I like to double check and select my 1 Ohm resistors for bias adjustment. Unfortunately even expensive Multimeter are not able to accurately measure small resistors. So I ended up to build a measurement device for small resistors. Idea is to use a adjustable voltage regulator LM317 I usually use for DC heater - in case I want to use DC for heater
With this regulator and a 1.25 Ohm resistor you can do a circuit that delivers exactly 1 Ampere (LM317 can deliver max 1.5 Ampere). Details can be found in the LM317 datasheet (https://www.ti.com/lit/ds/symlink/lm317.pdf) - the Precision Current-Limiter Circuit. For R1 use a 1.25 Ohm resistor to get 1Ampere of current. Between I(limit) and ground you can put the test object and in parallel your Multimeter and measure the Voltage across the test object. The value on the Multimeter in Millivolt represents the resistance of the resitor in Milliohm. As input voltage I use a wall wart that delivers 12V @ 3 Amps DC.
As you can see on the image I used a 100Ohm trim pot for calibration. I only had a 1.2Ohm resistor around so I measure a litte bit too high values at the moment. I will replace it with a 1.3Ohm resistor that should work fine.
I hope my explanation was not too confusing and this is of value for somebody. Feedback for improvements or errors are very welcome
Gerhard
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Very nice! I'd be surprised if you find resistors outside of their stated tolerance, but it would be interesting to know what effective resistance is seen when a 1 Ohm current sense resistor is wired into the cathode circuit. Have you tried measuring across the bias test jacks in an amp?
I didn't try measuring across the bias test jacks yet but I will definitely do on my next build. I am not so worried about the difference to 1 Ohm. My goal is to see same value on all 4 test jacks even if they are not exactly 1 Ohm.
At the moment I have slightly more than 1A in the meter so I need to change to a 1.3 Ohm resistor and calibrate with the trim pot. Than I will take a closer look to the repeatability.
The 1.25R resistor likely needs some tweaking and good 317 heatsinking to get better than 1.00A +/-1% due to line regulation, load regulation, and thermal regulation. I had a LM317 set for 0.1A that gave 0.05% variation with time after initial accuracy tweaking. Obviously keeping voltage drop across the 317 to a minimum, and maybe even tweaking the sense resistor tempco, and do some part swapping, may achieve some benefit. But of course you would need a way to measure 1.00A accurately. And then there is the voltage measurement accuracy across the 1 ohm resistors, plus there is the issue of the tempco of the 1 ohm resistors that you are trying to match given they are dissipating 1W.
Given all the measurement uncertainty, and even if the aim is for relative comparison, I can't really see you could get better than the +/-1% tolerance of the resistors you are trying to match (assuming you are using 1% tolerance resistors).
