Combination Volt /Amp Meter

[JohnC]

I'm seeking help trying to build a combination voltage/amp meter that can be determined by pressing a DC voltage button or a DC amp button.

I came across a Beckman analog meter. Scale on front reads, 0 to 15 and 0 to 50.

I would like to use the 0-15 for voltage and the 0-50 for reading amperage.

On the back of this meter is 4 brass pegs that read 15,50,100 UA and +C.

On the front of the meter it reads, "FS=100UA,15-50MA".

Could some one show me a simple schematic so that I could try building this.

Your help would be greatly appreciated.

THX,

John

Ottawa, Canada

[TomW]

John;

Well, I can offer some terms to search for:

"current shunt"

"voltage divider"

All very simple to do but not something I want to try to describe in text either.

I did a story or Diary [forget which] on this forum about homemade shunts and matching them to the meter you have. It would get you going on how to get a handle on the ammeter part. I couldn't find it with a quick search, but it is here some place.

The voltmeter part probably just needs a resistor or 2 to drop the voltage to the right value at the meter. "C" is probably "common" and the others go to the meter based on what you have for "sense" voltage. Never had one with multiple taps but the principle is the same for all meters. "FS" means "full scale" and means when you feed it that value it will be full scale reading. Do not put any "normal" voltage across it mv is millivolts [.0001 volts] or microvolts [the u looking thing and is .000001 volts]. Anyway others may jump in with instructions but I don't have the energy now.

Good Luck.

Cheers.

TomW

[jimjjnn]

Sounds like a job for Flux. If he can't give you the answer at least he usually can tell you where to find the answer.

[SparWeb]

I know of someone who is selling some old wattmemters from a university lab clearance.  They aren't really expensive, and you could take one apart to figure out how to replicate its workings.  Mind you, the one I bought from him looks so nice, I don't dare take it apart!

You can reach him at http://www.eece.ksu.edu/~gjohnson/

or by e-mail at gjohnson@ksu.edu

[Nando]

The meter has a circuit that is shown on the left wit 2 resistor one called R1 15 and the other R2 50, which for your circuit is not needed.

The meter is a 100 microamps for full display that corresponds to a meter with and internal resistance of 1000 Ohms ( 0.1 volts display).

The added circuit requires 2 resistors, for the 15 volts you need 149 kilohms @ 1 % or better precision that with the internal meter resistance of 1000 ohms makes the total of 150,000 ohms ,

For the amp meter side you need a resistor with a value of 0.002 Ohms ( two milliohms) at least with 25 watts dissipation.

The only form I had to make was as a .Jpg I hope is the proper size.

Nando

[Flux]

Nando has given you the basic details. The only thing I can add is that I think the thing as it stands has a "universal shunt" inside. The meter movement is 100uA but the shunt brings it to 15 and 50 mA.

Without seeing a circuit of the internal connections I can't tell what they have done.

I suspect the actual movement is below 100 uA and the shunt brings the first step to 100UA, in which case you just use the common and 100uA terminals.

If they have based on a true 100uA movement then there must be a way of breaking the internal shunt to obtain the 100uA figure.

Try the voltage series resistors that Nando gave you on the 100uA and common terminals. If it reads significantly low on volts then you will need to break the shunt connection inside.

Volts should work out accurately, the actual movement resistance may be rather nominal and you may have to play about with the shunt for accurate current. Most likely you will need to make the shunt a bit low and file it to get what you want, I doubt that you can measure its value accurately anyway unless you have a Kelvin bridge.

Flux

[mikeyduk]

Nice article on shunts and multipliers.

http://www.jaycar.com.au/images_uploaded/shuntmul.pdf

The watermark fades nicely on printing.

Low ohm meter.

http://members.tripod.com/~baec/DEC90/ohmmeter.htm

  Regards

          Mike

[JohnC]

Thanks brothers,

I got something to work with.

[Nando]

John:

The circuit that I have shown as the original part is quite common that most to the time is used in instruments for teaching.

I do have several of those types, some are 100 UA some are 50 UA basic meter.

They do not have a shunt across the basic meter to reduce its current, the front display is telling you that the basic meter is 100 UA.

I have shown you some of the circuits that are added by teachers to the basic meter to extend its capabilities.

Nando

[Nando]

I went to my work bench and examined 2 of those ammeters specially one that had the same values.

There are 3 resistors, one for 50,one for 15 and another one 1000 ohms then the meter, all in a loop.

My circuit failed to indicate that the 50 MA resistors loops back and connects to the Common of the meter. VEEERRRRYYY SSOOOORRRRYY for the error.

The meter is a 50 UA full scale and my meter shows 50 UA display.

So there are two ways to do what you want:

FIRST CASE, one removing, or opening the resistors current sampling chain or

SECOND CASE leaving they like the are.

First case : series resistor for the 15 voltage range needs to be increased to 299 kilo-ohms, INSTEAD of the 149k and for the SHUNT a standard 0.001 Ohms 50 amps shunt, INSTEAD of the 0.002 ohms, I indicated.

Second case: leaving the current loop connection as they are now and the values of the two resistors as I indicated.

Nando