low resistance DMM suggestions

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Donald

Donald

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While I have a variety of DMMs including the HF #98674 I am working on a used Winco generator and need to measure some resistance in the stator down to (0.12 - 0.13) ohms (not K or M).

My trusty Craftsman does not go below 1.0 ohms. The HF 98674 goes down to 0.1 but its doing a little wandering between .1 and .2

I need one down to 0.01
 
Is this an instrument that you'd be using regularly? If not, I'd rig something up for one time use....

Make your own current source.... put a resistance in series with a voltage like a car battery... at that low resistance, almost anything will work. Put a first DMM in series with the that path to measure current (make sure that you stay within its maximum limits). Use another DMM to measure the voltage drop. Close the circuit only long enough for the values to stabilize and then do some math... R = V / I.
 
Originally Posted By: Donald

I am working on a used Winco generator and need to measure some resistance in the stator down to (0.12 - 0.13) ohms.

The HF 98674 goes down to 0.1 but its doing a little wandering between .1 and .2

I need one down to 0.01


Did you cancel out the resistance of the test leads? If the stator winding is supposed to be 0.12 ~ 0.13 ohms and you're seeing 0.1 ~ 0.2 (not counting the test leads), the it's probably OK.
 
Originally Posted By: Skid
An ordinary DMM will not cut it. You need an ohmmeter with kelvin type connections (4 input). Basically, it separates the current source and voltage sensing (an ohmmeter drives current through a resistance and translates the voltage drop to a resistance measurement).

Stuff like this (I've not used any of these):

http://www.amazon.com/Instek-GOM-801H-Four-Input-Milliohm-Resistance/dp/B005DTHP88

http://www.amazon.com/Vici-Vc480c-Accuracy-Multimeter-Milli-ohm/dp/B0090Y3W5W


The Vici is within my price range.
 
I have no idea but I would think insulation needs to be very high resistance aka multiple mega ohms, so it it probably is not the thing you are looking for.
 
Insulation testers are megohm meters. That's the other end of the range. They usually use higher voltage to drive a small current through a high resistance. We have one at work, it's an old classic ( General Radio 1864 ). In general terms, the applied voltage goes up to 1000 volts and the maximum resistance it can measure is 200 terraohm (yes, terraohm). It's an interesting piece of equipment to play around with.

Skid, thanks for the thoughts on low resistance measurement. I'll try that technique if I ever need to make a low low resistance measurement.
 
I may be wrong but I think you need a vacuum tube volt meter
Also if this is a coil you may need a frequency generator to test it.
Run say 10k through it and up the voltage. If it loads the generator you may have a short
That the dmv won't catch.......
 
Last edited:
Originally Posted By: Ken42
I may be wrong but I think you need a vacuum tube volt meter
Also if this is a coil you may need a frequency generator to test it.
Run say 10k through it and up the voltage. If it loads the generator you may have a short
That the dmv won't catch.......


The troubleshooting info from Winco provides resistance values down to 1/100 of an ohm for stator coils and does not mention there is any special methods needed to measure it.
 
A VTVM is for High Z measurements as not to load the circuit under test.
Read this
http://www.testequipmentdepot.com/megger/pdf/low-resistance-testing.pdf
We do low resistance testing on our avionics, known as bonding resistance. Many times it is < 0.002 ohms. Our standard equipment for this is a kiethly 580 milliohmeter using 4-wire (kelvin) connections. This is down at the level where voltages generated by dissimilar metals can effect the reading, so the meter measures this voltage if present and compensates for it.
While your generator manufacturer may not outline any special equipment requirements, the numbers they are throwing out are beyond the realm of normal everyday DVMs, at least to do it accurately (Rule of 10's, instument accuracy must be 10x better minimum than the test limits)
 
Last edited:
Originally Posted By: Skid
Is this an instrument that you'd be using regularly? If not, I'd rig something up for one time use....

Make your own current source.... put a resistance in series with a voltage like a car battery... at that low resistance, almost anything will work. Put a first DMM in series with the that path to measure current (make sure that you stay within its maximum limits). Use another DMM to measure the voltage drop. Close the circuit only long enough for the values to stabilize and then do some math... R = V / I.


This is how I do it. I use a benchtop power supply with current limiting, but I've also used a car battery with a 21 or 55 watt bulb in series. Quick and easy to set up, and accurate with even moderately accurate dvm's
 
Originally Posted By: JetStar
A VTVM is for High Z measurements as not to load the circuit under test.
Read this
http://www.testequipmentdepot.com/megger/pdf/low-resistance-testing.pdf
We do low resistance testing on our avionics, known as bonding resistance. Many times it is < 0.002 ohms. Our standard equipment for this is a kiethly 580 milliohmeter using 4-wire (kelvin) connections. This is down at the level where voltages generated by dissimilar metals can effect the reading, so the meter measures this voltage if present and compensates for it.
While your generator manufacturer may not outline any special equipment requirements, the numbers they are throwing out are beyond the realm of normal everyday DVMs, at least to do it accurately (Rule of 10's, instument accuracy must be 10x better minimum than the test limits)


So the $100 one from Amazon (mentioned early on in thread) I ordered should do it. Its a 4 wire.
 
The Vici meter came today. China Post was the carrier even though ordered from Amazon. Its a 4 wire tester.

I put it on 2 ohms and adjusted the zero, then proceeded to measure the stator. The scale on 2 ohms is 1.nnn So it can go up to 1.999 ohms. It wandered a lot from .1xx ohms to .2xx ohms and back again. Never stopped wandering.

The meter seems OK, but its not a Fluke.
 
Now a question. The test probes that came with the meter are alligator clips not just a point. One test lead goes to each side of the alligator clip. Two sides to each of 2 alligator clips is the 4 wires.

I assume you either need to grapb the object to be tested BETWEEN the two halves of the alligator clip or leave them closed and use as a test probe.

What I do not think you can do is to (on an insulated connector) place one half of the alligator clip inside the connector touching metal and the other half on the outside of the connector touching only nylon.
 
No, because it is forcing a current on one wire, reading the resultant voltage on the other. All four wires have to make contact.
 
The device you need is called a milliohmmeter. The most reasonable one I know of is made by Extech. Basically it uses Kelvin clips and works by forcing a current through the resistance and measuring the voltage drop. I calibrate these things for a living at a calibration lab and know that a regular ohmmeter will not do the job. Google for a milliohmmeter that will meet your requirements.
 
A second Vici milliohm meter arrived on Sat. Unsure why. I am also learning that if anything like the rotor or stator is broken, its best to get a new gen head.
 
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