What 'chu know 'bout my meter?

[rave0035]

So I think I'm going to like this pedal thing.  I'm starting to amass parts and draw up plans for a test rig with lots of bells and whistles so I can get down to business this winter.

One thing I'd like to incorporate for eye candy and to confirm power on the rig is an old panel meter from a VTVM.  The trouble is, I can't find any specs on this particular meter online.

The meter is from a Jackson Model 645 VTVM.  I don't know the full scale mA rating of the movement.
When I supply 9v from a battery with a 10k resistor in series, the meter shows about 8vdc.
During this test, I measure about 46.3mV across the meter.

What do I need to know about this meter to get it to display relatively accurately in the 0-10V range from batteries and standard wall-wart power supplies?

Mike

[Jdansti]

I don't know what the current draw is, but I can say that that type of meter will be a pretty good drain on batteries if you leave it connected for long periods.  It should be fine for quick checks, though.

I could be wrong, but I think that if you're just using it across the supply voltage, there is no issue with amperage ratings (other than draining batteries as I mentioned above). It should be fine for AC/DC adapters.  Just connect it and read.

[GibsonGM]

Look at this:  http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/movcoil.html

I'd just make that 10K a pot (20K?), and use it to calibrate the meter, then replace with a fixed resistor.  Switch the meter in/out so as to not waste power.    For ammeter use, you need to look up "diy meter shunt" or some such thing...

[antonis]

For ammeter use, you need to look up "diy meter shunt" or some such thing...

+

Or measure with voltmeter the voltage drop across a 0.1R series resistor...
(and divide it by 10, of course..) 

[duck_arse]

I dunno nuffin bout chore metah, but people here and here might:

http://judyboxamp.blogspot.com.au/2011/07/wintage-parts-excursion-part-2.html
http://www.radiomuseum.org/dsp_hersteller_detail.cfm?company_id=8164

[or PRR will arrive and tell all you need to know]

[edit :] also:

http://www.antiqueradios.com/forums/viewtopic.php?f=8&t=231584
http://www.radiomuseum.org/r/jackson_el_vacuum_tube_multimeter_64.html

[amptramp]

You need to determine the resistance of the meter itself so you can use its full-scale deflection sensitivity (current) and select a series resistor that will add up to enough resistance so that the applied voltage will be shown on the meter.  An example:

Meter resistance: 500 ohms
Meter sensitivity: 200 µA full scale
Meter marking: 0 - 15 full scale

At this point you want a series resistance that will give you 200 µA at 15 volts.  E/I = R so total R is 15/.0002 = 75000 ohms.  Since the meter movement is 500 ohms, use a 74500 ohm resistor in series to get this total.

The Jackson 645 has scales that end in 1 or 4 so you can have an easily-read 10 volt or 40 volt scale.  I would suggest using the 10 volt scale (so change the meter marking to 0 - 10 volt full scale).  The 200 µA is close to correct - I have seen people get 230 µA s the full scale.  I don't know what the actual meter resistance is.  You can determine all these things and select your own resistor.

[PRR]

> meter shows about 8vdc.

On which scale?? It probably has scales from 1V to 300V. "8V" on the 300V scale is tiny; on the 10V scale it is nearly full swing.

If you want a Volt-meter, you kinda don't care about the meter internal resistance. As your test shows, with 9V across meter and practical resistor, the meter itself drops like 0.05V, less than 1% of the total. So long are you are working to several-volt scale and several-percent accuracy, the meter resistance is negligible. If you need smaller/better, you will be hand-trimming the resistor, and again the meter resistance can be neglected (it calibrates-out).

Although your 9V test with 0.0463 on meter already tells you the meter alone is near 51.7 Ohms.

My experience is that true VTVMs were *almost* always nominal 200uA movements; however this was not tightly controlled because they always had to be calibrated and a 234uA movement just meant another tenth-turn on the trimmer.

If that "8V" was near full-scale, and you want a 0-10 scale, it look like you want a 8.889K resistor. Take your 10K and tack a 100K across. Now does it read same-as your good meter? If it still reads low, tack another 100K across. If now reads high, take that 100K off and tack a 1Meg across, maybe another. If your resistors were perfect, 10K||100K||1Meg||1Meg gets you close enough for jazz (or rock or polka). With real resistors you may need a few more or less of the large resistors.

Don't go crazy. There is nothing magic about 9.355V. Any decent pedal will work from 10V to 8V just fine. What you really want to know: is it 6V (no wonder it sounds hungry) or 19V (no wonder it smokes)?

[rave0035]

Thanks to all for excellent responses!  I already love this forum, and I only joined a week ago!

The meter showed 8vdc on it's 10v scale, which is nearly full swing.  PRR - adding 100k resistors did the trick and we're right where we need to be from the battery. 

Although your 9V test with 0.0463 on meter already tells you the meter alone is near 51.7 Ohms.

Learning... how did you do this calculation?

Understood that there's nothing magic about 9v.  This is eye candy and burning through the parts pile as much as anything else, so sort of accuracy is all we're going for here.

If I ever use a 12v or 18v supply for a project, do I risk damaging the meter if it's calibrated at this scale? 

I'll post some photos once I'm done building.  Putting together a case for it so it might be a few weeks.
Mike

[PRR]

>> your 9V test with 0.0463 on meter already tells you the meter alone is near 51.7 Ohms.
> Learning... how did you do this calculation?

BASIC voltage divider.

You started from 9V (9.xxxx?V).

You ended with 0.0463V across the meter.

That is about a 194.4:1 division.

The 10K must be about 193.4 times the meter resistance.

10,000/193.4 = 51.71 Ohms.

+/- whatever uncertainty in your "9V" and "10K".

> use a 12v or 18v supply

The needle will go to "18" on a 0-10 scale.... be stuck against the top stop. Doesn't tell you much except it is over 10V.

The meter is sure to be happy in the 10V condition but will run almost 4X (3.24X) hotter with 18V across the string.

"Most" meters can live at 2X the rated current. So a 18V overload "might" not burn-up.

OTOH, if a meter is easy to get, it is hard to kill; irreplaceable meters die if you look at them sternly (Murphy's Law). So I would NOT push this guy.

For 18V supply use about 20K (maybe 18K) resistor, and read "0-10" as "0-20".

It will read your 300V supplies too, WITH the right resistor. As it appears to be a 1mA meter (9V thru 8.88K gives near full scale), a 300K resistor will make it read 0-300V; if it has 0-4 markings then a 400K resistor makes it come out nice.

[rave0035]

Much appreciated, Paul!
Mike