# Have meter that reads volts. Would like to...

Discussion in 'Electronic Basics' started by Don Bruder, Mar 15, 2005.

1. ### Don BruderGuest

Heya folks, got a cheap car voltmeter - it reads in (duh!) volts. 6-20
of 'em, to be exact. Nothing particularly fancy or ultra-accurate, just
"a voltmeter" that'll be handy for keeping a general eyeball on the
charge state of the system.

I know that a meter that reads volts and a meter that reads amps are
often constructed around the same meter movement, as is evident from the
analog VOM I've got that can read all three values of Ohm's law without
having to change the movement - just the setting of a selector.

Problem is, I've never managed to wrap my head around the "how-to"
involved in changing from one to the other. Every explanation of the
idea I've encountered leaves me scratching my head.

So what I'm looking for here is a way to set it up so that I end up with
a dual-purpose "When left alone, it displays volts, but when <something
- button push, switch toggle, etc> is done, it displays amps" meter. I
COULD go down to the parts store and buy its amp-reading mate, but I've
got this minor problem called "lack of place to put another one". If I
can make this one "convertable", I'd like to go that route.

What do I need to do to make a voltmeter read either volts or amps on
user demand? Obviously, I'm going to need to create/calibrate an "amps"
scale for it, but that's pure trivia - Part of the "trim" when it gets
done. I need the "nuts and bolts" at this stage.

2. ### Roger JohanssonGuest

When you open the instrument and inside you will find a meter
movement mechanism and a resistor in series with it, or two series
resistors which you can switch between, if there are two different
voltage scales.

For a meter mechanism to be used as a voltmeter you need a series
resistor which can absorb most of the voltage, because the mechanism
itself can only handle something like 50-300mV.

If you want to use that mechanism as an ammeter you need a parallell
resistor which can handle most of the current, because that mechanism can
only handle something like 1mA by itself.

Inside an analog multimeter you will find several series resistors for
the voltage scales, and several parallell resistors for the amps ranges,
all switched in by a multiposition switch.

The easiest way to find out what series resistor you need for a certain
voltage range is to connect a highohm pot in series and set it to its
full resistance. Then connect a suitable test voltage in series with the
pot and the mechanism. Adjust slowly towards lower resistance until the
meter shows the value you want for that voltage.

Measure the pot and replace with a fixed resistor. Or with a fixed
resistor and a small trimmer, if you want to be able to adjust it later.

For current you take a low value resistor, like a piece of wire, or a bar
of metal. Send a suitable current through it. Put the two connections
from the mechanism together and touch the wire with them. Pull the two
connections slowly apart until you see the meter show the value you want
it to show for that current, solder the wires there and cut off excess
length of the shunt wire/metal bar.

Or learn to calculate what resistors you need
Use another instrument to measure the voltage and current of the meter
mechanism when it shows full scale.

If it has a voltage of 100mV and a current of 1mA you know that the inner
resistance of the meter is 0.1/0.001=100Ohm

If you want a voltmeter for 100Volt you must add series resistance which
makes the total resistance a thousand times higher, because 100V is a
thousand times greater than the 100mV of the meter.

100Ohm * 1000 = 100kOhm. So add a 100k resistor in series.
(that will become a total of 100100 Ohm, a little too much, but a
difference of one tenth of a percent doesn't matter)

3. ### PeteSGuest

All moving coil meters (that's what's in your analogue VOM) ultimately
measure current. for current ranges, we shunt (parallel) the path
around the meter and for voltage we add resistors in series to set the
current for the meter. Although this type of movement is common, moving
iron meters are often used for your battery / charge indicator (in fact
it probably is as it ranges from 6-20V).

The first thing to figure out is which type of movement it is, and then
follow one of the various guides to designing custom ranges.
(There are a profusion of such guides on the web - try these search
strings:
moving coil meter design
moving iron meter design
)

A couple of things

1. Moving coil meters usually have a sensitivity of 20k / volt - i.e. a
full scale current of 50uA.
2. Moving iron meters are robust (which is why they tend to be used in
the automotive industry)
3. Moving iron meters are slower to respond, but can give an average of
a fluctuating line.

So, once you've figures out the movement type, come back and someone
will no doubt give you a full guide on making the necessary
modifications for your little project.

Cheers

PeteS

4. ### John PopelishGuest

Don Bruder wrote:
(snip)
(snip)

Meters that need only a small voltage to read full scale and also need
only a small current to read full scale are well suited to be made
into a volt-amp meter. The trick is to arrange with external
resistors to have the full scale requirement to be met with the
desired full scale measurement.

Lets say you have a magnetic movement that hits full scale at 1
milliamp of current and has 10 ohms of internal resistance. Ohm's law
relates voltage, resistance and current (Resistance = volts / amps) so
when 1 milliamp is passing through that internal 10 ohms, there will
be a 0.01 volt drop across the meter.

To arrange for this meter to be a volt meter that reads full scale at
20 volts you need to add more resistance in series so that only at
that voltage will the 1 milliamp full scale current pass through the
meter. Since that occurs with 20,000 ohms (ohms law again), total and
the meter already has 10 ohms, you would need to add another 19,990
ohms in series. (Resistor in series simply add up)

If, instead you wanted to make the meter read full scale for 10
amperes, you would need to parallel it with a big, low value
resistance, such that the parallel combination dropped the full scale
voltage of 0.01 volt when 10 amps passed through both the resistor
(mostly through the resistor) and the meter's 10 ohms. 0.01 volt/10
amps = 0.001 ohm. When working with parallel resistors, it is easier
to convert them to conductances (1/resistance), since conductances in
parallel simply add up. Then convert the effective conductance back
to resistance with a 1/conductance calculation.
So 1/10 ohms + 1/shunt resistance (that carries most of the 10 amps) =
1/ total resistance
So the shunt resistance needs to be about... well crap, it is so close
to .001 ohm ohm that the meter's 10 ohms makes almost no difference.

You leave the .001 ohm shunt in series with the current, permanently,
and switch the meter to either be connected across the to be measured
voltage with a 19,990 ohm resistor in series, or connected directly
across the shunt resistor. The shunt will lower the voltage to
whatever load is in series with it by 0.01 volt when 10 amperes is
passing through.

Now back to your meter. If it already has the equivalent of the
19,990 ohm resistor already inside the case, you will not be able to
use it without doing surgery on it, since this resistor cannot be in
series when you make current measurements. You need ot start with a
simple meter movement with both a low current full scale requirement
(so the voltage dropping series resistor will not get so hot) and a
resistance (equivalent ot saying that it has a low full scale voltage
requirement) because that full scale voltage is what the shunt must
drop top measure current.

By the way, the shunt resistor may be able to be a piece or wire that
is already involved in getting current ot the load. If you can pick
two spots on it that have the desired full scale drop you can just
connect two small wires to those points and take them back to the
meter. The difficulty comes in knowing where those two points are.

5. ### Andrew HolmeGuest

details.

If this is for a car, due to the extremely heavy current, instead of
splicing series resistance into the battery lead, you might want to
consider a clamp-on magnetic current sensor.

6. ### Roger JohanssonGuest

Surfing the web a little I found this tutorial about how to make shunts
to construct an Amp meter.