"Use only with external resistor"

[LeftyStrat]

Bought this old school voltage readout off of eBay to stick in a bass amp I want to build. On the back is a label "Use only with external resistor."

I assume this is to limit current? Any ideal what value I should use?

[tele_player]

A meter like that would normally include specs for its internal resistance and current required for full swing. You'd use those specs to calculate the series resistor required to limit the current.

With a bunch of resistors, a DC power supply, and a known accurate multimeter, you could figure it out experimentally.

[LOUDthud]

Don't use an ohm meter to measure the movement's resistance. Some DVMs will damage the movement.

[tele_player]

Yes, I suppose an ohmmeter might push too much current. I'd put a large resistor in series with the meter (start with megs), and put a small DC voltage across, and figure out how much current generates full swing. and the internal resistance of the movement.

[skyboltone]

Most meter movements are 50uV full scale. Soooooo. If you want say 100 volts to swing the meter full scale you'll need to put a resistor across the screws on the back that drops 50uV when 100 volts runs through the resistor. In order to do that you need to figure a load of course. If you're trying to measure the plate voltage on a class AB 1 or 2 the load is all over the place and you won't get an accurate reading. It's much easier to use a amp meter on plates because if your full roar plate load is (say) 210 mills you use ohms law to figure what size resistor drops 50uV at 210 mills.

I'm sure there must be a way to do the volts thing on plates too. Isn't there a multimeter schematic in the RCA tube manual? Check that out and see what sort of network makes it not load dependent.

[tele_player]

Meter movements are ammeters, it's the other parts you add to them which make them voltmeters.

[mhuss]

Not to be argumentative, but all coil meters are *voltmeters*, that's why they're rated for N volts at full deflection. One typically adds shunt resistances to make an ammeter (the voltmeter is then measuring the voltage developed across the shunt).

So, you add series resistance to vary the voltage measured, and shunt resistance to vary the current measured.

The limiting factor is the physical meter itself, as it can't measure a lower voltage then it's sensitivity rating, and it starts to become a significant parallel load at high enough shunt resistance.

If you have the settings wrong and apply 100v directly to the movement, you fry the voltmeter! None of this namby-pamby DVM input overload protection.

--mark

[tele_player]

Not to be argumentative, but that's not what it says in the textbook I'm checking. ( Electricity and Electronics, A. Mottershead, (c)1982)

A bare coil/magnet movement is a galvanometer, whose deflection is determined by the amount of current passing through the coil.

Everything else follows from that.

Anyway, the marking 'use only with external resistor' suggests it's a bare movement. Maybe it is, maybe not. That sticker doesn't look original.

[CaseyJones]

Not to be argumentative, but that's not what it says in the textbook I'm checking. ( Electricity and Electronics, A. Mottershead, (c)1982)

A bare coil/magnet movement is a galvanometer, whose deflection is determined by the amount of current passing through the coil.

Everything else follows from that.

Anyway, the marking 'use only with external resistor' suggests it's a bare movement. Maybe it is, maybe not. That sticker doesn't look original.

In the words of the inimitable Mr. "T"...

"Pity the fool!"

I've never known Mark H to be incorrect in Matters Electronic. Maybe this is the first time. Say it ain't so!

[skyboltone]

It's a 50uV movement. Every one I've ever seen is. Well, regardless. Attach one screw from your meter to ground and the other to the wiper of a 1 or 2 meg pot. Attach the far end of the pot to a known voltage and dial in the proper reading on your voltmeter. If you want to be extra careful start with a couple of extra meg in series there. <edit> no, this is wrong> You need a voltage divider right. Take your two meg pot and put one end on the voltage you want to measure and the other end on ground. Put on screw of the meter on ground and the other on the wiper. Start with full resistance. It will be touchy. You could start with a couple meg above ground on the pot, I think. 16 meg drops 50uV at 800 volts


Measure the series resistance that you need and use a fixed resistor. You are just trying to drop 50uV across the resistor at full scale deflection.

That's what I'd do anyway.

[mhuss]

I've never known Mark H to be incorrect in Matters Electronic.

If that's the case you haven't been reading enough. We all make mistakes, myself included!

The deflection of a galvanometer (an analog meter movement) is indeed caused by the current going though it. However one could argue that you need a certain voltage across it to cause that current to flow.

In any case, I stand by my assertion that you add series resistance to adjust the voltage measuring range and parallel resistance to adjust the current measuring range. You may need to do series/parallel to get the most accurate readings in low voltage/low current situations and/or get a certain desired measuring range.

--mark

[tele_player]

mhuss: now you're getting it. Nice try about the voltage Of course, yes, you'd have voltage across the meter if current is flowing, as a result of the meter's internal resistance. But it's the current which produces the magnetic field.

skyboltone: Yes, that's the procedure. No, it's still not about voltage. It's about the meter's full-scale current, and internal resistance, as I mentioned in my first response on this topic.

And I make mistakes, too - but when I cite a textbook, I'd bet on me.

[CaseyJones]

I've never known Mark H to be incorrect in Matters Electronic.

If that's the case you haven't been reading enough. We all make mistakes, myself included!

Yeah, fooey! You're the "go to guy" for reliable answers now. Tag, you're it!

There are a couple other "experts" who apparently are never wrong, never have been, never will be. If we learn from our mistakes one might infer that "they" don't know nuthin'!