Digital voltmeter for power supplies

Discussion in 'General Electronics Chat' started by hp1729, May 21, 2016.

  1. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    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?
     
  2. ScottWang

    Moderator

    Aug 23, 2012
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    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.
     
  3. MrAl

    Well-Known Member

    Jun 17, 2014
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    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.
     
  4. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    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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  5. crutschow

    Expert

    Mar 14, 2008
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    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.
     
  6. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    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: May 21, 2016
  7. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    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.
     
  8. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    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.
     
  9. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    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.
     
  10. ScottWang

    Moderator

    Aug 23, 2012
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    Then choose the second one that I mentioned, it has independent Vcc supply.
     
  11. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    There is nothing usual about that three wire configuration, the shunt resistor is in the DVM module.
     
  12. ScottWang

    Moderator

    Aug 23, 2012
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    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.
     
  13. ISB123

    Well-Known Member

    May 21, 2014
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    I use PM438 but it requires isolated supply.
     
  14. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    Hi Scott,
    I have them working with 3 wires on my 12V battery charging system, they work just fine.
    E
     
  15. ScottWang

    Moderator

    Aug 23, 2012
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    Really, do you have the datasheet or links?
     
  16. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    Very nice for many applications. Just not mine.
     
  17. crutschow

    Expert

    Mar 14, 2008
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    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.
     
  18. RichardO

    Well-Known Member

    May 4, 2013
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  19. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    PM438 looks good.
     
  20. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    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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