VU meters

[artv]

Hi all , I'm not sure if this is the proper spot for this Q....I salvaged some anolog meters out of an old cassette recorder,thinking I could put them in series with some leds to measure current.(duh).  It didn't work,  but I noticed that it slowed the discharge rate of the capacitor bank and also limited the output, regardless of stored voltage on the caps.  The discharge rate  and the limited output: Does this have to do with the size of the coil inside the meter   If the coil was bigger would the discharge rate be slowed even more??? The only markings on the meters were engraved on the plastic face  M-18B, and an ink stamp 72L .......if that means anything......... just wondering if this could be scaled up? .............thanks for any input..........artv

[Madscientist267]

You need a shunt to use them as ammeters.

They will work just fine, but you will need to know a few things first.

1 - Voltage across the meter to hit full scale. This is in the mV range; 50-100 is typical.

2 - The maximum current you need to measure.

3 - Then calculate the resistance of the shunt so that the voltage drop across the shunt and the full scale meter voltage line up.

4 - Mock it up to test it.

The higher the current, the lower the value of the shunt. Above several amps, you can get away with using just a length of solid copper wire (like the ground from some Romex).

To connect the meter to the shunt, connect the negative side of the meter to one end of the copper wire. The positive will go somewhere along the length of the wire toward the other end.

To calibrate it:

Tie a calibrated meter in series with your homemade meter and slide the positive meter lead along the copper wire. When the meter matches the range you intend to measure, solder it at that spot on the wire.

To achieve lower currents at full scale, the positive lead from the meter will go further away from the negative.

If the wire ends up being of any significant length, you can fold it in a zig-zag pattern and glue it to a substrate to make it more compact. Just don't let it short out on itself, or you'll throw the calibration.

Voila! Mark the scale appropriately and put it in a case!

Steve

EDIT - In case it's not immediately clear, the terminals to your new ammeter are on the outside of the copper. Like so:

   (-) ------X---------------------------------X----- (+)

The X's represent the connections to the meter movement.

[ghurd]

There are different kinds of analog meters.

If the needle went off-the-scale when powering the LEDs, it was intended to be used with some kind of external shunt.

Typically:  The slowed discharge rate is probably because of simple resistance.
The coil has a gazillion turns of microscopic wire, which means quite a bit of resistance.
The added resistance means the LEDs used less amps, and the cap voltage lasted longer.

Anticipating the next question:  "But the LEDs stayed on much longer, and were nearly the same brigfhtness..."
Dropping the current from 20ma to 10ma is not a whole lot different on a bench being rated by human eyes.
Same with going from 10ma to 5ma.  Or 5ma to 3ma.
20ma to 3ma is a big change, but it is less noticable when it happens in steps.

G-

[Flux]

If you had put this in series with a circuit having significant energy it would have burnt out but with your limited source you probably haven't damaged the meter.

It will measure current with a suitable shunt in a reasonably normal circuit but at your low power led level its volt drop will most likely be too high for you to be able to make much use of it even with a shunt.

Moving coil meters intended for ammeters have a typical fsd of 75mV. the VU meter will be fairly high resistance to give the characteristic needed for its original use, it may have several volts fsd.  It may have an internal series resistance that could be removed but you will need to know a bit about meters to make it into a useful ammeter.

Flux

[artv]

Mad267, thanks for that I'll have to reread it several times..........G ,thanks...so if the meter was made with thicker wire, it would have less resistance , meaning  the caps would drain quicker, and the led's draw more amps???........Flux ,thanx...."significant energy"..again thicker wire, bigger caps, to handle bigger #'s......to avoid "burn out"...........artv

[rossw]

Mad267, thanks for that I'll have to reread it several times..........G ,thanks...so if the meter was made with thicker wire, it would have less resistance , meaning  the caps would drain quicker, and the led's draw more amps???........Flux ,thanx...."significant energy"..again thicker wire, bigger caps, to handle bigger #'s......to avoid "burn out"...........artv

OK, just to clarify..... because reading this I *THINK* you've missed a key part.

If you installed the meter in series with your LEDs (and their driver or current-limiting resistor), then you will inevitably be adding *SOME* more series resistance. Even if the meter had very thick wire, it would still add some (small, possibly insignificant) resistance. No passive component you add in *series* with with the LED and resistor will make your battery/capacitors discharge *FASTER*.

(That said, a thicker wire, or shunt across your meter will make things discharge faster than if you used the existing meter alone, because it has a higher resistance. Does that make any sense?)