Have ammeter in need of shunt

[JohnC]

Hi every one, I think this is a good deal for this ammeter, that I picked up on ebay for $5. Tis a Weston 0 to 300 DC ampmeter, model 45.

Not sure of the manufacture date but was told early 1900's.

On the meter it reads, "use with external 50 MV shunt".

Here's some links to some images.

http://www.otherpower.com/images/scimages/3724/Ammeter1

http://www.otherpower.com/images/scimages/3724/Ammeter4

http://www.otherpower.com/images/scimages/3724/Ammeter3

I would like to know how I could make a 50 mv shunt, and could it be possible to make this a 0 to 150 amp meter.

Not great at this electrical stuff so I need to be spoon fed,.....still.

Appreciate any expertise advise.

Thanks,

John E Cox

Ottawa, Canada

[DingBat]

You could buy a 150A 50mV shunt

But I would not, the 50mV is for full movement on the meter, so unless you plan on replacing the scale, that is not what you want. If you want to use it with the current scale, you need a 300A 50mV shunt or a 150A 25mV shunt(not 100% about the 25mV).

You can make your own shunt from wire.

For 50mV at 300A, you'd need to get a resistance of .00016666666667 ohms.

resistance =(.05V / 300A)

4 AWG copper wire has a resistance of .000292 ohms per foot or .00002433333334 ohms per inch.

.00016666666667 ohms / .00002433333334 ohms = 6.8493151 inchs

So if you wire the meter so it has a roughly 7 inch piece of AWG 4 wire between the two wires going to the meter you should be pretty close.

[Drives]

John:

You have purchased a beautiful instrument, at a steal I might add!  IMHO Weston and Simpson were the best analog meter manufactures, and all their equipment was built with craftsmanship in mind.

You will need a 300amp 50mV shunt, not a 150amp 50 mV.

Don't let DanB see this...he collects this type of fine "antique" electrical equipment.    

Nice Find.

[JohnC]

Thanks guy's for the info. I wanted to use this for my windmill and as you can see the scale is quite large. This is why I wanted to adjust the scale to 150 amps max.

Playing around with a 12.8 volt 30watt headlamp I measured 2.2 amps on my digital meter. I then tried different lengths of 4awg wire to where my weston analog meter read close to 4.4 amps., roughly on this meter. This makes more usage of the scale.

I think.

Correct me if I am off base here.

Many thanks,

John E Cox

Ottawa, Canada

[Nando]

NICE display indeed.

There several ways how to make the 150 amps shunt.

You get 3 each 50 amps, 50 mv shunts and place them in parallel VOILA 150 amps shunt.

Also 1 each 100 amps with 50 amps, 50 mv shunts in parallel: VOILA 150 Amps shunt.

In either case you only read one 50 amps shunt or even you may parallel the readings point with equal wire lengths.

Also one can use a shunt, let's say 100 amps and with some other known calibrated ammeter in series, you generate certain, let's say 75 amps and you add a resistor in series with the ammeter leads until it reads 50 amps, there you have the display set for 150 amps, ( the shunt will be able to carry the 50 additional amps).

Again if interested let me know to give the steps necessary to attain such job.

To make an accurate shunt you need precision equipment and and alloy that has zero temp coefficient.

If you really are going to get the shunts and need to know how to parallel them, let me know.

You do not need to change the display, one can apply a multiplier factor like 1/2

to what the display say.

Nando