my milli-ohm meter

[dinges]

G'day,

After an overwhelming number of requests (by Bruce downunder) I've decided to make plans & photos of the construction of my milli-ohm meter available.

I have built it about 2 months ago, and already have had much use from it, like measuring resistance of coils, high-current connections (cable clamps etc.), measuring contact resistance of switches and relays, etc.

The instrument is basically very simple: a current source, and a voltmeter (range 0-199.9mV). I've made mine luxurious, with inbuilt DVM-module, but there's no reason to only build the current-source part and use an external multimeter. You put a known current (say, 1ohm) through your unknown resistance; you measure the voltage it drops (say, .123 V), then you know the resistance via ohms law: .123ohm

The schematic:

There are 3 current sources: one for 10mA (the 20ohm range), 100mA (2 ohm range), and a 1A source (for the 199.9milli-ohm range). They can be switched to the outputs via switch S1. This also controls the digital point for the DVM module, and lights the proper LED (either ohm or milli-ohm).

It uses the Kelvin measurement system; this means 4 wires: 2 for the 'excitation', the current source, and 2 for the DVM inputs. This way, transition resistances in the instrument itsself and the connecting wires are not a problem. There's a switch to go to the 2-wire system, so you can use just 2 wires, but this introduces a measurement error of (in my case) 113milli-ohm. Not a problem in the 20 ohm range, but a big error in the 200milli-ohm one.

Below is a picture of the instrument in use, measuring one of the spare coils for my bicycle generator. It is switched in the 100mA-range (2.000 ohm); if I had used the 1A range, resolution would have been even greater (132.x milli-ohm).

The picture above shows the inside of the instrument. The PSU is on the extreme right, it was saved from a defect DVD; perfect for this application. Also saved (from an old modem) was the case. The things people throw away...

In the middle is the 1A current source; this caused most headaches, to realise, because of cooling and the big resistors. On the left is the main circuit board, containing all connections to the DVM module, front and various other parts of the instrument. There's an extra power supply for the DVM module, because it needs floating power (gnd of the module may not be connected to the ground of the rest of the instrument; only the more expensive DVM modules can do this).

That's it, basically nothing much to it, but a very handy instrument for measuring low resistances. In combination with my valve-voltmeter (1 micro volt resolution, 100uV full scale), I can measure down to 1 micro-ohm :-)

Of course, you could just build one of the current sources (say, 1A) and be done with it; I wanted a stand alone instrument, including various ranges and a DVM, that could be used without too much fussing and connecting various wires and power.

BTW, once you've got such a meter, you'll wonder how you ever managed to live without it :-)

Peter,

The Netherlands.

[dinges]

Something apparently went wrong with the pictures; have done the same thing as usual, but they don't come out right, despite resizing and resampling. Have changed the pictures in my photo-uploads with smaller ones (with exact same name), but still isn't the way I want them.

At least it's now more or less bareable, but my apologies.

The hi-resolution schematic (100kB) can be found in my photo-uploads, near the bottom.

Peter,

The Netherlands.

[dinges]

Oops... Pictures come out ok now. Must press refresh... Duh.

Peter,

The Netherlands.

[drdongle]

Very profesional loking unit!

[Waterfront]

Nice piece of homebrew equipement! DaveW designed a very cool coil winder, then you have this... I tell you, the way things keep going we'll have some nice plans for building just about any machine related to RE!!

Keep on designing!

[Opera House]

Nice build. It is good to remind everyone what a nice current source the 7805 can make.  I own a nice high current sourcing milliohm meter and I find it one of the most usefull pieces of equipment in the shop.  Mine has a calculating ohm meter and it will calculate the ohms with any current from less than 1/4 amp up to 100.  I modified mine with some 300 ohm resistors from the source to sense terminals instead of using the switch.  Sometimes getting the value to 10 milliohms is close enough when you don't want to fool around with four wires.  Many might just consider building just the current source half of this since you probably own a digital meter with a good 200mv range.

[DaveW]

OK, Peter you've convinced me. I need to build one.  I don't see any fans. Do you have any problems with temperature drift or do you just take a quick reading and unhook?

[dinges]

No fans are needed: the 10mA (BC559) and 100mA(7805) obviously don't, while the 1A current source(7805) has such a large heat sink you hardly feel the temperature rise (a bit overdesigned, I'm afraid). The resistors (3) in the 1A current source do get warm, but not too hot to touch.

However, there's a little drift in the 7805 (or the resistors?) when using the 1A current source; it drifts from 1.00A to .99A, a 1% error (over a 5 minute period).

I can live with this, most of my other (commercial) instruments are worse than this :-) Besides, I don't mind if my screwdriver has a resistance of 18.8milli-ohm or 18.6-20.0 ; but if you've got the knowledge, there would be no reason to design a better stabilized current source. But I think a 1% error is good enough for all practical purposes, short of a laboratory situation.

I still limit the use of the 1A source though; usually only use the 100mA, and when it gets critical or I need more accuracy, I switch to 1A. That's the reason the picture shows the 100mA mode, I had wanted to show the picture the 1A mode, with 199.9mV full scale display. But there's no real reason not to use the 1A mode continuously, the machine can handle 1A at 100% duty cycle.

Peter,

The Netherlands.

[dinges]

Besides, you could even compensate for any temp. drift, should it be an issue; just hook a 1A meter in series with the current source, and measure not only volts but also the current; if it's .99A, use Ohm's law R=U/I, with I=.99A instead of 1.00A. But haven't felt the need for that yet.

Peter,

The Netherlands.

[ghurd]

Hey Peter,

Nice!

Just curious, but why do you want to know the resistance of 'high-current connections (cable clamps etc.), measuring contact resistance of switches and relays,etc.'?

I have only experienced appreciable thermal drift with RF circuits. But that was many moons ago, back when I could remember what RF was.

G-

[ghurd]

(PS- A razor blade popped the 6 coils off the VCR motor plate with no lost windings. Looks like a 8/6 dual rotor with 12.5mm round neos and 7cm magnet disks.

I always think big.

[dinges]

to make sure they're clamped properly, not just look good. These things could overheat if transition current is too high?

Some time ago my father wanted to install heavy duty welding cable in his sailing boat for an anchor winch; didn't know which was better (thicker would be better of course, but there were other limitations as well). So, out came my (then new) trusty milli-ohm meter. A quick check gave us all the information we needed on the difference in resistance of several sizes of 10m of this cable (25mm^2 vs. 35mm^2). Normally this kind of data should have been provided by the manufacturer of the winch (an Italian company), but they didn't. They also didn't feel it necessary to answer my requests on the subject.

Also, when installing new cable harnesses in my UPS, with external connectors for connecting to an outside battery bank, it was reassuring to know that the resistance of the wires, including the clamp connectors (AMP), was 5.1 m-ohm for one cable and 6.2m-ohm for the other one. Just an extra check to make sure no strange things have happened while manufacturing the new wire harness :-)

Peter,

The Netherlands.