Re: Meter Shunts

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Bob (R.A.) Jones" <a1accounting@xxxxxxxxxxxxx>

Perhaps this brief description will help.  You will need to understand Ohms
Law and how current flows in series and parallel resistors.

Assuming its a moving coil meter the meter actually measures the current
flowing thru. i.e. the position of the pointer is determined by the force
generated by current flow in the moving coil caused by the magnetic field
the coil is in. The force acts against a spring so the deflection is
approximately proportional to the force and therefore the current.

The coil and its connections have a particular resistance. A particular
current produces full-scale deflection (fsd) of the pointer I will call this
fsd current

To measure voltage a high value resistance is put in series with the meter
such that at the desired voltage, for fsd, the fsd current will flow.

To measure current a resistance is point in parallel with the meter to
"shunt" the majority of the current thru the parallel resistor.
The current flow in the meter is inversely proportional to the ratio of the
shut resistance to the resistance of the meter.
The shut resistance is chosen such that when the desired total current for
fsd is flowing only the fsd current is flowing in the meter.

I hope the above helps.  There is a lot more to it than the above but that
is the basics.

Probably the most important additional info is.  Meters designed to indicate
voltage tend to have there coils wound with many turns of fine wire so they
require for example only 50uA of current to produce full scale defection and
are not ideal for measuring current because they would require  relatively
high resistance shut that would have to dissipate a lot of power.

Meters designed for measuring current tend to have only a few turns of much
heavy wire so require for example 50mA of current to produce full scale
deflection but have a much lower resistance and so require a much lower
resistance shut to measure a given current.

Robert



----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Thursday, February 17, 2005 10:30 AM
Subject: Re: Meter Shunts


> Original poster: dave pierson <davep@xxxxxxxx>
>
>
> >Anyway what I find rather odd is that I called up a store and ask if I
can get
>
> >a meter and then shunt it.
>
>     It can be done.
>
> >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.
>
>     I would describe it differently.  Use of words to describe this is
>     tricky.
>
> >This is odd because he said this cannot be done, why is that?
>
>     He may not have been familiar with shunts, many people aren't,
>     Or he may not have understood the description.
>
>
> >The person didn't explain to me why either, I tihnk they didn't know.
>
> >Anyway before I was told that I don't need to use a shunt to measure the
> >voltage
>
>     'shunt' is a term used when discussing current meadurements.
>     (When extending a voltmeter range 'multiplier' is the word...)
>
> >nor resistors, but wouldn't the current have some effect on the meter's
coil?
>
>     Is current or voltage being measured?
>     Both?
>     (When using coil typoe meters, there is always some current
involved....
>     the discussion can get complicated....)
>
>     best
>        dwp
>
>
>