Digital Ammeter from Ebay?

Discussion in 'General Electronics Chat' started by geratheg, Dec 15, 2014.

  1. geratheg

    Thread Starter Member

    Jul 11, 2014
    107
    3
    Hello does anyone have experience with digital ammeters from ebay?
    I'm thinking about getting one like this:
    Digital Ammeter

    Will it be appropriate for it to be powered by the same power source that powers the circuit from which this ammeter will be measuring?

    If anyone has experience I hope you can throw some tips in on use and calibration and such.

    Thanks!
     
  2. DerStrom8

    Well-Known Member

    Feb 20, 2011
    2,428
    1,328
    What do you plan to use it for? That ammeter is designed for use in a car, which is a modification that would not be allowed here at AAC according to the ToS (which hopefully you have read).

    Do you need a module like that, or can you simply get a multimeter? Difficult to answer without knowing your application.

    Regards,
    Matt
     
  3. geratheg

    Thread Starter Member

    Jul 11, 2014
    107
    3
    This is going to be used for a project. I will be building a solar powered boat and wanted to monitor the current coming from the panels. I want it built in so I can monitor the current without manually connecting a multimeter.
     
  4. sdowney717

    Member

    Jul 18, 2012
    385
    13
    I looked at that myself and do not like it for the simple reason, it cant be calibrated - adjusted. I asked them and that is what they said.
    There are no pots on the back.

    I am interested myself in one of these.
    Buy this meter
    http://www.ebay.com/itm/191401117588?_trksid=p2060778.m1438.l2649&ssPageName=STRK:MEBIDX:IT

    buy the 75mv shunt, 200 amp shunt. Higher amps here is better for the shunt.
    http://www.ebay.com/itm/230974566668?_trksid=p2060778.m1438.l2649&ssPageName=STRK:MEBIDX:IT

    Buy these both is cheaper.

    At least if it is off reading you can fine tune it. I have bought meters from Ebay and one set non adjustable reads .7vdc too low.
     
  5. geratheg

    Thread Starter Member

    Jul 11, 2014
    107
    3
    I think cutting the shunt or adding a parallel resistor (adjustable pot) with the shunt will allow for calibration.

    Which meters have you bought from ebay? Thanks for those links.
    Also doesn't the amperage reading depend on the shunt? So at 75mV it will read 100A, if you put 200A shunt won't it read 50A since it measures the voltage and has a pre-programed resistance in the software (I don't really know how these ammeters work so I may be wrong here)? Voltage / Resistance = current.
     
  6. sdowney717

    Member

    Jul 18, 2012
    385
    13
    I bought two cheap 2 wire LED voltmeters. While it had a nice display and was cheap it was really way off. I dont want to link to it but it was under $2, I got a full refund and they have a lot to sell and have dropped the price. 3 wire volt meters are better, or at least get something that can be adjusted. I mean if you get it and it is off, then your stuck.

    Other meters were these here, and it is adjustable. these were also ok with no need to be adjusted.
    http://www.ebay.com/itm/201204685005?_trksid=p2059210.m2749.l2649&var=500399379626&ssPageName=STRK:MEBIDX:IT

    http://www.ebay.com/itm/291241481557?_trksid=p2059210.m2749.l2649&var=590394305409&ssPageName=STRK:MEBIDX:IT
    For this one, I bought blue, it is very bright, but due to white unlit LED segments in sunlight hard to read. So I used a piece of blue translucent plastic and it made it a lot more readable. They are adjustable and I played with the pot and it works.

    The shunts, if they 75millivolt, then it will work with all the ebay ammeters. The amp rating is not what determines the readings, the millivolt drop is what allows the readings as ammeters are like voltmeters and read a voltage drop across the shunt.
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    I've used meters similar to the ones sdowney717 mentioned off EBay. Mine came with the shunts for around 15-20 USD free ship.

    They self power from the same source as they measure, no sepaate power souce needed.

    However, as they are just there to indicate activity (is my unit dead or alive?) I never checked the calibration.
     
  8. geratheg

    Thread Starter Member

    Jul 11, 2014
    107
    3
    Okay thanks for the information.
    One of the meters you linked to is used for AC circuits.

    With the 200A shunt, is the voltage drop across the shunt 75mV when the current is 200A?
    With the 100A shunt, is the voltage drop across the shunt 75mV when the current is 100A?

    My question is: when the voltage is 75mV across the "resistor" the ammeter will read 100A or 200A?

    Another question: Do you use anything to cover the shunt with insulation? Any ideas?

    Gera
     
  9. sdowney717

    Member

    Jul 18, 2012
    385
    13
    http://www.ebay.com/gds/A-Guide-To-Current-Shunts-/10000000001675505/g.html
    What is a shunt rated meter?

    Just match the meter to the shunt for millivolt, whatever the meter was designed to measure against, ampacity is capacity a higher amp shunt will work if it has the same millivolt reading. If I am wrong, then someone will say so. I have been wrong before, so keep on with the researching.
     
  10. sdowney717

    Member

    Jul 18, 2012
    385
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  11. geratheg

    Thread Starter Member

    Jul 11, 2014
    107
    3
  12. Blackbull

    Well-Known Member

    Jul 26, 2008
    70
    6
    A 75mV 100A shunt will have been calibrated to read a 75mV drop when 100A is passed through it. It is normal to select a shunt that will be used at 2/3 its stated Amps; exceeding this will overheat the shunt making the calibration inaccurate. The larger the Amp rating the cooler the shunt will work. The mV drop is calculated from the internal resistance of the meter. The shunt will need to be open to the air to dissipate heat, some sort of cage/guard is required.
     
  13. geratheg

    Thread Starter Member

    Jul 11, 2014
    107
    3
    Thanks for the information. I won't be having more than 40amps passing through the shunt for sure. Is the shunt conductive on the outside? Any ideas on how to enclose it?
     
  14. sdowney717

    Member

    Jul 18, 2012
    385
    13
    Hi, that looks like a good one, adjustable if needed.
    I like the higher rated shunts because smaller ones might not be as accurate at higher amp flow.
    And I have also read they should not be used continuously at their rating which kind of limits the usefulness.
    On another forum, someone said you can simply use your battery cable (or large wire) with the meter input spread apart the length of the wire. I think it would work, can be used at higher amps, but I dont know how you can calibrate that unless you have a DC clamp on amp meter. If you do, and you have an adjustable meter, maybe you can reasonable readings without needing a specific shunt. I do have an Elenco clamp on DC amp meter. I might some day look at doing that.
     
  15. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
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    there are two kinds, one has the parts under the display, the other on the backside. These are often adjustable or you can modify it.
     
  16. sdowney717

    Member

    Jul 18, 2012
    385
    13
    The ebay picture for his meter, I can see mini pots on the board. There two, my guess is one for the volt meter, one for the ammeter.
    [​IMG]
     
  17. geratheg

    Thread Starter Member

    Jul 11, 2014
    107
    3
    I think you're right. One for the voltmeter the other for the ammeter.
     
  18. Esposito

    New Member

    Apr 21, 2015
    4
    0
    I just love these cheap LED voltmeters and ammeters you can get on eBay.
     
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