[Boatanchors] Help - I'm stumped!

[Gary Schafer]

Most meters that are used with large current shunts have a spec of mv full
scale. Common are 30mV, 50, 100 mV.

Think of the current shunt as a zero ohm voltage source when figuring for
the meter. It has such a low resistance compared to the meter circuit that
it can be ignored.
Then in this case find any meter that will read 100 mV full scale with a
series resistor. A meter with 50 ohm movement and a 50 ohm series resistor
etc.

73
Gary  K4FMX

> -----Original Message-----
> From: boatanchors-bounces at mailman.qth.net [mailto:boatanchors-
> bounces at mailman.qth.net] On Behalf Of Bill Cromwell
> Sent: Friday, October 05, 2018 11:44 AM
> To: boatanchors at mailman.qth.net
> Subject: Re: [Boatanchors] Help - I'm stumped!
> 
> Well I'm confised. You said it is a 100 mA meter and THEN you said it is
> a microammeter. Maybe that is the source of the trouble you are having.
> 
> Meters are spec'ed for full scale current. I have never seen a meter
> movement spec'ed for full scale voltage. For that to happen there MUST
> be additional parts even if they are mounted inside the housing.
> 
> 73,
> 
> Bill  KU8H
> 
> On 10/05/2018 12:21 AM, Michael D. Harmon wrote:
> > I have the parts of an old Simpson Model 375 ammeter.  Some time in
> its
> > checkered past, it was dropped, thrown, or run over, smashing the
> meter
> > movement beyond repair.  I decided to save the shunts and reconstruct
> > the meter in a new box.
> >
> > When I downloaded the manual, I discovered that the original meter was
> > described two different ways.  In the electrical specs at the front of
> > the manual, the meter movement is described as a "1 mA annular
> > instrument with integral shunt".  The (external) current shunts for
> each
> > range are designed to provide a 100 mV drop at full scale on the meter
> > movement.  In the schematic on the back page however, the meter is
> shown
> > as a 100 mV meter.
> >
> > If the meter is designed to be 1 mA full scale, and provides a 100 mV
> > drop, then according to Ohms Law, the internal resistance must be 100
> > ohms. I have boxes of meters, but I have NOT found a 1 mA meter that
> has
> > an internal resistance of 100 ohms!  I have measured them all!
> >
> >  If you try to use a microammeter as a millivoltmeter, using the the
> > multiplier formula (R= Efs/Ifs -Rm), most of the time you end up with
> a
> > negative number (depending on the internal resistance of the meter
> under
> > test)!  You just can't build a millivolt meter out of a microammeter
> > without some trick of mathematics!
> >
> > What am I missing here???  Was this some kind of special meter?  I
> have
> > Jim Tonne's meter face design  program, but it doesn't do much for me
> > until I find a suitable meter movement!
> >
> > Anyone know where I can find a 1 mA meter movement with a 100 ohm
> > internal resistance??
> >
> > Thanks for the help!
> > Mike Harmon, WB0LDJ
> > mharmon at att dot net
> >
> >
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> >
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> --
> bark less - wag more
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