Super low Ohm meter

[katou]

I've looked on ebay, but I think I'm using the wrong search terms.

I'm wanting an ohm meter that will measure the resistance of wires and contacts.

Eg. I use 10 awg wire and I splice it to two 12 awg wires. I would like to know if my connection is as low in resistance as the wires.

Someone point me in the right direction?

Katou

 

[torker]

Those get really pricey. I found 300 dollars the lowest :x http://www.globaltestsupply.com/category.cfm?category_id=39

 

[swbluto]

Get a power supply with constant current and a volt-meter. That's all you need.

OR

Get an ammeter, voltage meter, power resistor and voltage source(like a battery). Use the power resistor to limit the current, and measure the current using the ammeter. You could use a lightbulb in place of the power resistor.

Then use Ohm's law to your advantage.

 

[liveforphysics]

The only reason to get a high dollar meter is if the things you're measuring can't handle currents in the 10A+ range. Get a constant current supply in the 10-20amp range that does maybe up to 5v or so, doesn't trip when short circuited (or else you need a series resistor in your testing). Then get a precision volt-meter (available for cheap). Like swbluto said, use V=IR, and you're set.

 

[katou]

Okay, I see. I have a PS, but it is not current limited. I found that out by shorting it, and blowing the fuse. Good thing it had a fuse I guess.

I will get the PS and go to it. Thanks for the guidance.

Katou

 

[texaspyro]

I'm wanting an ohm meter that will measure the resistance of wires and contacts.

What you want is a meter with 4-wire "Kelvin" connections. Two wires drive the test current and two wires independently measure the voltage drop.

I usually use either an HP 3458A 8.5 digit meter (over $15,000 new), and ESI 2150 VIdeoBridge ESR meter ($25,000 when new), or Valhalla digital igniter testers ($3000+ when new).

I have several of the Valhalla units that I refurbed... these can measure down to milliohms. PM me if you have a hundred bucks or so burning a hole in your pocket and don't live in an export controlled country...

 

[swbluto]

Okay, I see. I have a PS, but it is not current limited. I found that out by shorting it, and blowing the fuse. Good thing it had a fuse I guess.

I will get the PS and go to it. Thanks for the guidance.

Katou

You can use your "non-current limited" power supply just as long as you put a power resistor or something similar (Like a light bulb) in series with the connection. This would effectively limit the current, preventing the fuse from blowing. Although... I believe bulbs have a pretty low resistance when "cold" and so there may be high startup currents, so a bulb might be a problem.

But, yeah, having a current limited power supply is nice for measuring tiny resistances. Just clamp it, set the current, and measure using a voltmeter. Calculate and you're done.

 

[bigmoose]

I have a HP4328A Milli Ohm meter with Kelvin probe that I use for these purposes, and can highly recommend it. Very accurate and very useful.

 

[Toorbough ULL-Zeveigh]

I have a HP4328A Milli Ohm meter with Kelvin probe that I use for these purposes

don't wanna brag how much it cost??

 

[bigmoose]

don't wanna brag how much it cost??

I will not take offense and for your information, it was a couple hundred bucks off of eBay and then a NIST traceable calibration. I have my own electronics consulting business, and these are tools of the trade for me, business necessities. I am not a hobbyist when it comes to electronics. The gent wanted to know how to measure in the milliohm range, I politely provided a recommendation that I can personally vouch for.

All the best Toorbough ULL-Zeveigh,
Dave

 

[Toorbough ULL-Zeveigh]

wasn't directed towards u but a previous poster who seemed to feel it was important to parenthetically note their price when new every time.
i guess i should have put an 'also' in the middle of my comment.

if anything seeing that the gloves had been thrown down, i was prodding for an arms race so people would whip out who's got the biggest, baddest, most expensive unit.

 

[bigmoose]

Thanks for the clarification, that was kind of you to post your reply. Note that you quoted my post and not the other gent...

All the best,
Dave

 

[John in CR]

Throw the gloves down, but instead the winner is who finds a good quality tool for the job for the best price. Bigmoose is telling us a price that's not too far from what cheapo me would pay. Can someone top that, preferably with double digit price?

 

[deronmoped]

You can use a ESR meter to measure ohms.

These meters read from .01 to 99 ohms. I think I paid like $60.00 new when I bought it a couple of years ago. Looks like someone bought out Dick Smith Electronics and it's now going for $79.00 under a new design and name.

My meter is so accurate you have to zero it out by touching the test probes together and pushing the reset button. The test leads measure like .12 ohms if you do not zero it out.

http://www.flippers.com/esrktmtr.html

Deron.

 

[redorblack]

I don't know the level of electronics experience the original poster has, so I'm just going to throw this out there.

Super low resistance ohm meters are both expensive and there is a good chance one isn't the best solution for what you are wanting to test.

Look up "Voltage Drop Test" in your favorite search engine until you find a hit that gives the explanation in a manner easy for you to understand.

Doing a voltage drop test is much easier and cheaper than using an ohm meter... you just need a loaded circuit and a cheap volt meter. Light bulbs make great loads in place of expensive circuitry.

Here is a video showing how to test voltage drop across sections of a circuit.

http://www.youtube.com/watch?v=-ybelDXySc8&feature=related

When testing across a connection it's the same thing, you load the circuit, put your meter on millivolts, and measure across the connection in parallel. Any resistance in the connection will show up as a voltage drop (ie. your meter will read higher than 0.00mv). Remember you are measuring in parallel with the wiring on either the positive or ground side of the circuit. If the reading is high, there is a problem. How high is a problem depends on if it's a high or low current circuit. I generally don't want to see more than 50mv across a low current circuit (under 10 amps), something high current like a car starter(around 150 amps at 12v) is ok up to about 450mv. The lower the better... remember that any voltage reading is voltage pressure pushing through your meter because it's easier than pushing through the circuit. Determine what is ok for the use of the circuit... if you have a suspiciously high number across your power or ground side, just narrow it down by moving to measuring across sections until you find the culprit. You can use backprobe terminals to get at either side of an Anderson power pole or equivalent, on screw type terminals measure from the screw to the wire itself to see if the connection is weak or corroded.

If the OP knows this... I apologize, but I figure it might help someone. I do automotive testing and have found that the majority of techs don't know this simple way of testing a circuit for resistance.

 

[battman]

I think redorblack's advice is excellent.

If you still want to pursue low ohm meter you could try this:

http://www.aeroelectric.com/articles/LowOhmsAdapter_3.pdf

I built it and it worked well.

 

[katou]

Top notch! This is all excellent information! Thank you all. I like many things about the various solutions presented, at varying levels of knowledge and price.

Now I just have to decide which one! For low cost, and tidyness, I think that last suggestion wins. But for ultra low cost, I can do the trick with the resistor and the non-current limited PS I currently have.

Hmmmm.

Katou

 

[texaspyro]

If you still want to pursue low ohm meter you could try this:
http://www.aeroelectric.com/articles/LowOhmsAdapter_3.pdf
I built it and it worked well.

Definitely a workable solution if you have a high quality meter to go with it. For milliohm resolution you need a meter that can read to 100 microvolts (1/10 a millivolt)... Most 3.5 digit meters might resolve down to a millivolt with a few counts of noise thrown in at that level.

Using a current limited bench supply at 1 amp will allow you to read 1 milliohm with a millivolt capable meter. Again, you will have a few counts of noise to deal with plus errors due ohmic heating due to the high test current (which might be a good thing when testing power connections).

The Valhalla igniter testers are designed for testing things like blasting caps and rocket igniters. On the milliohm range they are designed so that absolutely no matter what goes wronnng inside them they cannot and will not put out more than 15 milliamps of current.

The HP milliohm meter is a very nice device. I sold mine when I got the 3458A's. I should have kept it... I occasionally have the need to measure things it can do well, but the 3458A's don't (or is not worth risking damaging a high dollar meter on)

Ebay can be a wonderful source of very sophisticated test equipment at dirt cheap prices. My lab has a couple million dollars (retail) worth of equipment that was acquired for under $30,000. It can also be the source of high dollar, you bought it... it's yours, boat anchors that belch nasty smoke when you plug them in.

I do electronics for a living, so can easily justify most of the rather extravagant equipment. If I need a piece of equipment for a project, I can almost always buy it there for less than I could rent it for a week! But a couple pieces were acquired more for the "wow" factor... like wow, that's neat... I'll bid a few hundred bucks on a $250,000 gizmo... and occasionally wow... nobody else wanted it more that week. Who knows, someday I may actually NEED that 500 gigasample/sec transient digitizer.

 

[katou]

Is that the one from Tron? I think that's what sent him inside the computer.

Careful.

Katou

 

[amberwolf]

This one has a 100mV range; should it work ok in combination with that adapter circuit?

If so, I'll build that adapter for it, as it would be interesting to see how much resistance my bolt-together connections have, vs soldered ones.

 

[texaspyro]

This one has a 100mV range; should it work ok in combination with that adapter circuit?
If so, I'll build that adapter for it, as it would be interesting to see how much resistance my bolt-together connections have, vs soldered ones.

That should work OK. With a 100 ma test current you should be able to resolve 0.1 miliohm... about the minimum that I would want when testing connections. One milliohm res is really not adequate for properly evaluating even mediocre connections. If you have a 1 amp current limited supply that would be even better... 10 micro ohm res.

 

[amberwolf]

I have a 55A current limited supply if that helps.
http://www.endless-sphere.com/forums/viewtopic.php?p=283087#p283087

 

[texaspyro]

One thing to watch out for when using any of these current limited voltage sources for measuring milliohms is your voltmeter. When the probes are not across a low resistance, the full supply voltage is put across the meter (which is probably set to millivolts). A decent meter will not mind at all... YMMV (your meter may vaporize).

 

[jateureka]

.... try this:

http://www.aeroelectric.com/articles/LowOhmsAdapter_3.pdf

I built it and it worked well.

Wow, cheap DIY test kit for the home workshop, love it!