milliohm meter

[mazur50]

Does anyone know how i can use my RMS Multimeter to read milliohm I want to check the resistance in my armatures.

thanks
Mike

[AndyOne]

A way to measure very small resistances is to pass a large current and measure the voltage drop across the resistance under test with a sensitive voltmeter. Using a Kelvin connection which means connecting the voltmeter to the points to be measured across and connecting the current source separately and further out so none of the connections touch each other. I suggest you Google for Kelvin connection or 4 terminal resistance measurement to see how this is actually done. Then using Ohm's law calculate the resistance, QED.

Andy.

[mazur50]

A way to measure very small resistances is to pass a large current and measure the voltage drop across the resistance under test with a sensitive voltmeter. Using a Kelvin connection which means connecting the voltmeter to the points to be measured across and connecting the current source separately and further out so none of the connections touch each other. I suggest you Google for Kelvin connection or 4 terminal resistance measurement to see how this is actually done. Then using Ohm's law calculate the resistance, QED.

Andy.

I want to make somthing like this to check the resistance in a armature.

http://www.ruzamracing.com/webpics/arm.gif

[AndyOne]

Mazur,

You'll need a power supply to act as the current source and probably some way of measuring the current if it isn't built into the power supply, see...

http://www.allaboutcircuits.com/vol_1/chpt_8/9.html

Andy.

[Acetronics]

Mr Cam's µMetros ... with a Nicad in series !!!

Then substract the Nicad internal resistance ...

Alain

[AndyOne]

The problem with trying to use internal resistance of any kind of battery is that it's very difficult to pin down. Figures quoted by manufacturers are measured under specific conditions and unless you match these it will be different. The internal resistance will change with state of charge, rate of discharge or charge, age and temperature amongst other things. An ammeter is the only way to be certain how much current is flowing.

Andy.

[asm_]

Like Andy said, the most effective way for DIY to test resistance below 1ohm is to pass current through the measured device and measure the voltage different at the input and output terminal.

If you pass exact 1 amp, the voltage reading difference would the the ohm value you are searching for. Thanks to LBMiller5, here is a DIY on how to construct such device.

http://www.rcgroups.com/forums/showthread.php?t=580151&highlight=diy+motor+resistance

~asm

[Acetronics]

The problem with trying to use internal resistance of any kind of battery is that it's very difficult to pin down. Figures quoted by manufacturers are measured under specific conditions and unless you match these it will be different. The internal resistance will change with state of charge, rate of discharge or charge, age and temperature amongst other things. An ammeter is the only way to be certain how much current is flowing.

Andy.

Andy ....

1) Step

You measure the Batt internal resistance

2) Step

You measure the Batt internal resistance plus winding resistance ( connected in series ...)

3) Step

You substract the Batt resistance measured at 1) from the total measured at 2)


.... Boooooooooooo :censored:

There's another way ... but prepare you credit card !!!


Alain

[AndyOne]

I wouldn't trust the battery internal resistance not to change during the time between the two tests. Do two internal resistance tests one after the other and you will get different results because of the temperature rise or polarisation inside the cells etc.

Andy.

[vintage1]

all this was covered in a thread a while ago.

You can make an accurate - say - 100mA current source, and read the answer in millivolts..or I did an analogue circuit as well.

It worked and I must get round to boxing it up. Go a big panel meter to go with it.

However much to my disgust it was no better than a LCR bridge for brushed motors..it seems nothing is sensible but you actually have the motor spinning, on those.

My LCR bridge* proved especially good for checking hand wound motors though..the resistance is super low, but the inductance is not. since inductance is turns squared, an extra turn shows up very quickly.

So much so that I may well use it in the end instead of a turns counter ;)

* off Ebay for about £50 IIRC. A good buy.