Wayne said:

I just learned that you can measure the change in resistance in a shunt

connector between source and load as the voltage or amperage changes due to

on/off, and load characteristics. I don't understand why this happens. If

you power a machine like a vacuum cleaner, does the resistance of the

extension cord change when turning off to on? and when tackling a tough

spot?

If this is true, I should have learned this fifty years ago and it would

have saved me a lot of time and would have helped in trial and error

experiments.

Anybody?

Wayne

Hi, Wayne. Placing a small series resistor, or shunt, in series with

the load is an easy way to measure current. You just measure the

voltage across the shunt, which is proportional to the current.

The problem is that resistances change with temperature. Copper in

particular is pretty bad here -- its temperature coefficient of

resistance is about +0.4% per degree C.

Let's say you've got a 1 milliohm copper shunt, and you're measuring 0

to 50 Amps = 0 to 50mV. At 50 amps, the shunt will be dissipating 2.5

watts. Let's assume that 2.5 watts causes the temperature of the

copper to rise 20 degrees C. That means the actual resistance of the

shunt will be more like 1.05 milliohms, leading to a 5% error in your

reading.

This is a limitation in measuring current by measuring the voltage drop

across an extension cord. Yes, as the current increases, the extension

cord heats up, and its resistance does change. But the change in

resistance is due to the change in temperature. You can put a shunt in

an oven, with no current, and measure resistance change just the same

with an ohmmeter.

A plain old resistor has a temperature coefficient of around 300

ppm/degree C, or 0.03% per degree C. Good wirewound resistors have

temperature coefficients better than 25 ppm/degree C, or 0.002% per

degree C.

Most high current measurement shunts are made of alloys of metals whose

postive and negative temperature coefficients cancel out, leading to a

more accurate reading at high current/power (close to 20ppm/degree C).

Does this answer your question? If not, feel free to post again.

Cheers

Chris