Elenco XP-720 Voltmeter

[madcitygeek]

Okay, I'm a noob and I built myself an XP-720 power supply. It works well, tests within reasonable ranges. I had a great time building it, my 6-year-old son helped.

I'd like to add a pair of volt meters -- one for positive, one for negative. There seems to be ample room in the case -- I might be able to dremel out a couple holes.

I was looking at these displays from Adafruit -- https://www.adafruit.com/products/705 -- the "Mini 3-wire Volt Meter (0 - 99.9VDC)" I figured I could solder them in between the outputs of the LM317, wiring the "red, power" wire to the 5v output from the 7805.

Am I wrong? Is there a better way to add a meter?

[Dragonfly]

I don't see why that wouldn't work for the positive supply but you wouldn't be use this meter on the negative supply.  It's range is 0-99.9VDC so it can't measure negative voltage.

[rstofer]

Duplicate response, see above!

[madcitygeek]

Good point. Could I wire it in backwards? That is, put two meters in and wire one to positive and one in backward for negative? Or use a switch to flip between them?

[Dragonfly]

That wouldn't work with this 3-wire meter because it expects the voltage between the red wire and the black wire to be a constant 5V.  If you tried to wire the meter in reverse for the negative side (i.e., black to the negative output and white to ground) then the voltage between the black and red wire would vary with the voltage setting of your negative supply.  An alternate approach would be to use the 2 wire version on the negative supply (https://www.adafruit.com/products/460) wired backwards.  The downside is the 2 wire meter won't work when the negative supply is set between 0 and -3.2V.  Anything beyond that should work though.