kevwalsh said:

Hi,

Anyone know a simple way I can measure a small, unknown resistance? I

am guessing it is about between 1ohm and 0.5ohms, roughly. It is part

of a car, so I can't bring it inside to test it.

I have several little multimeters, but none go down anywhere near as

low as half an ohm -- the cheap analog one goes as low as about 100ohms

at the very tinies graduation, and I don't think my digital ones go

much lower than that either.

I can also put 12v to the thing and try to measure current draw, but

obviously 10A or more is too much for just sticking my little

multimeter inline with the resistance.

I have access to plenty of simple parts, resistors and such, to build

some kind of simple measuring circuit. Any tips?

Thanks,

Kevin

BTW -- it is a rear window defroster, if you didn't guess that already.

My problem is that the new rear window draws too much current (how much

I am trying to discover) and melts things, like the on-off switch.

You would have some very unusual meters if they can't measure resistances of

less than 100 ohms. As digital multimeters go they almost universally (from

the very cheapest to the most expensive) have a minimum resistance scale of

200 ohms. On the 200 ohm scale the maximum resistance that can be measured

is 199.9 ohms. The minimum resistance that can be measured is limited by

the accuracy of that scale. On the 200 ohm scale, this means the minimum

resistance that can be measured is 0.1 ohm. The last digit is not very

accurate, so it could be off by a little. Measuring a resistance between

0.5 ohm and 1 ohm will not be too especially accurate, but it will likely be

accurate to about as good as within 0.1 ohm. Sometimes the meter will not

be calibrated to zero and will thus produce a fixed offset to your

measurement. You can manually compensate for this by shorting the meter

leads together on the 200 ohm resistance scale and observing the result. It

may for instance measure 0.2 ohms, but for a hard short circuit it should be

0.0 ohms. So, take you measurement of your 0.5ohm to 1 ohm resistance and

then subtract 0.2 ohms from the displayed value.

To get better accuracy and to further extend the measurement range down to

lower values you need another technique. One way is to build a 100mA

constant current source using an LM317 linear regulator. Then apply the

100mA constant current to the unknown low value resistance and measure the

voltage across it. Ohms law predicts V=IR, so for I=100mA, the voltage that

appears will be one tenth of the resistance. IE: if the meter reads 50.0mV,

then the resistance is 0.500 ohms.