Re: Question re. shunts to measure amperage


"Eeyore" <rabbitsfriendsandrelations@xxxxxxxxxxxxxxxxxxxxxx> wrote in
message news:44DE4255.6E8B3BE5@xxxxxxxxxxxxxxxxxxxxxxxxx


Wayne Lundberg wrote:

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?

Yes.

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.

Whatever it is, be it a shunt resistor or a cable, has a temperature
coefficient
of resistance. As the current increases, so does the temperature from I2R
heating effects, so the resistance changes ( upwards ) and in the reverse
direction as the load reduces. High accuracy shunts may be made of
materials
with negligible tempco to minimise the effect though.

Incidentally, the same happens in loudspeakers and causes thermal 'power
compression' at high volumes as the voice coil resistance increases.

There is also a power compression effect in loudspeakers if the voice coil
ends
up 'excursing' out of the magnetic gap or at least the linear part of it.
A
popular way of reducing the effect is to use long voice coils.

Graham

Graham, Cris - I have some high resistance wire used in electric kilns. If I
took a couple of inches of this stuff and connected it between my source and
load, then measured what... voltage? ... resistance? along the heating wire?
Or... should I bandage some thermocouple pairs and measure voltage changes
from temperature changes?

Wayne




.

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