# [EVDL] Expanded scale voltmeter

1076 2
Having a zero-center +/- 5 volt meter I used a multiplying
resistor to make it read +/- 25 volts. Then I built a
bridge circuit consisting of two 50 volt zener diodes

and two resistors (plus a couple forward biased diodes
to trim the voltage and ad some temperature compensation).

Now it reads 100 volts at center, and +/- 25 volts at the
extremes. For my 96 volt pack I can treat it as voltage
or percentage. (100 volts +/- 25%)

It lacks some linearity at the extremes, but not enough to worry about.

My next version will use a normal (not zero centered)
voltmeter. The circuitry will be remote, sending only
about 5 volts through the firewall, fairly well isolated.

New subject:

High quality meters are often delicate, and may not last
long in a mobile environment. I look for aircraft meters

on surplus. (I remember dropping in from about 25 feet
in an Aircoupe!)

John in Sylmar, CA
PV EV

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John in Sylmar CA wrote:

> My next version will use a normal (not zero centered)
> voltmeter. The circuitry will be remote, sending only
> about 5 volts through the firewall, fairly well isolated.

A common way to make an expanded-scale voltmeter out of an ordinary
voltmeter or milliammeter is:

- Put a zener diode in series -- its voltage moves the zero of the
meter to the zener's voltage. For example, a 0-10v meter with an
80v zener in series becomes an 80-90v meter.

- put a resistor in series to increase the voltage range. For example,
if that 0-10v meter measures 10k ohms, add another 30k resistor in
series and it becomes a 0-40v meter. With the 50v zener, you now
have an 80-120v meter; just about right for a 96v pack.

- Split the 30k resistor in half; two 15k resistors. Put one at the
battery end of the wires from + and - terminals to the meter and
zener in the dashboard. Now if you touch either wire, or they get
shorted or grounded at the dashboard end, the 15k resistors limit
the current to a safe level (about 5ma for a 96v pack).

> New subject:
> High quality meters are often delicate, and may not last
> long in a mobile environment. I look for aircraft meters
> on surplus.

High quality meters are often *more* robust than the common cheap ones.
Look for "taut band" movements, or well made D'Arsenval movements.
Military and aircraft surplus ones are good, or even the ones from
quality VOMs (my old Triplett and Simpson meters have been dropped many
times and survived).

Another aspect is damping (how fast the pointer moves). Some meters are
designed for fast response, and will bounce around quickly in a moving
car. If this is a problem, put a large filter capacitor across the meter
to damp its movement.
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