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Old 6th May 2006, 08:08 PM   #1
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Default how to measure and match source resistors 0,22-1ohm?

sorry for starting new thread, but I found lot info about how to measure and mach mosfets but how to measure and match resistors. throught multimeter can not measure 0,22-1ohm precisely.
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Old 6th May 2006, 09:16 PM   #2
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I think you need an ESR meter.
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Old 6th May 2006, 09:29 PM   #3
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what is esr meter? my multimeter measures Volts,ampers, inductivity, capacity,,temperature, diode mode, hfe mode.

somehow I thought that I need to build some power suply to hook there resitors and measure it across..
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Old 6th May 2006, 09:30 PM   #4
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A practical way would be to connect in series a higher value resistor like 10R or 100R that your multimeter can read. Make sure you measure this resistor first. Your meter should be able to read 2 to 3 decimal places.
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Old 6th May 2006, 09:33 PM   #5
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Build a current source, somewhere between 100ma and 1a. Then just measure the voltage across the resistor (~22mv-1V, depending on resistor value and the current you chose), plop the values into ohm's law and...

You don't even need to calibrate the current source or use ohm's law to find the actual value, if you're just matching emitter resistors. Just match voltages.
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Old 6th May 2006, 09:36 PM   #6
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thanks for quick reply, could You post simplified schematic for this current source, I have no imagination what it is. some dc voltage shortened with this resistor or not?
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Old 6th May 2006, 09:49 PM   #7
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Q1 can be any medium/high power transistor. Q2 can be almost anything. R1 should be between 0.1 and 10. R2 should be Vcc/((0.65/R1)/Hfe'q1)*2, or around 10 to 100 times larger.

P.S. Sorry about the image quality, I compressed it rather quickly without thinking and compressed it into nothing (20kb from 420kb).
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Old 6th May 2006, 09:49 PM   #8
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How to measure resistors below 1R
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Old 6th May 2006, 10:04 PM   #9
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ok Tim thanks I think I will not blown more transitors with this circuit.its enought at this month.
am I too naive to thinking this way- I solder input terminals for source resitor can be solder/desolder without dismounting pcb from heatsinks. I just turn on amp after 10 sec measure voltage arround this resistor and write value(lets say 0,4-0,5V) and then try to match pairs . this would be pretty ok?
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Old 7th May 2006, 12:45 AM   #10
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Quote:
Originally posted by elviukai
what is esr meter?
esr = equivalent series resistance

An ESR meter is used to measure the ESR of an electrolytic capacitor, but it also can be used to measure low valued resistors, typically in the 0.01 to 99 ohms range. I use mine for this purpose and it seems to be accurate.

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