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Old 26th June 2011, 09:37 AM   #1
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Default Practical Mosfet testing

Hi Friends,

I refer to the Papa article "Practical Mosfet Testing for Audio" where he presents this diagram for testing theses devices:

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I would just like to know:
-what is the recommended wattage for the resistors ?
-same for the V+ voltage (approx) ?

What kind of regulated power supply do you use ?..

Thanks,
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Old 26th June 2011, 10:59 AM   #2
AndrewT is offline AndrewT  Scotland
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Calculate the dissipation for each resistor.
Then use a resistor rating that is at least double the required dissipation. If possible use 10times the dissipation for minimal resistance change.
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Old 26th June 2011, 12:47 PM   #3
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Thanks Andrew, but if I were able to calculate this, I would not have asked the question ...
(I just begin building power amps, my experience/expertise relates only to other kind of devices (DAC, etc), so I am an idiot here ...)
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Old 26th June 2011, 04:04 PM   #4
AndrewT is offline AndrewT  Scotland
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Power dissipation = Volts times Amperes = Amperes squared * Resistance = Volts squared divided by Resistance.

P = V*I = I*I*R = V*V/R
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Old 26th June 2011, 04:34 PM   #5
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Assume that the MOS-FET has failed and that it is a short circuit.

The variable voltage supply can be almost anything +24V is a good place to start.

R3, R4. R5, R6. R7. R8 are then directly connected across the 24V PSU.

From that you can use Andrew T's calculations to work out current and power ratings.

WORST CASE R3 or R6 are in circuit with the other side SHORT CIRCUIT.

V**2 / R = 24 x 24 / 10R = 57W.

I've got 10R 100W resistors for sale on E-Bay at 99p each at the moment.

Last edited by Andy5112405; 26th June 2011 at 04:39 PM.
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Old 27th June 2011, 05:45 PM   #6
h_a is offline h_a  Europe
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Are you sure you need the full circuit? I mean it provides a lot of functionality and for example its possibility to test for transconductance might not be applicable to you as you would need a signal generator and an oscilloscope.

Hannes
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Old 9th July 2011, 07:52 PM   #7
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Thanks Andy, for the reply !

Quote:
Are you sure you need the full circuit? I mean it provides a lot of functionality and for example its possibility to test for transconductance might not be applicable to you as you would need a signal generator and an oscilloscope.
I have a signal generator and an oscilloscope , (you can laugh: i have received them last week and they are still in their box ) but i am not sure I will conduct all these tests in the near future, as I just start my F5 assembly...
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Old 10th July 2011, 04:29 AM   #8
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Lol, I also got me a new scope and sig gen, so far all I have done is found the signals > Thank god for autoset.
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Old 10th July 2011, 08:25 AM   #9
h_a is offline h_a  Europe
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That's nice to hear! Having some equipment really helps in tuning amps!

Enjoy! Hannes
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