vu meter question

[mich]

hello again.
i found a chip meter wich i have no specs on, and would like to try and use with the ssl clone.
i know its a DC meter cause its marked "+" on one of the pins.
the scale shows numbers from 1 to 10.

i found this:

Quote:
Aah, mystery meters...

Well, when I get some unknown meters I usually set up a little jig. The chances are they are "straight" DC meters. These can come in any range from 100uA to 1mA. These are the basic "movement" sensitivities- any higher and you can bet that they're a basic 100uA/500uA/1mA movement with a "shunt" resistor added in parallel with the movement...

To test for the sensitivity, it's always safest to assume it's a 100uA meter. Things you need for testing: DMM set to 2000uA range, 1M LIN pot (or a fixed resistor), 1.5V battery, other value fixed resistors.

Connect the +Ve wire from the battery to the +Ve lead of the DMM. Then connect the -Ve lead of the DMM to the end-track terminal of the 1M variable resistor. Then connect a 1M fixed resistor to the wiper of the 1M pot. Connect the other end of this 1M fixed resistor to the "+Ve" terminal of the unknown-meter to be tested. Finally, connect the -Ve terminal of the meter to the -Ve terminal of the 1.5V battery.

Okay, before connecting the battery, make sure the wiper of the pot is hard-against the end of the track not connected. The pot is being used as a series resistor, and setting it here means that the full 1M of resistance is in the circuit. Connect the battery, and then slowly rotate the 1M pot. You'll see the needle of the meter move. If it appears to "peg" backwards- swap the connections to the meter- it's polarity is reversed, and the meter could be damaged by applying more current!

There will be a setting of the pot where the needle will point to the max. position on the scale- this is the Full Scale Deflection, or FSD. The value of the current flowing through the meter will be the current registered on the DMM. If this setup doesn't move the needle much, it obvioulsy has a higher FSD than 100uA. Swap out the 1M fixed resistor for 470k, 100k, 50k, 10k, 4k7, 1k etc and adjust the pot until you get the FSD. The series resistor helps to protect the meter in case you go "too far" with the pot!

tried it, and the meter didnt move even with lowest setting of resistors.

but, when i atached it directly to the dmm,measuring resistance (200K scale) it floats to the middle point more or less.

and when i fed it a sine wave of 780mV it went directly to number 10 on the scale.

any ideas? thanks

 

[mich]

i found the answer....

after conecting the meter through a 15k resistor to the 1.5V battery, i got to the full scale in the meter...and then calculated I=E/R (1.5/15000=0.0001) = 100 uA.
if u feed it 100 uA it will touch the full scale point....
the fun of learning.... :roll:

 

[PRR]

> i know its a DC meter cause its marked "+" on one of the pins.

No, they use the one meter-case mold for all meters, AC or DC. Normally you know what you bought, and if it is an AC meter you know to ignore any polarity marks.

BTW, most AC meters read DC just fine. Quick-check: tap an audio signal. An AC meter will dance up with the music. A DC meter will just jerk around the zero mark (and if it jerks hard it will jump out of its pivots). Or if you already rigged that fine DC test, after you find the full-scale calibration, reverse the battery (or meter). A DC meter will now try to swing "below" zero and bang the peg. An AC meter will read the same either polarity. (There may be a few AC meters with half-wave rectification: these will read DC one way but always zero on reverse polarity.)

Thanks for posting the meter-test process. Are you all sorted?

 

[mich]

thanks for responding PRR :grin:
yes, im sure its a dc meter now,cause the niddle jumps back below 0 when i conect it the other way around...
the only thing i didnt figure out yet, is why does it say to shunt a 330R resistor for a 100uA meter?(in the ssl schematic) :roll:
thanks again

 

[gyraf]

To make an attenuator with the 2K series resistor.

 

[mich]

yes,but why 330? i expected a number 3 zeros larger or smaller....
for the diferences between 1 mA meter and 100 uA meter. ..

 

[gyraf]

I guess it's experimentally found. 100uA/1mA is a factor 10. Take into account DC resistance of meter.

Try it.

Jakob E.