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AndrewT
R.I.P.

Join Date: Jul 2004
Location: Scottish Borders
Quote:
 It is very difficult to get an accurate measurement of such low value resistors.
Get a couple (or more) of 1% 1r0 and use them to "define" the current passing. This is REF = reference resistor.
Place some DUT resistors in series and feed them from the 1r0||1r0 resistor pair. DUT = Device Under Test.
Pass ~100mAdc through the long string of resistors.
Measure the voltage across the 1% 0r5 to find the TEST current. Pq for REF is 5mW (2.5mW each)

Measure the voltage across each DUT.
Now you can calculate the DUT resistance to <2% error. Mark each DUT with new value.

You can use this METHOD with a more accurate REF and get to within +-0.1% for <1r resistors, (if the DMM voltage across REF ~= to the DUT voltage then DMM errors are almost eliminated, i.e. use three 1r0 0.5% to get your results to +-0.7%).
__________________
regards Andrew T.

Last edited by AndrewT; 16th September 2014 at 09:33 AM.

 16th September 2014, 10:34 AM #1002 Cambe diyAudio Member   Join Date: Aug 2001 Location: USA,NorthCarolina Thank you, Alex.
 16th September 2014, 02:40 PM #1003 Bonsai   diyAudio Member     Join Date: Jul 2003 Location: Europe Very nice. Thanks for that Alex. __________________ bonsai https://www.ovationhifidelity.com/ and DIY here http://hifisonix.com/ Ground Loops http://hifisonix.com/ground-loops/
still4given
diyAudio Member

Join Date: Oct 2004
Location: Victorville, CA
Quote:
 Originally Posted by alex mm Thanks , a lot ,also I suggest you to mount Q3 under PCB in contact with heatsink ...... Regards,Alex
That sounds difficult to do. I might try changing it to a TO-225 and mounting it on top of one of the outputs.

still4given
diyAudio Member

Join Date: Oct 2004
Location: Victorville, CA
Quote:
 Originally Posted by AndrewT Get a couple (or more) of 1% 1r0 and use them to "define" the current passing. This is REF = reference resistor. Place some DUT resistors in series and feed them from the 1r0||1r0 resistor pair. DUT = Device Under Test. Pass ~100mAdc through the long string of resistors. Measure the voltage across the 1% 0r5 to find the TEST current. Pq for REF is 5mW (2.5mW each) Measure the voltage across each DUT. Now you can calculate the DUT resistance to <2% error. Mark each DUT with new value. You can use this METHOD with a more accurate REF and get to within +-0.1% for <1r resistors, (if the DMM voltage across REF ~= to the DUT voltage then DMM errors are almost eliminated, i.e. use three 1r0 0.5% to get your results to +-0.7%).
Can you draw up a little schematic for that? I'm having trouble forming a picture.

Thanks, Terry

AndrewT
R.I.P.

Join Date: Jul 2004
Location: Scottish Borders
a string of resistor all in series.
Attached Images
 low value resistor measurement.png (857.4 KB, 569 views)
__________________
regards Andrew T.

Last edited by AndrewT; 16th September 2014 at 05:06 PM.

 16th September 2014, 05:04 PM #1007 still4given   diyAudio Member     Join Date: Oct 2004 Location: Victorville, CA I understand that. How do you set the 100mAdc? Also, any issue using BD139 in place of BC547 for the VBE multiplier?
AndrewT
R.I.P.

Join Date: Jul 2004
Location: Scottish Borders
Quote:
 Measure the voltage across the 1% 0r5 to find the TEST current.
__________________
regards Andrew T.

 16th September 2014, 05:09 PM #1009 AndrewT   R.I.P.   Join Date: Jul 2004 Location: Scottish Borders I never use a To126 for Vbe multiplier. To92 or sot23 __________________ regards Andrew T.
still4given
diyAudio Member

Join Date: Oct 2004
Location: Victorville, CA
Quote:
 Originally Posted by AndrewT You don't set the 100mAdc.
You said "Pass ~100mAdc through the long string of resistors."

That was confusing.

Quote:
 I never use a To126 for Vbe multiplier. To92 or sot23
On what amps did you do this and how did you attach them to the heatsink or outputs so they could help control thermal runaway?

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