To: mail11:;-at-cimcad.enet.dec-dot-com (-at-teslatech)
From: I am the NRA <pierson-at-cimcad.enet.dec-dot-com>
Date: Sun, 26 Mar 95 10:59:21 EST
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I do a bit of meter hacking. An AC Ammeter, depending on type (I can think
of four off hand) will give varying kinds of indications.
1) Rectifier type: dunno if exist commercially for 10-50W level. Should
be fairly accurate up to the 100s KHz.
2) Moving Iron Type:
Common, accuracy will degrade away from design freq, usually 60Hz (in US). If
used only to peak a system, more or less irrelavant, since the peak is all that
matters. Typically meter will read low at higher freqs. that is 10A indicated
might be 15A actual.
3)Repulsion type: Essentially same comments.
4) Thermocouple type:
Should be NICELY accurate, since that is what they were designed for. Nice
side effect in that meter can, with care, be mounted remote from base of coil,
with "dc" leads run out to meter. (There is some detail design involved...)
I would think in terms of running the leads inside a hollow, grounded tube.
Nasty tendency to be fragile (blowing of heater). If used, esp in power work,
i would provide fuses/gaps/etc to protect.
5) Hot wire ammeter.
Obscure, these days. If calibrated at the frequency, thould be nicely
accurate. Hard to remote.
Any of them will be ok at typical coiling freqs IF CALIBRATED AT THOSE FREQS.
(Note that calibration may require large currents, it does not need to involve
large POWERS. The trick is to get a standard, at the frequency, to calibrate
AGAINST. A nice 1 ohm, noninductive resistor (or 0.1, depending on current)
Any of them can be remoted by use of shunt or current transformer. Care in
design & protection, esp protection, of shunt.
(Some auto gauges (fuel, voltmeter esp) are hot wire. The economy minded might
look at those for sources, if feeling experimental.)
(I can expand on this, if there is interest....)