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Old 8th June 2002, 10:05 AM   #11
UrSv is offline UrSv  Sweden
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Since you have fairly low restistance it is better to measure the voltage across the resistor rather than the resistance itself (for us folks that do not have super gear). If you can measure current and/or voltage accurately then that works just fine but if you can't then you can match using a bridge set-up like a wheatstone bridge. Simply put you make two voltage dividers by connecting two resistors in series in parallell with another pair in series. Swap around your resistors until you find a pair with a reading of 0 volts between the voltage divider outputs. When that happens you have two dividers with the exact same ratios but not necessarily the same values. By swapping one the lower end of on of your dividers with the other lower end you can find a set which has similar restistance as they would not be producing a 0 reading unless they were the same resistance. Swap away and find the lowest readings (I remember something about easy matching to 0.1 % doing this but I could be wrong). You have no idea what the value is (which is irrelevant as you know it close to the marked value but you just wanted to match)

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Old 8th June 2002, 08:45 PM   #12
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Hello Edwin,

I wouldn´t worry about the small differences in bias current. I´ve got even bigger differences in my Aleph5 (matched for 5 minutes to 0,1V). As long as not one Mosfet does most of the work this can´t be dangerous and you´ve only got 5% of mismatch!!

groetjes,

william
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Old 10th June 2002, 08:00 AM   #13
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Hello,

some info for all you matcher out there:

I did first the 10 second test, so I read the VGS after 10 seconds of running current through them. From these readings I made 4 sets of 6 IRFP240 (all within 0.01) and 2 sets of 9610 (within 0.001).

I re-tested 2 sets of IRFP240 and 2 sets of 9610's with the 5 minute test to see how this turns out:

5 minutes:

3.91
3.90
3.90
3.89
3.90
3.90

and

3.871
3.879
3.877
3.874
3.885
3.884

and

3.821
3.823

and

3.803
3.803

So pretty close to the 10 seconds matching ... So perhaps Nelson is right after all

I will build the second channel with these mosfets so I will let you know. I am testing my resistors tonight.

Edwin
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Old 11th June 2002, 06:27 PM   #14
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Second channel does work also !!! I still have a minor bias difference between mosfet's however I did not match the resistors. My DC offset is very low: 0.012V . So I am going to put another matched IRF9610 pair in the first channel to see if this helps!

Edwin

P.S. YEAH!!!!!!!!!!!
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Old 11th June 2002, 09:22 PM   #15
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Quote:
Originally posted by Edwin Dorre
Second channel does work also !!! I still have a minor bias difference between mosfet's however I did not match the resistors. My DC offset is very low: 0.012V . So I am going to put another matched IRF9610 pair in the first channel to see if this helps!
Just changed the IRF9610's in the first channel....... A DC offset of 0.007V it is a miracle!

Edwin
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