# Digital voltmeter for power supplies

#### hp1729

Joined Nov 23, 2015
2,304

Last edited:

#### hp1729

Joined Nov 23, 2015
2,304
If you use the power supply ground as the plus input to the meter and the minus voltage connected to the meter common, then you should be able to measure the negative voltage as well.

Here's one that measures either plus or minus voltages with 1mV resolution to 19.999V.
Super on the meter. Not a bad price either.

Regarding your hookup, I'm not sure how I can measure negative voltages. Meter positive power to power supply ground. Ground of the meter to -5 V (has to be 5 V difference on power). My input could go negative but not very far. Negative 5 V in would read 0 V???

The inverter works better.

#### hp1729

Joined Nov 23, 2015
2,304
hi,
I use these for general purpose VA indicators, from Amazon.
As others have pointed out, consider isolation and lowest voltage range.
E
Three wire, measures voltage and current???
Measures power in? Only 4.5 V to 30 V measurable? Okay for a power supply that only goes down to 4.5 V. Not much good for exercises at lower voltages.

#### hp1729

Joined Nov 23, 2015
2,304
I got a similar one from a friend, the Vin around 0~30V, but when the input voltage less than 3V then it won't work any more, I have design the circuit to measure from 0~3V with that kind of voltmeter, but I haven't try it yet, this is the two wires type.

There are some other 3 wires type can be measure 0~99V, one wire is the Vcc for voltmeter, I connected this type to my 0~50V/4A power supply, and there are some other types can be measure voltage and current.

Maybe you can buy two of them to built dual voltmeter, if compose into one maybe the price could be higher.
That 3 V limit is a problem for me.
My intended use was for basic electronic component exercises, so I spend a lot of time below 1 V. The G21310 seems accurate at 5.0 Volts and below.

#### ScottWang

Joined Aug 23, 2012
6,933
That 3 V limit is a problem for me.
My intended use was for basic electronic component exercises, so I spend a lot of time below 1 V. The G21310 seems accurate at 5.0 Volts and below.
Then choose the second one that I mentioned, it has independent Vcc supply.

#### ericgibbs

Joined Jan 29, 2010
10,817
Three wire, measures voltage and current???
There is nothing usual about that three wire configuration, the shunt resistor is in the DVM module.

#### ScottWang

Joined Aug 23, 2012
6,933
Three wire, measures voltage and current???
Measures power in? Only 4.5 V to 30 V measurable? Okay for a power supply that only goes down to 4.5 V. Not much good for exercises at lower voltages.
If the meter including voltage and current then it can't be just using 3 wires, at least using 5 wires, as this one and this.

#### ISB123

Joined May 21, 2014
1,236
I use PM438 but it requires isolated supply.

#### ericgibbs

Joined Jan 29, 2010
10,817
If the meter including voltage and current then it can't be just using 3 wires, at least using 5 wires, as this one and this.
Hi Scott,
I have them working with 3 wires on my 12V battery charging system, they work just fine.
E

#### ScottWang

Joined Aug 23, 2012
6,933
Hi Scott,
I have them working with 3 wires on my 12V battery charging system, they work just fine.
E
Really, do you have the datasheet or links?

#### hp1729

Joined Nov 23, 2015
2,304
There is nothing usual about that three wire configuration, the shunt resistor is in the DVM module.
Very nice for many applications. Just not mine.

#### crutschow

Joined Mar 14, 2008
25,432
.................
Regarding your hookup, I'm not sure how I can measure negative voltages. Meter positive power to power supply ground. Ground of the meter to -5 V (has to be 5 V difference on power). My input could go negative but not very far. Negative 5 V in would read 0 V???
You connect the meter input to the common output (which is positive to the meter referenced to the minus output).
The meter will then measure the -5V output.

To measure down to 0V with any of the meters you will need one that can operate from a separate (but not an isolated) supply.
You can generate this meter supply voltage from the power supply unregulated DC input using a simple regulator, such as an LM317.

#### hp1729

Joined Nov 23, 2015
2,304

#### hp1729

Joined Nov 23, 2015
2,304
You connect the meter input to the common output (which is positive to the meter referenced to the minus output).
The meter will then measure the -5V output.

To measure down to 0V with any of the meters you will need one that can operate from a separate (but not an isolated) supply.
You can generate this meter supply voltage from the power supply unregulated DC input using a simple regulator, such as an LM317.
Your description misses me. Drawing, please. It sounds like you are describing more leads than I have. I have one ground. One power input, which I can measure with the meter. One input.