Mosfet Testing -Wall wart ok?

[george a]

Hi All
I need to test / match some Irfp240 mosfets.
I have found some stuff on the net and a diagram as to how to wire up the test setup.

see attached
Question - Can i use a 15V 15VA wall wart instead of the 22v?
and if so - what size resistor needs to be used from V+ to the Drain?

Thanks in advance
Cheers

 

[FoMoCo]

For the first circuit: 10 Ohm 20W
For the second circuit: 1.1K 0.5W
That's if the currents given are correct.

How I figured that out:
1.125A times 16 Ohm drops 18V.
That leaves 4V (22-18) across the FET.
If we use a 15V supply and still want 4V across the FET, the resistor must drop (15-4) 11 volts.
11V divided by 1.125A is 9.78 ohms. Use 10 Ohm.

Just a simple application of Ohm's Law.

 

[george a]

Many Many thanks FoMoCo

This should make life a little easier .
I was concerned as to the suitability in terms of quality of the DC output from a wall wart.
Cheers

 

[FoMoCo]

No problem.

Unless the wall-wart has a ton of ripple or noise there's no need to be concerned for a simple DC biasing test. It'll be fine.

However, I did notice that the schematic is not for the IRFP240, you'll probably need to calculate new resistor values based on the curves in the data sheet. Barring that, just start with a high value resistor and reduce the value until you see a combination of current and voltage you want to match for.

 

[AndrewT]

Geo,
do you realise that the test jig in post one allows you to measure Vgs when Vdg=zero?

This is a very rare condition in a working amplifier. It does not model actual mosFET behaviour well.

But in your favour it is better than not measuring at all.