Low Resistance Measurement

Thanks Tom, that's something I did not think of. I remember finding my fridge unacceptable many years ago, but was told by a reliable source that fridges are notoriously bad at current leakage. Then I thought of my new fridge with its box of electronicals on top. I won't check that! I was thinking of checking the windings of old transformers that I am rewinding. No point wasting electricity melting them if I've breached the insulation anywhere :) Same goes for old motors that I might want to resurrect. And then there is elderly house wiring... jack

Ahh, thanks, Phil. Come to daddy, li'l ol' analogue meter :)

Sort of how you can check earth wiring. Apply a decent load (electric radiator eg) between active and earth and see if it works as per normal.

Are there any DMMs out there that use a larger test current?

jack

Your DMM will certainly have a hard time with measurements below 1 Ohm. For precise low ohm measurements a bridge meter designed for that purpose is used. As for problems with the megger, you could fry things if you don't know what you are doing.

Tom

** Use lotsa current. ** The analogue jobs use far more test current.

The simplest way for the odd low R measurement is to use TWO meters at once.

One for current and the other for voltage.

........ Phil

Your old analog meter seemed to better because it can't display the very small changes in the resistance of the connections to the unknown resistance (contact resistance). The digital meter will show these variations and as a result the reading will wander quite a bit. A much better method is what is called 4 wire or Kelvin sensing. With this method 2 wires are used to supply a regulated constant current through the unknow resistance so that the current flowing through the resistance is independent of the contact resistance. The other 2 wires are used to measure the resulting voltage across the unknown resistance and that voltage is scaled to represent the resistance. The resistance of the circuitry used to measure the voltage is very very high compared to the resistance being measured so it has very little effect on the the voltage appearing across the the unknown resistance. Some of the more expensive meters have this capability, but you can use this method without such a meter by using a vaiable current regulated supply and setting the current through the resistance and measuring the voltage across it then calculate the resistance. You can build a simple current regulator pretty easiy just for this purpose.

Mike

"I think, therefore GOD is." J.P. Moreland

I didn't ask you anything, Herr Controller

jack

ps I can't honestly think of how to create one of these, but then I'm just a newbie who has ventured onto your hallowed turf, apparently

** No.

Create your own.

Do NOT come back and ask ME how to do that.

Use your brain.

..... Phil

changes in the resistance of

meter will show these variations

what is called 4 wire or Kelvin

current through the unknow

the contact resistance. The

resistance and that voltage is

measure the voltage is very very

the the voltage appearing

capability, but you can use

and setting the current through

resistance. You can build a simple

Thanks so much for that, Mike. So much more helpful than sending me away to "create" a DMM that uses a higher test current. jack

ps I have a nice digital bench PS, so I will use that method to check my transformer winding resistances.

Here's an better description of Kelvin sense resistance measurment.

You don't really need a lot of current to get reasonably accurate measurments. You'll find that the winding resistance reading will creep upward as the current you send through the winding heats up the wire, so it's best to use a little current as you can and still get a decent voltage drop. You might start with 0.1A so that your volt meter will read .1V/ohm. That way you have no math to do except moving the decimal point. Hopefully your power supply will allow you to set the current and not just the voltage. If so just set the current to 0.1A and the voltage as low as you can and still maintain the 0.1A. Just be sure to measure the voltage drop directly on the windings. I used to build and sell milliohm meters to guys who race slot cars so they could check the windings on the little armatures. I used 0.01A constant current and still that would warm the wire up enough to see the readings creep up a bit. The Kelvin clips they show on the link above really do make things a lot more convenient especially if you will doing a lot of measurments.

FWIW, phil allison and rest of his ilk are best totaly ignored. Those fools troll around doing little more than trying to stir up trouble.

Your's was very reasonable question that deserves a reasonable response. I'm no engineer, but I would be more than happy to lend any assistance that I can to help you.

Mike

"I think, therefore GOD is." J.P. Moreland

I do it this way. I have a current limited PSU with an adjustable current limit that I set to 1A. I then connect the item to be measured across the output terminals and measure the volts across the item in question. Be sure to use a Kelvin connection when measuring.

Graham

That's asking a lot these days. Kids today are spoon fed their education mostly.

Graham

Wonderful. Thanks. I've filed that as a keeper.

0.1A

except

the

armatures.

see

measurments.

I imagine that the test current used on my DMM is somewhat less than

100mA, and yes, my PS can set current. Problem solved. Thanks.

He apparently doesn't want me asking him certain questions, but I understood that usenet allowed you to answer what you wanted and to ignore what you were not interested in. Phil seems to have a compulsion to answer everything and object when some of this "everything" includes stuff that he doesn't want to answer. He needs to understand just what he can change and what he can't. If he does not like a question, then he would do himself a favour to just ignore it, like every other sane member of this group has learned to do. But perhaps he has a very empty life and usenet is a big part of it, and so he wants it his way. Yep, your advice is sound. Thanks.

Thanks again, Mike, rgds... jack

The same sentiments of 2000 years ago :)

jack

Great advice, thanks, Graham.

jack

If a megger is what you want so be it how ever, in case you didn't already know. Most meggers generate HV in the average of

500 Volts. Others will do more depending on the range of test you're doing. for doing G/T ohms, our units at work can go up to 2500 Volts to perform the test.

I guess if you were just doing test on transformers and things of that nature, a basic meter of 500 volts will do fine.

I've seen some newer one's use lower voltages for test.

The general idea is to use a voltage on the fixture to which the item under test was designed to handle. Also the HV makes it easier to get High R readings.

--
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