old amp meter

[stingray_65]

Rummaging through some stuff this week, I found an old simpson amp meter.

face reads 0-25A DC and in the corner, in very small print it reads .625mvFS

so does this mean it is not an ammeter and is actually a voltmeter?

if it is a volt meter, I would measure voltage drop across a resistor to determine the current? like a bias probe?

if I use ohms law and have a voltage drop of .625mV @ 25A my resistor should be .0025 ohms.

so I found a wire resistance calculator and found that a 1.5" piece of 23ga wire is .0023 ohms. (close enough?)

but that seems a real small ga to pass 25A through.

so far am I on the right path? I mean the calculations and how it operates?

is 23 ga too small? what else could I use?

This is all just for funzies, I can't imagine any DC circuit (besides maybe a battery charger) that I'd use this meter on.

Ray

[PRR]

> not an ammeter and is actually a voltmeter?

It's all relative.

All amp meters lose voltage. All voltmeters leak current.

> face reads 0-25A DC and in the corner, in very small print it reads .625mvFS

So this one apparently loses 0.6V at full 25A. Since it is big-marked A, it is probably intended for a circuit >>0.6V, like 60V (or 115V) so that this loss is insignificant.

However. 25A at 0.625A is over 15 Watts. Is the meter big enuff to be dissipating 15+ Watts?

High-current metering is usually a LOW current/voltage meter and an external "shunt". In this case, something near 0.025 ohms. 99% of the current flows in the shunt, keeping the big current off the panel and meter-studs, avoiding line-loss when panel is not inline with the main line. The shunt is a large block with two large lugs and two smaller lugs. (At 10A maybe 25A they may use the same lugs for line and meter; when you get to 200A you use half-inch studs for the main line and something cheaper for the meter tap.)

Tap it with your DMM ohms setting. If the needle slams, stop. If the needle is calm, see if you can get a resistance. This with the 0.625V will tell you the actual current scale of the meter movement.

Knowing these numbers, you can create your own shunt for any larger current.

I'm suspecting it is a 1mA movement (standard low-price mechanism) and hat something like 0.63 ohms parallel will give a 0-100mA scale suitable for tube biasing. Or a rectifier and 10K resistor will read voltage on your speaker (bad 'VU' meter).

[PRR]

> is 23 ga too small?

Yes. If you only have copper, use a longer length of a fatter wire. If you are in the Iron Age, fence-wire has much higher resistance, although you will have to measure because every mix of steel has different resistance, from 4X to 50X the resistance of copper. Iron oxidation is a nuisance... actually at 15A it can become a hazard when rusty connections go red-hot. There's other metals used for real shunts, though few are laying around the shed.

There's also temperature effects, though if your shunt is not burning tissue and you are not obsessing over accuracy you don't need the exotic alloys shunt-makers use.

> 0.625mV @ 25A my resistor should be .0025 ohms.

Your dog ate a decimal-place.

[billcreller]

I haven't used fence wire on a project yet.