Re: Volt meter vs Amp meter

Posted by Joe on April 22, 1998 at 13:26:47

In Reply to: Volt meter vs Amp meter
posted by Joel Rapose on April 22, 1998 at 12:25:48

: I would like to replace my amp meter with a volt meter...does anyone know where to find a small volt meter that would be the same size as a Model A amp meter


This will surprise you but your amp meter is actually a voltmeter.

What Ford (and most others) have done is use a d'arsonval meter movement (i.e. a low voltage voltmeter) in parallel with a shunt resistance (likely a steel or possibly a copper slug) and use the meter movement to determine the voltage drop across the shunt resistance.This voltage drop is small but proportional to the current flow and when connected in this way the small range voltmeter becomes a large range ammeter.

What you need to do is remove the meter movement from the shunt, discard the low resistance shunt, and provide a resistance of the proper value in series with the meter movement to read a higher voltage.I would imagine a small but very high ohm resistor would be required, probably a 25 cent component.

There are two ways to determine the proper resistance.

You can do this with some experimenting with different resistors against a standard 6 or 12 volts (whatever you have).It may be possible to use a variable resistor for this experiment on which you use a volt-ohmmeter (VOM) afterwards to determine the actual ohms.Then go buy the standard resistor of the same value to replace the trial resistor.This is the experimenter's way to do this.

You can also use the full scale reading shown on the face of the dial and the actual resistance of the shunt (if it is known or determined by a VOM) to calculate the ohms-per-volt constant of the meter movement.Once you have this you can calculate the series resistance to get a full scale deflection of the meter with 6 volts.But this is the electrical engineer's method to do this.

I will admit that my description above is a bit technical but the possibility exists to use the same meter in the same hole and do it for less than $1.A cheap VOM is available at Radio Shack for less than $10 and once you own it, you'll be amazed at the other neat uses for the meter.

Someone familiar with electronics should have no problem with playing around with the meter and getting it to the scale you want.

Another factor you may want to consider is that you won't be able to connect the voltmeter to the same wires as it was connected before.In fact the two wires that formerly were connected to the thumbnuts will now likely be connected to each other.

You will probably want the addition of a third wire from this new connection to one terminal on your "voltmeter" and the other terminal on the "voltmeter" should be connected to car "ground" to actually measure battery voltage.I would suggest you not ground to the gas tank or instrument panel in the interest of safety (imagine if the "voltmeter" should short circuit somehow - all that current going through your gas tank.Shades of Flight 800!)Instead go directly to the car frame if you can.

Also, you may want to consider to place a fuse or fusable link in the circuit where the ammeter used to go.It is thought by some that the Ford ammeter doubled as a fuse in the event of a short circuit somewhere in the system.The "hot ammeter" you comment on is the precursor to melting out of the low resistance shunt under heavy load.The rating value of this new fuse should probably be the same or slightly larger than the full deflection value of the your former ammeter.

I hope you are not overwhelmed or daunted by all of this.Technically it's not all that difficult and I enjoyed the mental gymnastics necessary to its consideration.But I certainly understand if you decide not to confront the technical challenge.I myself continue to use the meter as it was originally configured and have been content with it as such.But there are those who consider a voltmeter as a more accurate indicator of battery charge - almost like taking a hydrometer reading since a direct correlation exists between voltage and specific gravity.

Wherever you end up, have fun.

Best regards,

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