Using those freebee Harbor Freight DVMs as panel meters

[wa2ise]

Have an old homebrewed linear adjustable power supply with a meter that was hard to figure out. Volts and amps output, switchable. I figured I could use one of those freebee Harbor Freight DVM to replace the old meter, to become this power supply's panel meter. The power supply can provide up to 27VDC at around an amp. I picked two ranges I'm going to use, 200VDC and 10A. Then I had to figure out how to do the switching the big knob usually does, so I could have a two position switch. I found I could do this with a 4 pole double throw slide switch. Two of the poles would have wires connected to all 3 connections, and the other two poles just two wires each. Another normally switched connection was always connected, so a solder jumper took care of that. Below are some pictures detailing the meter wiring and switching.



The power supply connections to the meter goes as such: The positive output goes to the volts input jack. This positive also goes to the external output jack of the power supply cabinet. The negative output goes to the common jack of the meter. And the negative output of the power supply cabinet goes to the current jack on the meter. There's a current shunt inside the meter that measures voltage developed across this shunt. Thus I don't need to plug or unplug anything when I want to switch from volts to amps on the meter.

There's a trimpot on the DVM's circuit board that I used to adjust the voltage reading. And as the current reading was a little high compared to other DVMs I have, I was able to get it close by adding a 0.1ohm resistor in parallel with the shunt.

I needed to provide a separate 9V power supply to the DVM. Used the guts of a wall wart, with a voltage regulator added to it to get a steady 9V source.