Question about analog meters reuse

[kagliostro]

I've two very nice analog meters, the scale is rated in Kv and one of the two pin is labeled as +, however if I connect a small battery the index goes to full scale

I would like to use it as a 200mv instrument as to read bias voltages

I remember that many years ago I've seen someone that was using a power supply and a rheostat to "calibrate" an unknown instrument as to be used that way, but I didn't remember anything as to proceed to achieve that result

Someone knows what I must do as to obtain a 200mv meter (if possible) ?

Thanks

K

[sluckey]

Look for some small writing on the meter face that tells what the full scale current is. If you're very lucky, it might read "FS = 200ma".

If no markings, you can determine the full scale (FS) by connecting a battery, variable resistor, your meter, and your VOM (set to measure milliamps) all connected in series. Adjust the pot until the meter deflects to full scale. Then read the current on your VOM.

[kagliostro]

Ciao Sluckey

thanks for your indications

on the instrument no labels about current handling ability

only: the brand + Kilovolts Direct Current + hm3 + 31369-1

I'll try to measure it the way you explained

Thanks

K

[jjasilli]

It appears that your meter is not for direct readings, but was part of an instrument, like a VTVM.  Your meter may require the use of active circuitry, like in a VTVM, to knock KV down to mere volts or 10th's of a volt, to then run through a shunt resistor across the meter lugs.  The circuitry would be designed so that the reduced voltage & current sensed by the meter would match the readout printed on the meter face.  So, I think you need to build a VTVM around the meter!

[kagliostro]

Hi Jjasilli

yes, the instrument was part of an apparatus, don't know which one

I've try with a 1.5v battery and the index go to full scale, so I hope his sensitivity is high enough for my purpose (to read 200mv in a bias meter)

I'll try to discover more using the method explained by Sluckey

K

[jjasilli]

Maybe it can handle millivolts, under 1000mV = 1V.  Maybe put its lugs across a 1Ω bias sense resistor for a power tube.  Use test leads, or a momentary SW, to protect the meter.  If the reading is on the meter's scale, then it becomes a matter of calibration.

If this doesn't work, then I think the meter is not good for your intended purpose, because any other method of hooking the meter up to the cathode circuit may be too much of a hassle. 

[sluckey]

That exact meter was used in my old radar. I never had a need to see what the full scale current is. Look underneath the HM3 writing in the lower left corner. The current should be on the meter face. You may have to remove the curved glass bezel to see it. There should be another glass underneath the curved bezel.

[kagliostro]

I tested the meter using a power supply, a pot as rheostat and my VOM

the index go to full scale with 100mv

Can I use a resistor in series as to obtain the index to go to full scale at 200mv ?

Thanks

K

[sluckey]

yes. That's exactly what was done to use that meter to indicate 10KV.

Use your VOM to measure the resistance of the meter. You need a resistor that same size to go to 200mv.

[kagliostro]

Thanks

at the moment the instruments are in my "laboratory" and so I can't read the label someone put in the back side, if I'm not wrong the label say 95ohm

if I can solve the problem simply with a resistor (may be better with a trimmer) I'm thinking to use a  switch to have more scale

As I don't build a bias meter every day, do you think I can go with 100mv - 200mv - 300mv or the 300mv is excessive ?

K

[stratele52]

This meter can't work alone , IMO  it need to be in parralle on a resistor.

[kagliostro]

The idea is to read mv,  so in the amp or in the probe, there will be a resistor in series with the cathode

the instrument will be used to read the voltage at the extremities of  that resistor

may be that I decide to increase the maximum readable mv, so as to be compatible with both resistors 1ohm and  10ohm, as also 10ohm can be found

K