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New photo shows meter operating with wall wart and resistors on ac power

enter image description hereI have a large Weston Station Meter 0-300 VDC made around 1901. Made for a power generation plant, inside of the housing was some sort of mica wafer assembly wired in series with the +binding post. I would assume that this was some sort of resistor, but it is no longer conducting current. Applying just one volt runs the meter to 125V, 3 volts and the needle pegs. Is there an inline resistor I can choose which can be used to make this meter function at line voltage? All I have come up with so far is to use a 6vdc wall wort transformer and some resistors, but surely this was not the way that the meter was set up back when.enter image description here

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    \$\begingroup\$ Would definitely like to see photos/videos of this in operation! \$\endgroup\$ – Ryan Griggs Jan 2 '17 at 2:43
  • \$\begingroup\$ I have uploaded a photo showing the meter with the wall wart and resistors. It reads outlet AC voltage accurately, but becomes less accurate when voltage is reduced with variac. Can't post videos on this site. \$\endgroup\$ – jan Jan 3 '17 at 16:57
  • \$\begingroup\$ That's a beauty. \$\endgroup\$ – Ryan Griggs Jan 3 '17 at 20:50
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You probably DON'T want to set it up like it was "back then". Those were the bad old days when we didn't have wall-warts to make handling mains voltage safe, etc.

I think you have a great idea to use a wall-wart. I would use a 12V wall wart and a ~100 ohm pot to "calibrate" the meter reading. And then you could replace that incandescent bulb with a nice array of LEDs powered from the same wall-wart.

I would probably throw a 1000 ohm resistor between the source and the "top" of the pot so that I could use an ordinary pot (not a more rare, costlier high-power wire-wound pot).

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  • \$\begingroup\$ Thanks for your suggestions. I found two resistors in my box and when I put those together in series I was able to get the voltage right at 125 V. That's using the six vdc wall wart.I wanted to use a handblown carbon bulb that I had on hand to illuminate the scale- it fits the era of the meter quite nicely. \$\endgroup\$ – jan Jul 3 '16 at 13:35
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...inside of the housing was some sort of mica wafer assembly wired in series with the +binding post. I would assume that this was some sort of resistor, but it is no longer conducting current.

I've never heard of these but a series resistor would be required for the high voltage.

Applying just one volt runs the meter to 125V, 3 volts and the needle pegs.

So full scale should be at \$ \frac {300V}{125V}1V = 2.4~V \$.

Is there an inline resistor I can choose which can be used to make this meter function at line voltage?

Assuming there is a scale on the other side of the meter and that you want to keep it as a 300 V DC meter we need to calculate the series resistor required.

  1. Measure the meter coil resistance using a multimeter.
  2. The full scale current can be calculated using this value and the 2.4 V full-scale (FS) that we calculated above.

$$ I_{FS} = \frac {V_{FS}}{R_{COIL}} = \frac {2.4}{R_{COIL}} $$

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Voltage range conversion is as simple as adding a series resistor.

  1. The series resistor needs to drop the remainder of the voltage at \$I_{FS}\$.

$$ R1 = \frac {300-2.4}{I_{FS}} $$

I'd be interested to know how to wire it more directly w/o a wall wart so that it could read higher voltages.

schematic

simulate this circuit

Figure 2. Using a wall-wart to convert the meter to measure AC mains voltage safely.

You just need a transformer with a voltage rating greater than or equal to the highest AC voltage you will be measuring.

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  • \$\begingroup\$ I found two resistors in my box- the first moved the needle on the scale too high but I added another and when I put those together in series I was able to get the voltage right at 125 V. That's using the six vdc wall wart. Lucky find- I don't have many resistors on hand. \$\endgroup\$ – jan Jul 3 '16 at 13:37
  • \$\begingroup\$ If it's just for show that will be fine. If you want to measure mains voltage it may not be much good. e.g., If the 6 V wall wart is regulated it will give out steady 6 V for mains between 100 and 140 V at a guess. If it's unregulated it might do the trick but the output capacitors would prevent you seeing any short dips when you switch on a heavy load, or lightning brownouts, for example. \$\endgroup\$ – Transistor Jul 3 '16 at 14:33
  • \$\begingroup\$ Thanks. I had not thought about regulation. However I did power the Wall wart through a powerstat: the voltage reading on the Weston corresponded with a multimeter as the input voltage to the wall wart was reduced.The weston showed that voltage variation between floors. I confirmed that the Meter was reading correctly by comparing to a wooden boxed Weston Bench meter- That read 119 v and the weston 118. The meter is for show. I'd be interested to know how to wire it more directly w/o a wall wart so that it could read higher voltages. That is, if possible and if it's simple and inexpensive... \$\endgroup\$ – jan Jul 4 '16 at 15:23
  • \$\begingroup\$ See the update. \$\endgroup\$ – Transistor Jul 4 '16 at 18:18
  • \$\begingroup\$ Update: I checked the Weston Station Voltage Meter again with the Powerstat and Weston Bench meter. Turns out that at reduced voltage, there is a significant deviation- about 10% at 90V. As I am not running an electric generating plant, the reading at line voltage is good enough. Curiously, my Greenlee CMT-80 reads line voltage too high! \$\endgroup\$ – jan Jul 4 '16 at 20:13

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