Voltage + current meter

Discussion in 'General Electronics Chat' started by John Berry, Jun 20, 2017.

  1. John Berry

    Thread Starter Member

    May 17, 2016
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    Hello, I am using my meter for the first time to try and test for current flow in some led's and then through my 12v motor.
    It would not work for the led's and the motor gave a false reading?.
    Aafter checking back on the spec in the advert it says that it cannot read below 1amp, (only betwwen 1A - 10A )
    I have just seen another meter that looks identical but the spec is different?
    It says minimum resolution 0.01A, does this meen I can measure as low as 0.01 A?
    At the moment I only use 12v dc or less for learnig and thought one of these might be ideal?
    Thank you very much
    John.... :)
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Just stating "minimum resolution" .01A does not specifically guarantee it reads below 1A..
    It could just mean that it can show the difference between 1.00A and 1.01A but cannot read below that..
    You would think and hope it could but there are no guarantees..

    For temporary voltage/current readings you should just buy a multimeter and use that.. Its also handy in other situations too so its like the Swiss Army Knife of electronics..
    most if not all have the ability to read milliamps as well as amps (usually up to 10A or so)
     
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  3. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Resolution is the size of the step that meter uses to display measurements.

    The word you are looking for is Range. Since second meter does not list range, it is worthless to you.

    Since you tried to be cheap and failed. Now is time to get smart. Get multimeter. if you need simultaneous measurements, get two meters. Something in 20-30 pounds range should be half decent. Uni-t 61 is usually recommended at this point.
     
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  4. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    You can modify it by altering the Shunt resistor inside, increasing the resistance to give a lower reading on current.
     
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  5. DickCappels

    Moderator

    Aug 21, 2008
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    Or, if the voltmeter has a reasonable input resistance (100k or higher) you can use resistors as shunts -just put the resistor in series with the load and measure the voltage across it.

    The sensitivity in volts per amp is the resistance, so:

    A 1 ohm resistor is 1 volt/amp so if your meter can measure 1 millivolt the resolution is 1 millliamp.

    A 100 ohm resistor is 100 volts/amp so if your meter can measure 1 millivolt the resolution would be 10 microamps.

    And so on...
     
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  6. John Berry

    Thread Starter Member

    May 17, 2016
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    Ok thank you for your help.
    I have a multimeter but I am wanting to make a basic psu running off a 12v battery ( just for testing purposses ) WP_20170621_001[1].jpg
    Thanks John.... :)
     
  7. John Berry

    Thread Starter Member

    May 17, 2016
    63
    4
    Hi, Thanks for your quick reply. I have a DMM but I am wanting to make a basic PSU running from a 12v battery
    Thanks again. John.... :) WP_20170621_001[1].jpg
     
  8. John Berry

    Thread Starter Member

    May 17, 2016
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    HI Dodgydave and thanks for the reply, I have seen these on ebay 10a 20a 30a 40a 50a Which one would I be better off using?
    Thanks again. John... :) WP_20170620_002[1].jpg
     
  9. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    @john Those shunts are higher than your meter is now, as you said your meter wont read below 1Amp.
     
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  10. crutschow

    Expert

    Mar 14, 2008
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    The meter in your picture looks like it should read down to 0.01A.
    Can you post the meter spec and/or the part brand and model?
     
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  11. John Berry

    Thread Starter Member

    May 17, 2016
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    Hi mate, this is what I found from my purchase history. I can't believe I did not ammeter.jpg WP_20170621_002[1].jpg notice the bold red lettering... :)
     
  12. John Berry

    Thread Starter Member

    May 17, 2016
    63
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    I am confused now Dave, You said I could modify it by changing the shunt resistor?.... :)
     
  13. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Yes you need to remove the shunt that is inside, and replace it with a higher resistance, (lower current) this will give you a larger voltage developed across it, so your meter can read currents under 1amp,..

    An Ammeter is just a Voltmeter with a shunt in parallel with the terminals, the lower the resistance of the shunt the Bigger currents it can measure, the higher the resistance the smaller the current it can measure, you need to take the cover off and post pictures of the pcb,
    Ideally you need another dvm to calibrate it.
     
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  14. John Berry

    Thread Starter Member

    May 17, 2016
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    Hi dave I have just taken these pics with my usb microscope, let me know if you need more info.
    Thank you very much. John... :) Snap_001.jpg Snap_002.jpg Snap_007.jpg
     
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  15. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    The shunt is the Silver Zigzag thick wire. Ideally you need to pass 1amp through it and measure the voltage across it with another dvm, this will give you its resistance.
     
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  16. John Berry

    Thread Starter Member

    May 17, 2016
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    Hi Dave, I presume that i must first desolder it from the board. Could I then connect my battery charger to my LM2596 cv/cc and adjust it to 1A output? and what voltage would I set it to? Thanks John.... :)
     
  17. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    No need to Desolder it, just set it to 1amp constant current, and measure the voltage across it, this will give you the resistance,

    so if you get say 65mV across it, the resistance is 65milli ohms... You can then choose a shunt with higher resistance like 650mOhms to give a lower current range of 1amp.
     
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  18. John Berry

    Thread Starter Member

    May 17, 2016
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    Ok Dave, thank you very much for your help. I will have a go and see what results I get. Thanks again.. John... :)
     
  19. John Berry

    Thread Starter Member

    May 17, 2016
    63
    4
    Hi, Dave, I have used a 12v 20w-27w car bulb for my load, it looks like the voltage drop is only 10.2milliv WP_20170701_002[1].jpg I would appreciate it very much if you could check my readings for me as I am not sure I've done it right? One reading I can't understand is the V meter reading 4.68v I have attached a picture for you.
    Thank you very much ... John.. :)
     
  20. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    No good, you must use a Constant current source, this will put a stable voltage across the shunt, then you can measure it and choose a shunt with a higher resistance.
     
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