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Re: DC meters with AC Re: Ignition coil (anti-parallel pairs)array and other questions
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> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: DC meters with AC Re: Ignition coil (anti-parallel pairs)array
and other questions
> Date: Wednesday, July 21, 1999 10:26 AM
>
> Original Poster: "Bill Noble" <william_b_noble-at-email.msn-dot-com>
>
> for both meters - a single diode will work, but I agree, a simple bridge
> rectifier (full wave) is best - that is what you find inside analog VOMs.
>
You'll need a bridge for the ammeter for sure, unless it is a very
sensitive meter (say a 50 uA unit). Consider a case where the basic meter
is a 5A full scale (these are fairly common, with an internal shunt for use
with a current transformer), and you want to make it 50A full scale. The
shunt would be 1/9th the resistance of the meter (and it's internal shunt,
if any)
Say you put the diode in series with the whole assembly, meter and all...
The diode would a) have to carry the entire load current; and b) would turn
the line current into pulsating DC.. arghhh
OK, say you put the diode just in series with the meter, and the shunt is
outside the diode/meter combination. The meter will work just fine,
except... On one half cycle you'll have a series resistor of X milliOhms
and on the other half cycle, you'll have 1.1X milliohms. Now you are
drawing asymmetric current (a small amount, I grant you), but there will be
a DC component in your transformers, inductors, etc....
Bridge rectifiers are just too cheap to not bother with.
I agree that you don't need a filter cap most of the time. The analog meter
movement is a pretty good low pass filter. I've been fooling around a lot
with those little $10 LCD meters and was thinking more about those, than
the more traditional ones.