Re: Measuring tiny unknown resistance?
From: Fritz Schlunder (me_at_privacy.net)
Date: 01/28/05
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Date: Thu, 27 Jan 2005 23:05:01 -0700
"kevwalsh" <kevwalsh@gmail.com> wrote in message
news:1106889503.256797.171490@c13g2000cwb.googlegroups.com...
> 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.
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