> Peter Chadwick wrote:
> >A lot depends on what the meters are. If the shunt is internal and it's a
> >'standard' 100mV meter, then increasing the drop isn't very practical.
I was GM at Prime Instruments and directly headed the precision
meter department.
Standard meters stateside, and as far as I know worldwide, are 50
mV, not 100 mV.
> Many "1A" meters are actually a 100uA, 1mA or 10mA movement with an
> internal shunt. Open the meter and snip the shunt, and you're back on
> track.
100uA meters are rare, and if 50 mV, are very low torque. They
require weak springs so friction or trash in the movement as well as
balance and static are problems. So is meter response time. The
pointers move slowly. About the most sensitive 50 mV movement
with a reasonable cost and performance are the 1 mA movements.
10 mA is better if you are measuring high current, but most
amplifiers use 1 mA since the meters often read voltage.
> > A 5x
> >overload may not be a brilliant idea for the meter, but it's not normally
> >disastrous - or not as much as no protection. A Schottky rectifier across
> >the meter gets around the problem
>
> Only partly - remember that the voltage drop at surge currents of 40-50A
> will be a lot higher than the rated Vf, for all types of diode.
Most meters will suffer mechanical damage, like bending the
movement or pointer, first. The high spike from a HV fault is easy
to suppress.
> Hmmm... I thought about this long and hard. Can you see any way that a
> surge from HV+ to the HV- rail could go in the opposite direction?
I can't. All I've even done is use a diode from negative rail to
chassis.
> >Incidentally, I'm assuming that 100mV is the standard drop for moving
> >coil meters in the US.
> >
> It hardly seems to be a standard anywhere, if it ever was. High-cost
> meters (surplus of course) seem to vary considerably, but have a lower
> voltage drop than the far eastern imports which typically run 300-400mV.
The cheaper the meter the higher the mV rating. For a given
current, that means more torque. 400 mV is a standard for high
torque meters. Those meter really move fast when current is
applied, and tend to have excessive "bouncing".
> >Do people still manufacture moving coil meters, by the way, or have
> >electronic meters taken over completely?
> >
> Come on, Peter - they're still in the component catalogues, but good
> ones cost a fortune at full price. That's why they're worth protecting.
Moving magnet meters had a BOM cost of about 35 cents,
including labor, in 1980. That's why automobiles and other
consumer devices use moving magnet meters, better known as
"wigglers". Many of the automobile meters are thermal movements!
Meters of all types are still being made, the more expensive meters
have taken a hit from digital displays but odds are they will never
be totally pushed out of the market. Many applications, especially
where "adjusting" to specific values is required, work better with an
analog indicator.
73, Tom W8JI
w8ji@contesting.com
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