Digital voltmeter for power supplies

Thread Starter

hp1729

Joined Nov 23, 2015
2,304
G21310 from Electronic Goldmine. About $5.00
Runs on 5 Volts. Nice, but only does positive voltages, 0 to 30 V or so. Okay for positive power supplies at this price.
Does anybody know of one that does positive and negative voltages?
 

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ScottWang

Joined Aug 23, 2012
6,933
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.
 

MrAl

Joined Jun 17, 2014
7,812
Hello there,

Be careful when buying these little meters. Some of them require a totally ISOLATED power supply to run them. That means you cant use the same supply you are measuring, such as 12v even if you regulate it down to 5v. You would need a separate wall wart to run the meter. The isolated types are a pain to run so look for the other type that dont need that.

For a while these types swamped the marketplace but now there are types that will measure the same voltage they are running from, such in an automobile. I have a few of these too, and i dont need a separate power source unless it has to measure less than 3.3 volts, and then it can be common ground with the source i am measuring. You can get 4 1/2 digit types these days for under 20 dollars USD.
 

ericgibbs

Joined Jan 29, 2010
10,817
hi,
I use these for general purpose VA indicators, from Amazon.
As others have pointed out, consider isolation and lowest voltage range.
E
 

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crutschow

Joined Mar 14, 2008
25,432
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.
It just won't have a negative sign in front of the readout.

Here's one that measures either plus or minus voltages with 1mV resolution to 19.999V.
 

Thread Starter

hp1729

Joined Nov 23, 2015
2,304
Hello there,

Be careful when buying these little meters. Some of them require a totally ISOLATED power supply to run them. That means you cant use the same supply you are measuring, such as 12v even if you regulate it down to 5v. You would need a separate wall wart to run the meter. The isolated types are a pain to run so look for the other type that dont need that.

For a while these types swamped the marketplace but now there are types that will measure the same voltage they are running from, such in an automobile. I have a few of these too, and i dont need a separate power source unless it has to measure less than 3.3 volts, and then it can be common ground with the source i am measuring. You can get 4 1/2 digit types these days for under 20 dollars USD.
On the G21310 it is not so isolated. To measure negative voltages I hooked up an inverter (LF356) running from + and - 15V, so I can measure 0 to about -13 V. I am mounting them on my breadboard.
Accuracy is nothing to brag about. They are about 0.1 V off on the high side at 6.7 V in.
They (Electronic Goldmine) have a purchase limit of two at a time. $10 minimum order.
 
Last edited:

Thread Starter

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.
It just won't have a negative sign in front of the readout.

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.
 

Thread Starter

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.
 

Thread Starter

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.
 

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.
 

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.
 

Thread Starter

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.
 
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