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blakktalon 10th December 2010 03:03 PM

home made low resistance tester, for under 10 bucks.
If there are any of you on a budjet, and cannot afford the 4 wire dmm. Here is a pretty simple jig for testing low resistances. I was using it for .1ohm, and .22 ohm resistors in car amplifiers.

If you have a plug in power supply that puts out 4 to 6 volts it works better than the three battery method. I used batterys, and got one good reading and then started getting strange ones when the battery dropped below 4 v.

here is the first link for the project.
Simple Low Resistance Measurement | MediaDir Electronics Projects Blog

This one looks a bit better and easier to use.

c2cthomas 10th December 2010 03:29 PM

Thanks for the links blakktalon - the second one buy Bob Nuckolls looks very nice.

Once upon a time I was a depot level (II) repair and cal tech and performed the repair and calibration of 4 wire resistance meters - dmm's - etc. and when you get things down to measuring 0.1 ohms life can get tricky. Keep in mind that when you are calibrating test equipment that the standard you are using must be at least a power of 10 more accurate than the unit being calibrated. Some of the stuff we were calibrating was used in labs so the standards we were using were pretty far out there. With some of the high accuracy meters the resistance standards were in a metal tank to shield from EMF/RFI and immersed in a thermally stabilized oil bath. Lead length and capacitance can be important and any more than a couple of feet can get nasty because of the lead resistance (which needs to be nulled out and balanced if you want to get waaaaay out there in accuracy). If you are only going out .1 this stuff should be OK tho.

BTW - cute doggy in the avatar!

blakktalon 10th December 2010 04:28 PM

The second one looks the best for sure, I am gonna prolly make one more like the Bob Nuckolls one. mine is just on a piece of bread board with some smd resistors. Pretty ugly.

I am using a fluke 112 ebay special. The .22 ohm resistors, were reading .4 and subtracting the .2 ohm for the test leads. The tolerance at 5% for the resistors is .209 to .231 I was wondering if that mere 9thousandth was gonna lead to catastrophic failure.

c2cthomas 10th December 2010 05:04 PM


Originally Posted by blakktalon (Post 2393774)
The tolerance at 5% for the resistors is .209 to .231 I was wondering if that mere 9thousandth was gonna lead to catastrophic failure.

:rofl::rofl::rofl::rofl::rofl: Ohhhhhhhh fret and worry!!! That's very much like wondering if a brick in the road will slow down a M1 Abrams tank. OTOH - if that brick is made outta an explosive like C4 then all bets are off.
I think you have a good hand on things - and there comes a time when good enough is good enough. ;)

That 9 thousandths could be caused by a cool breeze - or just the resistor heating up while under test - or basic variation between readings.

AndrewT 10th December 2010 05:25 PM

using a constant known current for resistance measurement is very good and very simple.

You can extend that CCS measuring technique by stringing together many resistors for test. Lets say you have 20 off 0r1 3W 5% resistors to check.

All in series have a total resistance of 2r0+-5%

If you have a 2r0 500mW 1% resistor, then add that to the string. You now have ~4r0.

Apply your 100mA test current.
Measure the voltage drop across the 1% resistor. You can now calculate your test current to <=1% accuracy.
Now measure the voltage drop across each 0r1 resistor taking care to ensure your voltage measuring probes always measure the same length of resistor lead outs. Compare all these test Vdrops to the 1% Vdrop. You have an absolute accuracy on the 5% resistors that is <=1%. The 2r0 resistor is dissipating ~80mW or about 16% of it's maximum rating.

Now increase the test current to 195mA. Compare the Vdrop across each 0r1 to all the others. You can get 1 part in 200 matching (0.5%) if your DMM has a resolution of 0.1mVdc. More CCS current or a more sensitive DMM cn get to better than 0.1% matching.

But the absolute accuracy can never be better than the tolerance of the standard resistor. If you are lucky enough to have some 0.1% standard resistors then absolute accuracy can be improved, but watch for temperature effects in your high accuracy resistor. I would suggest that your high accuracy resistors never dissipate more than 10% of their maximum ratings.

jackinnj 10th December 2010 05:41 PM

For the LDR's current sources I have used the Linear Tech LTC1990 Difference Amplifier -- and have on order some Analog Devices parts -- -- I was very pleased with both.

My male pattern baldness took a turn for the worse when I tried to replicate Janneman and Walt Jung's results for measuring the impedance of the super-regulators and commercially made competitors. Micro Ohms? It can be done, but don't jostle the cables!

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