Very low mili ohm DVM attachment DIY Works Great! / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Very low mili ohm DVM attachment DIY Works Great!

previous 1 2

Move Thread

LAN_403

Dr. Slack

Wed May 27 2009, 07:04AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

EDIT: just figured you could produce a quite-possibly-lethal shock with a sub-C size NiCd cell and a practical human-body-MRI-size superconducting magnet,

So there you are people, don't try this at home. If any of you have a practical MRI superconducting magnet to hand, don't mess with it with a battery!

Wavetuner

Wed May 27 2009, 07:37AM

Registered Member #1500 Joined: Sat May 24 2008, 04:38PM
Location: Ojai, Ca.
Posts: 44

I can say after using this easy to make device that I was measuring .1mv increments on the 200mv scale. This is NOT like reading .1 ohm after zeroing the probes which was not at all good enough for what I was reading. Lets see: 100ma constant current or.1*.0001 volts =.00001ohm best case increments. Like I said it was very stable for my needs. I am NOT arguing this is as good as a 4 wire and dual clips, but it really is a slick and cheap add on to a DMM. Try it. It clearly can't do pico ohms.

It earned a place with my HV probe.

Proud Mary

Wed May 27 2009, 08:32AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Sounds useful to me! How often do we actually need a 4-wire anyway?

klugesmith

Thu May 28 2009, 05:07PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Wavetuner wrote ...
100ma constant current or.1*.0001 volts =.00001ohm best case increments.

Oops, R = V/I not V*I. So 0.1 mV resolution with 0.1 A test current means 0.001 ohm resolution.
If full scale is 199.9 mV, that indicates 1.999 ohms.

I do admire the practical value of your battery-powered, regulated current source that literally plugs into a DVM.

Along those lines, have been intending to make a 1.000-amp fixed current source for 4-wire measurement
of resistances on the order of 10 milliohms. It would replace an adjustable, regulated laboratory power supply
that requires calibration for accurate work.

Steve Conner

Fri May 29 2009, 08:55AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

For low resistances, I've always just used a ghetto 4-wire technique with a bench power supply. I connect one pair of clip leads to the supply, clip it across the DUT and set the current limit to 1 amp. If the meter on the power supply is inaccurate, you can use another DMM on the amps range in series with your "current source" to set it.

I then use another pair of prods connected to a DMM on the millivolts range, to poke across the parts of the DUT whose resistance I'm interested in. Solder joints, terminal bolts, crimps and so on.

Using this technique I was able to measure the resistances of everything in my OLTC2 primary circuit to the nearest 0.1 milliohm, and so get a rough idea of the I2R losses in operation. I think I used 5 amps because the resistances were seriously low, and my PS didn't go up to 10 amps.

And yes, you can get quite a jolt from inductive kickback (and potentially blow up your millivoltmeter) when trying to measure the DCR of transformer windings with this method.

GeordieBoy

Fri May 29 2009, 01:47PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

A note on the Inductive kickback thing: I started a discussion on this on the Pupman TCML a few years ago to warn people about the dangers of stored energy in things like pole pigs. DC resistances are so low that you can actually trap a current in the winding for some time if you short circuit it!

For instance, I was able to short-circuit the HV-secondary of a big power transformer, power the primary from a dinky 9v battery for a several seconds, remove the battery, and then draw an arc from the secondary side as I removed the short circuit! You could still produce a spark on open-circuiting the secondary several seconds after removing the initial source of power (battery).

It is also quite common that a DVM will refuse to measure the resistance of windings on really big power transformers. Both my fluke and Tek multimeters would just repeatedly range-switch when asked to measure the primary resistance of a 2kW HV power transformer.

It is the high inductance of the winding which confuses the multimeter by not allowing the test current to change rapidly enough when it does it's automatic range switching thing. The solution to this is to short-circuit the other winding of the transformer and then perform the resistance measurement. Short circuiting the secondary winding kills off most of the inductance seen by the meter at the primary side. Only the leakage inductance remains, which should be small for any decent power transformer, so the meter is then able to measure winding resistances fine. You might still have to wait a few seconds of settling time though.

-Richie,

previous 1 2

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob