Modifying the panel mount V & A meters

[john444]

I really like the Panel mount volt-meter (ID: 575) and amp-meter (ID: 574).
They were easy to mount, easy to modify and at a good price.
I wanted the volt-meter to measure voltages below 4.5-v so I powered it (and the amp-meter) from a separate 6-v battery and un-soldered one side of R9 (see the excellent photo on the 575 product page).
A wire connected to R9 became the volt-meter's new input terminal.
I also changed the shunt in the amp-meter from a 10-a shunt (0.01-ohm) to a 1-a shunt (0.1-ohm) and removed the decimal point resistor. Now the amp-meter indicates in mA.
I also have plans to put the meters on a lead-acid battery bank. For this project I will again remove the 10-a shunt and connect the (thick) red/black wires to a external 100-a shunt. Not sure how to change the decimal point though.
These were perfect for my recent projects, Thanks.

BTW it is worth repeating to users, the black, negative leads also connect to your external circuits.
You may not be able to connect these meters the same way you can connect a handheld multi-meter.

[john444]

There was a question about modifying the volt meter so here is some clarification:



When looking at the photo of the back of the meter - R9 is on the left side near the middle of the PC board. Un-solder it. Orient it vertically instead of horizontally and re-solder it to the lower pad.
The exposed end of the resistor is the new input terminal. I soldered a short length of #40 wire-wrap wire on it.
The red and black wires on the connector now provide power.
Red (+) min 4.5-V, max 30-V @ 4-5-mA.
Black (–) or ground is also common to the input.

I have not measured the input resistance. It is at least 330-k (the value of R9) but it may closer to 1-M.
It looks like R9, the adj-pot and R11 are all in series. R11 (apparently) goes to ground with the connection between the adj-pot and R11 going to the A/D. Unless you provide batteries dedicated to powering the meter, watch out for ground loops because of the common ground between the input and the supply.

I don’t understand the A/D’s input resistors. They appear to make a 20:1 to 40:1 attenuator instead of a 10:1 as I would have expected. It would be interesting to solder another 24-k, vertically between the adj-pot and R11. Even though the input resistance would only be 48-k, it may provide a 3.00-Vfs meter.
Selecting a new decimal point is also an unresolved issue.