10 ma to .01 ohm resolution DVM adapter

Dr.killjoy

Joined Apr 28, 2013
1,196
No, but there is a 1 amp driver in the Completed Projects if that will do your job better than this one.
I designed this for 10 ma so we could use (one) 9V battery. Good enough for LEDs and emitter resistors in power amplifiers. In my opinion, trying to measure tighter than 0.01 ohm is not necessary, but you can re-design it for 100 ma if you use bigger batteries.
Thanks for the update ...
I would love help redesign but I wouldn't know where to begin and wouldn't have come as great as yours ....
 

Thread Starter

#12

Joined Nov 30, 2010
18,223
First, pick a voltage that is large enough for any range you want to measure. Second, pick a chip that can handle 100 ma and the heat it will produce if the entire voltage source is heating it (the output gets shorted). Third, set up a resistance on the regulator that is 1/10 ohm per reference voltage of the regulator chip. 4th, add an adjustment to account for the tolerance in its reference voltage and the variable idle current the chip uses.

Some chips use 1.2 volts to 1.3 volts as their reference, so you would need 12 ohms to 13 ohms, except that some 50 microamps come out the reference pin of the regulator chip. If the chip is going to heat up much, you will want to use a separate amplifier like the 1 amp design.
 

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Dr.killjoy

Joined Apr 28, 2013
1,196
Well I am trying here ..
Well Bill Marsden talk about using a Lm117 and I know when I read the lm317 datasheet it had really tight tolerances.. I am going to start playing with one or trying something else too Thanks for the help..
 

k7elp60

Joined Nov 4, 2008
562
This is an interesting thread. I think the original design is great. I thought I would build one until I looked up the max6035 price and found it is a surface mount part. I used to be able to handle surface mount parts, but even the dip's and original sized parts are getting a little difficult to handle. In a couple of days I will have my 74th birthday.
A long time ago I purchased a Fluke 2012 because it had some low ohms reading. One is 2 ohms full scale and one is 20 ohms full scale and I could 0 out the resistance of the test leads. I purchased it because I was designing and building a lot of power supplies that used low value resistors for current limiting.
At present those 2 low ohm scales do not work correctly, that's why I was going to build the circuit in the first entry thread.
I have a Fluke 87V so I tried measuring some low value resistors....and it works great on low value resistors. It even measured the resistance of the test leads of 0.2 ohms.
I try to build small projects with parts I have on hand, so if I decide to build this circuit perhaps a TIL431 set to 2.5 volts might work, as I have about 1/2 dozen of them in stock.
Thank you all for the information
Ned
 

Thread Starter

#12

Joined Nov 30, 2010
18,223
Here's the math for an LM317K at 100 ma, but you're right on the hairy edge of having thermal drift problems, even with a TO-3 case. That's why the 1 amp design used a separate amplifier. It's a lot more stable.
 

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