Weston Ma Meter

[stokes]

This is a bit off topic,but I figure maybe one of you guys would know.I have an old Weston Panel meter that reads dc milliamps.It has 2 scales on the face,one reads 0 to 5 ma's and the other reads 0 to 50 ma's.As it is right now it reads on the lower 0 to 5 ma scale,too low for my needs.Does anyone know what it would take to get it to read on the higher 0 to 50 ma scale?The posts on the back have a .001 capacitor across it,I have checked it with and without this on it and it reads the same.On the lower right hand side of the face there is FS=1MA printed and right below that it says 5000 ohms.I want to try to use this on a tube matcher I have so I dont have to hook up my meter to see what the tubes are drawing.I assume I need some kind of resistor network between the terminals to get it to read the higher scale.Anybody got any idea of how to acomplish this?

[tbryanh]

From "Make Your Own Shunts" http://www.uoguelph.ca/~antoon/gadge...ts/shunts.html
we get this formula

Rsh=Rm / (n-1) (Eq. 2)

For the 0 to 5 ma scale
Rsh = 5000 / (0.005 / 0.001 - 1) => Rsh = 1250 ohm
This means you must put a 1250 ohm reistor across the meter.

For the 0 to 50 ma scale
Rsh = 5000 / (0.050 / 0.001 - 1) => Rsh = 102 ohm
This means you must put a 102 ohm reistor across the meter.

"FS=1MA" means that the meter requires 1 milliamp of current to move the needle to a full-scale reading.

As your meter is now, it does not have a shunt resistor across it, so it is reading 0 to 1 ma.

Expect your meter to drop about 5 or 6v across it when it is reading full scale.
0.005 x ~1250 = ~6v
0.050 x ~102 = ~5v

The 0.001 capacitor is probably used to bypass high frequencies across the meter. The meter movement uses an inductor coil to deflect the needle. The inductor opposes high frequencies, and the capacitor keeps this from being an issue.

[Tom Phillips]

Hi Stokes,
I have changed the response of those old meters just as you describe.
You need to add an external shunt. It appears that the meter already has an internal shunt that is changing it from the native 1ma FS to the 5ma full scale.
In this case I think it's easiest to just experiment with the shunt value until you get the scale you want. That will take into account all the characteristics of the meter and the internal shunt. I'm not sure what resistance value I'd start with because the 5000 Ohm making sees high to me. After the first try you will have a good idea of the shunt resistance value required. I have a 1ma FS meter here that I changed to 1A FS. For that I used a short length of nichrome wire for the shunt.
Regards,
Tom

[stokes]

Okay,so I want to leave the internal shunt as it is or should I open it up and remove it?When I read the dc resistance across the meter at the connections on the back I read 4.9k,that would tell me the internal shunt is 5000ohm as it says on the face,right?So if a 5k takes me from 1ma FS to 5ma FS,if I want 50ma FS,my target shunt value would be about 500ohms,and I need to parallel a resistor with that 4.9k resistance I am reading so I read about 500ohms.If that is the case,am I correct in assuming that if I add a 500ohm resistor between the meter posts I should be in the ballpark,since the current will take the path of least resistance?Or am I missing something here,Tom?

[Tom Phillips]

Sounds good to me. I'd just give it a try and crank it up slowly on the test bench with your own current meter in series as a reference. I see no need to remove the internal shunt. Just add an external shunt in parallel.
T

[stokes]

Thanks Tom,that did it.I just took a 500ohm rheastat connected it across the meter,had the tube set at 25ma's in the matcher and dialed in the rheostat till I read 25ma's on the meter and measured the 104 ohms on the rheostat,just happened to find a 100ohm 5watt that read 104ohms,pefect.Re-reading your post tbryanh,you hit it right on the nose,I guess those formulas do come in handy sometimes,Bob.

[stokes]

"As your meter is now, it does not have a shunt resistor across it, so it is reading 0 to 1 ma."
tbryanh,that statement threw me off,because the meter was reading 0 to 5ma's not 0 to 1ma,it had an internal shunt of 5k,but the 104 ohm resistor in parallel with it did put me on the 0 to 50ma scale.

[tbryanh]

The 5000 ohms printed on the back of the meter refers to the DC resistance of the inductor coil that cause the meter movement, not to some internal shunt resistor.

If the meter has an internal shunt resistor for the 5 ma scale (1250 ohms), and if you removed all the external resistors, capacitors, etc. from the meter and measured the resistance of the meter, you would read 1000 ohms, 1 / (1/5000 + 1/1250). (You must be careful when measuring the resistance of the meter. You can damage it. The link explains that you should not use an analog meter.)

If there is not an internal shunt resistor, you would measure 5000 ohms.

If the meter had an internal shunt resistor for the 5 ma scale (1250 ohms), then you would have needed to use a 111 ohm resistor to make the 50 ma scale work 102 = 1 (1/1250 + 1/111).

Because you used a 105 ohm resistor, it appears there is not an internal shunt resistor.

When there is not an internal shunt resistor and no external shunt resistors, "FS = 1MA" means 1 ma of current deflects the needle all the way. This means your meter reads 0 to 1 ma. So if you were reading the 5 ma scale in this situation, 5 ma would equal 1 ma, 2.5 ma would equal 0.5 ma, etc.

It appears when you first tested the meter you were able to peg the scale easy. You concluded you were on the 0 to 5 ma scale as a result. In reality, it appears you were on the 0 to 1 ma scale for which the meter does not have any markings for.

[tbryanh]

The reason you required a 105 ohm resistor instead of a 102 ohm resistor is because the permanent magnet inside the meter has lost a little bit of its magnetism.

This means you need a little bit more than 1 ma to get the needle to deflect full scale. The 105 ohm resistor bypasses a little less current around the meter than the 102 ohm resistor would. This provides the little extra current the meter needs.

Magnets lose some of there magnetism over time. This is well known. Vibration and stray magnetic fields from AC power supplies, etc. cause this.

Old guitar pickups that have lost some of their magnetism is what helps give them their "vintage" tone.