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Old 2nd August 2005, 08:20 PM   #11
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Quote:
Originally posted by steenoe
2,2k is way too much resistance! You need like 10r for the irfp240 and something like 560r for the irf610 (or9610)!! I remember NP said that they just measured long enough to get a reading! I normally "Juice" them for 30 sec's. The resistor needs to be at least 10 watts. And a steady supply at that!

Steen

BTW 15v for the irfp240 and 10v for the irf9610!!!
Take a closer look at the link DW gave!!
2.2k is not good for 9610. that was what Nelson said.
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Old 2nd August 2005, 08:41 PM   #12
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Use whatever voltage you have handy, then subtract 3.5V for mosfet vgs, then use ohms law to figure the resistance to get the current you want to match for. Match the fets at a current close to where you plan to run them.

Think about it and don't just mindlessly follow someones recomendations.
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Old 2nd August 2005, 09:12 PM   #13
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Thanks Brian, you' re right.
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Old 2nd August 2005, 11:30 PM   #14
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When possible, I test the devices with the same voltage and current that they will see in the actual circuit. That said, there doesn't seem to be too much shuffling going on when I do the 15V thing. Sometimes devices will shift (relatively speaking) one or two places compared to their fellows, but not a lot.

Grey
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Old 4th August 2005, 08:21 PM   #15
Eric is offline Eric  United States
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Not to say that my way is best, but I just finished matching some IRF9610's for my Aleph-X. I have a detailed web page on my progress so far that you can find here

Scroll down to the section on Matching Components.

Since I'm still a relative beginner, I'm trying to document my project and methods as thoroughly as possible so that others can easily follow it.

Eric
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Old 4th December 2005, 09:02 PM   #16
Dr.Gone is offline Dr.Gone  Netherlands
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Default other matching idea

Hello everybody,

Today I started matching mosfets for my own little aleph-x, but I wanted to be able to match the components at a higher precission than 10 mV. Unfortunatly I have no acces to highend equipment so my standard multimeter will have to suffice.

The idea is as follows:

Instead of measuring the Vgs of 1 mosfet, I use one fixed mosfet and measure the difference in Vgs between this mosfet and the test mosfet (running parallel).

The result is that you can measure much more precisely using the standard voltmeter (because you can set the precission of the meter to millivolts instead of volts). I'll use a heatsink on the "fixed" mosfet to minimize the diference in temperature. But now my real question: are there any disadvantages of using this method?

See attachment for schematic.

Regards,
Jacco
Attached Images
File Type: gif matching.gif (2.6 KB, 1406 views)
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Old 5th December 2005, 02:18 AM   #17
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Not a bad idea. Just make sure to let your "control" fet run for a few hours before your test so it can reach equilibrium tem, and make sure there is no AC/Heater vent blowing or anything to cause a draft. Any variation in temp causes a big change in readings.

Also make sure the test fets are all at room equilibrium temp and you keep them in the test the same amout of time because the testing will heat them and change their Vgs.

You're setup should also help make PSU instabilities less of an issue.
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Old 5th December 2005, 02:49 AM   #18
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forget about the resistors -- you can use an LM317 with a 2.5 ohm resistor between the adjust pin and output. this will current limit to 0.5 A in a classic current limiter setup. I use a TEK PS5010 but the LM317 will work in a pinch.

it's important to measure all your devices at the same ambient temperature and for the same period of time.
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Old 5th December 2005, 02:57 AM   #19
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And if you use resistors, make sure they are way over rated, or they'll get hot and the resistance may not remain constant.
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Old 5th December 2005, 08:10 AM   #20
Dr.Gone is offline Dr.Gone  Netherlands
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Quote:
Originally posted by Brian Donaldson
And if you use resistors, make sure they are way over rated, or they'll get hot and the resistance may not remain constant.
Thanx, I will keep that in mind.

Regards,
Jacco
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