RE: Meter Shunts

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Steve Conner" <steve.conner@xxxxxxxxxxx>

>The meter will be for measuring the current,
>but the before the shunt I would use two resistors to drop the current
>evenly since the meter can't handle the whole load. This is odd because he
>said this cannot be done, why is that? The person didn't explain to me why
>either, I tihnk they didn't know.

Again I'm not too sure what you mean.

When you want a meter on your TC control panel to measure the line voltage
and current, you go to Mouser or DigiKey or wherever and buy an analog panel
meter. (Analog means the old sort with a pointer- the digital ones tend to
malfunction around TCs) There are different kinds of meters- moving coil for
DC and moving iron for AC. The moving coil ones won't work on AC without a
rectifier and even then they can read funny.

These meters are all intended for DC, or line frequency (50 or 60Hz)
current. If you want to measure the pulsed RF current in your Tesla coil
primary that is an altogether different beast. You need some fairly complex
hardware to do that, like oscilloscopes and broadband current transformers.
An ordinary meter won't give any meaningful answer, in fact you'd be lucky
if it stayed in one piece ;)


Anyway a bit about ammeters:

The wire in a moving coil meter is hair thin and it only needs fractions of
a milliamp to move the pointer. It's not feasible to make the coil thick
enough to carry say 10 amps. So to measure high currents, a low value
resistor is connected across the coil. Something like 99.9% of the current
goes through the resistor and the remaining 0.1% goes through the coil. This
resistor is what we call the shunt.

For currents up to about 20 amps, the meters are usually made with the shunt
built in. All you need to do is connect your circuit to the two large lugs
on the back of the meter. When 20 amps flow between these two lugs the meter
will read "20" and that's it.

For currents much above 20, you'll probably find that the meter needs to be
used with some kind of external shunt. (the shunt gets too big and chunky to
fit inside the meter case) A meter meant for DC will need a shunt, and a
meter for AC will need a current transformer. There is no need for any other
resistors or anything.

You can usually buy a shunt at the same place you got the meter. If you want
to read 100 amps DC, say, then you might buy a 75mV meter with a 0-100
scale, and a 75mV 100A shunt. For 100 amps AC you would get say a 5 amp
moving iron meter with an 0-100 scale, and a 100:5 current transformer.


>Anyway before I was told that I don't need to use a shunt to measure the
>voltage nor resistors, but wouldn't the current have some effect on the
>meters coil?

If you buy a meter for measuring AC or DC voltage it almost always comes
with a "multiplier resistor" already built in the case. A multiplier is to
voltage what a shunt is to current. You buy a 0-300V meter, apply 300V to
the lugs on the back, and the pointer should show "300".

The exception is if you want to measure very high voltages up in the kV, the
multiplier resistor is always separate (again because it's too big to fit
inside the meter, and for safety reasons)


Steve C.