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Old 16th September 2014, 09:24 AM   #1001
AndrewT is offline AndrewT  Scotland
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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%).
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Last edited by AndrewT; 16th September 2014 at 09:33 AM.
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Old 16th September 2014, 10:34 AM   #1002
Cambe is offline Cambe
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Thank you, Alex.
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Old 16th September 2014, 02:40 PM   #1003
Bonsai is offline Bonsai  Europe
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Very nice. Thanks for that Alex.
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Old 16th September 2014, 03:23 PM   #1004
still4given is offline still4given  United States
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Quote:
Originally Posted by alex mm View Post
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.
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Old 16th September 2014, 04:48 PM   #1005
still4given is offline still4given  United States
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Quote:
Originally Posted by AndrewT View Post
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
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Old 16th September 2014, 04:52 PM   #1006
AndrewT is offline AndrewT  Scotland
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a string of resistor all in series.
Attached Images
File Type: png low value resistor measurement.png (857.4 KB, 569 views)
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Last edited by AndrewT; 16th September 2014 at 05:06 PM.
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Old 16th September 2014, 05:04 PM   #1007
still4given is offline still4given  United States
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I understand that. How do you set the 100mAdc?

Also, any issue using BD139 in place of BC547 for the VBE multiplier?
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Old 16th September 2014, 05:08 PM   #1008
AndrewT is offline AndrewT  Scotland
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You don't set the 100mAdc.
Quote:
Measure the voltage across the 1% 0r5 to find the TEST current.
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Old 16th September 2014, 05:09 PM   #1009
AndrewT is offline AndrewT  Scotland
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I never use a To126 for Vbe multiplier.
To92 or sot23
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Old 16th September 2014, 05:14 PM   #1010
still4given is offline still4given  United States
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Quote:
Originally Posted by AndrewT View Post
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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