reactance (plus resistance) mosfet would see. I can measure inductance quite easily but not resistance of wire if below 10 ohms. I think the best is to have a bit bigger inductance so the resitance is smaller by some factor then reactance for given frequency of switching...
We need a lot more of helpful tips from TinselKoala to avoid damaging precious mosfets. Mine mosfet can switch in impulse some 200A but that is not very big value unfortunately....
For solace be sure Tesla had the same problems.... plenty of his patents are for circuit breaker...