measuring DCR of coils

[Duck-Twacy]

Last year I bought four 2.2 mH tritec coils to use in a Pi filter of my Aleph power supply (still under construction )



I ordered the ones with the 7*0.8 mm wire (3.51 mm^2 total).
According to the Intertechnik catalogue these should have a (DC?) resistance of 0.24 ohms
(see also http://www.speakerland.nl/TRITEC-SPOELEN.htm )

Because the coils arrived rather damaged (more than half of the black tire wraps were broken) I decided to measure the resistance with my Fluke multimeter. Even after cleaning the wire with some sandpaper I find a resistance of 0.8 mm??

Do I measure them in the wrong way or are these specifications of Intertechnik just false ??

(btw the size of the coils is the same a in the cataloge, so they are the 7*0.8 mm types. Wire diameter is about 3 mm, coil diameter about 13 cm and 3 cm high).

[gmarsh]

The lowest ohm setting on your Fluke is probably 600 ohms full scale.

On my 179's 600 ohm scale, there's 0.1 ohms of resolution, with a 0.9% + 2 error on top of that. You can't reliably measure fractions of an ohm by any stretch of the imagination, you need a meter that's designed for measuring resistances that low.

[powertriode]

DMM's are not ideal for measuring resistance of less than 100 ohms. You would be better off with either an LCR bridge or meter or at the very least a bench DMM (such as a Fluke 8040) which can measure resistance using 4-wires.

Bear in mind the resiatance of your DMM leads will most likely be around 0.2 ohms or so.

I have a top of the line Fluke DMM (Fluke 187) and its still pretty useless for low ohms measurements.

[Duck-Twacy]

Yes you are (both) probably right.

On the picture you also see a vishay resistor,. Is is supposed to be 0.47 ohm (about 2%). The Fluke 111 measures 1.6 ohm


Should have know that, just forgot.

What is a good way to measure these low impedances (except for mega $$$ DMMs)?

[byteboy]

I think you meant a resistance of 0.8 Ohm instead of 0.8 mm ???

Maybe a better way to measure the DC-resistance of such low-ohmic devices like coils would be to use this coil in series with a stable DC-voltage source, eg. a car battery, and a resistive load, eg. a car headlight lamp, and measure the voltage drop over the coil and calculate the DC resistance?

Say if you would use a 55 Watt bulb and a 13.8 Volt car battery wich would give you a 55 / 13.8 = about 4 Ampere current through the expected 0.24 Ohms of the coil, you would have to measure about 4 X 0.24 = about 1 Volt over the coil.

Of course you will have to measure the actual current through and the actual voltage over the coil to get a precise value.

To avoid having the multimeter in the circuit to measure the current and thus influencing the current (internal resistance of the meter itself) if it is taken out to measure the the voltage over the coil (if you have only one MM), you could instead use a known value resistor (say one of the source resistors (0.33 Ohm/5 Watt/5% ?) for the Aleph(-X) in series and by measuring the voltage drop over it (about 4 X 0.33 = 1.22 Volts) to calculate the actual current.

I think 4 Amps/1 Volts can be measured fairly accurate with an average "household" type of (digital-) multimeter?

[powertriode]

The best way to measure low resistance is with a measuring bridge such as a Wheatstone bridge.
You can easily do this yourself, you just need a couple of high accuracy metal film resistors and a DMM.

[rcavictim]

Duck,

On the bright side the DC resistance of the coil cannot be changed by the neatness factor of the winding pattern. You claimed that the coil was damaged and the ty-wraps were broken. Only if the wire has been physically nicked or cut into somewhere along the winding length, or turns cut off (less length of copper wire) can there be cause for DC resistance change.

The others are right, it is not possible to measure fractional ohm measurements on the common ohm meters.

[Duck-Twacy]

Originally posted by byteboy
I think you meant a resistance of 0.8 Ohm instead of 0.8 mm ???

Yes,

thanx for the tips

Originally posted by powertriode
The best way to measure low resistance is with a measuring bridge such as a Wheatstone bridge.
You can easily do this yourself, you just need a couple of high accuracy metal film resistors and a DMM.

http://www.dwiarda.com/scientific/Bridge.html
Will have to study that a bit

Originally posted by rcavictim
Duck,

On the bright side the DC resistance of the coil cannot be changed by the neatness factor of the winding pattern. You claimed that the coil was damaged and the ty-wraps were broken. Only if the wire has been physically nicked or cut into somewhere along the winding length, or turns cut off (less length of copper wire) can there be cause for DC resistance change.

The others are right, it is not possible to measure fractional ohm measurements on the common ohm meters.

All 4 where in a large box with some newspaper in between, most of the tire wraps were broken and some hars (don't know the english word, glue?) pieces were broken of.

However all 4 give more or less the same resistance, so I think they are still okay
Still they cost 45 euro a piece, don't like that shop anymore..

[sam9]

If you are going to try the Wheatsone bridge, take a look at the calculator and/or wheatstone bridge pages at

http://www.electronics2000.co.uk

It is the clearest explanation I've seen for someone who just wants to get a result. It should be obvious you can do the measurement by one of two means A- use a multiturn pot for one of the know resistors ans adjust it for 0V across the bridge, then measure the pots resistance ans plug in the values to the calculator, or B-work backward use thre fixed resistors,read the voltage and figure out what the unknown resistance has to be. It's just a matter of which is the easier method for you. This will make more sense one you take a look at the site and see how the on-line calculator is set up.

[raincheck]

With just the Fluke mm you have, are you connecting the two probe leads together and subtracting that from your measurements?