Using 5 wire LCD display to measure Voltage AND current from a PC power supply

Discussion in 'Test & Measurement Forum' started by arishy, Apr 6, 2018.

  1. arishy

    Thread Starter Member

    Apr 26, 2014
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    I have a PC power supply I am using as Bench power supply that gives me 12,5,3.3 V. I am attaching to it the LCD display See attachment.
    I am facing a very confusing situation and I need you to explain the behaviour. The five wires from the display are in two groups. One group to supply the power for the display ( thin Black and Red) the other to connect to the PC power supply to display the voltage and current of one of its outlets (3 outlets 3.3,5, 12)
    ( Two Thick Red and Black and a yellow wire). And here is the key to the problem:

    I decided to use the 12 V outlet to provide the needed power for the display. ( Instead for a SEPERATE power supply . a battery , to power it)
    Made the connects as shown in the attachment ( very important to see), Here is the weird thing :

    1. The Display is OK
    2. When I connect as suggested, I get SHORT CIRCUIT !!!!
    3. I floated the THICK black and the SHORT CIRCUIT is GONE , but I do not get current reading!!!!! and the voltage is displayed correctly.
    4. If I ground the THICK BLACK wire to the casing of the power supply I get VERY HIGH reading for current NOT TRUE. ( The load is in ma's )

    I have tried to x-ray the situation for you to SEE what is going on ; if I failed please ask me to further explain the situation.

    P.S. The display is working perfectly, I tested it before AND after in a seperate circuit.

    Your input will be greatly appreciated
     
  2. danadak

    Distinguished Member

    Mar 10, 2018
    2,773
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    Sounds like you connected the wires that come from the shunt (for measuring
    current) inside the module across the load batteries or load, which cause a short
    because the shunt is a very low resistance.

    The current measurement leads go in series with load batteries, whereas the
    V measuring leads go across the load batteries or load.

    Something like this -

    [​IMG]

    You may have blown the fuse/ shunt inside the module used for current
    measurement.

    Regards, Dana.
     
  3. arishy

    Thread Starter Member

    Apr 26, 2014
    82
    0
    Thank you Dana for your response. Did you have the chance to look at the wiring I attached to my post.?
    I love to comment on your response if we both talk about the SAME thing. By that I mean yellow wire to what terminal etc.
    The reason I am saying this is; it is acting very weird especially according to the diagram the RED THICK wire is connected to the NEGATIVE side!!!
     
  4. danadak

    Distinguished Member

    Mar 10, 2018
    2,773
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    The large image wiring show looks correct. The yellow line
    should be a high Z input for measuring V with respect to
    the thick black ground line for the load power.

    Note that diagram seems to imply isolated supply ground and load
    battery power ground should not be common, manual should discuss
    this ?

    Do you have a user manual URL for this module ?

    Regards, Dana.
     
  5. arishy

    Thread Starter Member

    Apr 26, 2014
    82
    0
    The good news is that you approve of the diagram, which means I AM doing something wrong and I hope with your help I get it right.
    In spite of the fact it is an expensive item ( $8 ) there is no manual for it. It has a button on the board which I have no clue how to use it. But this is a side issue.

    Regarding your comment BOTH groups of wires SHARE common Ground ie Both Black wires (Thick and Thin) are connected to each other.
    I have a feeling this post will be a long one because I hope to start isolating the problem. At the moment I am getting the VOltage reading OK but no current reading ( ZERO) when I touch the casing with the BLACK thick wire I get wrong High reading and very erratic.
     
  6. arishy

    Thread Starter Member

    Apr 26, 2014
    82
    0
  7. danadak

    Distinguished Member

    Mar 10, 2018
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    I keep thinking maybe you blew the internal shunt used for current reading. You
    could open the case and trace the current leads back thru internal board to see if
    it is open/blown.

    Or this needs an external shunt ? Some of these meters need external shunt, but
    I think its internal because they used heavy wiring which carries the load current.
    If it was external shunt wires for current leads would be just sensing, so like the yellow
    wire gauge.

    Contact the seller. Also try searching youtube to see if anyone else posted issue/solution.

    Is load current < rating of this panel meter ?


    Regards, Dana.
     
  8. arishy

    Thread Starter Member

    Apr 26, 2014
    82
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    The shunt is part of the module I actually can see it. I have two of these displays and both behave the same way.
    So, I assume no blown fuses so far.
    The problem of youtube is there are several of these modules with DIFFERENT hardware. I am afraid I have to do it the hacker way. one step at a time.....Any takers.... I need help
     
  9. ElectricSpidey

    Active Member

    Dec 2, 2017
    587
    129
    The power for the display must not share the ground of the metering connections.

    it clearly states that in the instructions.

    Assuming you are using the connections in the image you posted.


    EDIT:

    Well I see in that link the unit is sold as 100 volt meter then in the description it says 0-33.

    Also I see in that image posted it describes two black thin wires and no thick.

    So I’m sorry if I confused the issue further.
     
    Last edited: Apr 6, 2018
  10. dendad

    Distinguished Member

    Feb 20, 2016
    2,521
    676
    Yes. The red thick wire is the +ve end of the shunt resistor. The load -ve is actually a bit +ve above the power supply -ve connection.
    In your posted picture, there is a greyed out pic of using it with a common supply. Have you hooked it up like that?
    Can you post a photo of the way you actually have it hooked up, not a diagram from their paperwork.
    I have used these before with no problem.
     
  11. arishy

    Thread Starter Member

    Apr 26, 2014
    82
    0
    Let me clear the wires first. The group that contains two Red and black are thin wires to supply power to the display.
    The other group is THREE wires RED , BLACK THICK, and Yellow wires to be connected to the power supply.
    Very important: Both Thin Black and THICK black are connected INTERNALLY. Funny enough I think that makes sense;you have TWO circuits ( one for the display for LEDs to be on, the other to measure both voltage and current). Both circuits MUST have common grounds
    To proof it When I connect the THIN RED to 12 volt It lights up. I leave it's black THIN wire floating.

    Now MY WIRING. Imagine 12 Volt Bus line and Ground Taken from the PC power supply:
    • Yellow line Goes to 12V bus
    • RED THICK wire Goes to Ground bus
    • BLACK THICK wire Goes to Ground bus
    • Red THIN wire goes to 12V bus.
    • Black THIN wire goes to Ground Bus
    With these wiring arrangement I get reading for both Voltage and current The voltage is correct the current is bit weared:
    I get 6.4 for .66 A LOAD ( which off by 10 approx) Also as soon I put the meter in series to measure the load current the current reading drops to 3.7.

    As I said before I have two of them and they behave the same way...I want you to discard the possibility of bad unit.
    Also there is a button in the back which I presume for calibration but no clue how to use it. ( No manual)
     
  12. arishy

    Thread Starter Member

    Apr 26, 2014
    82
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    While you are helping me out to solve this mystery. Here what I am going to do.
    I will disregard the PC power supply. Also I am going to use ONLY batteries but same wiring as above And I will be back !!!!
     
  13. arishy

    Thread Starter Member

    Apr 26, 2014
    82
    0
    Thin Red ( LCD ) and Yellow to 6 v battery
    Thick RED and Thick Black and thin Black to ground.
    I get reading for Voltage BUT no current. Put serial Meter and the load gives me .6 A and no current reading in the display!!!!
     
  14. arishy

    Thread Starter Member

    Apr 26, 2014
    82
    0
    The million Dollar question:
    What wires responsible for the current reading.
    I know the yellow is for VOLTAGE. I tested it.
     
  15. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I saw this thread back when it was fresher, but couldn't find my photo until now. Maybe it will help.

    As you see, my setup uses an external shunt, and in that case the current is determined by measuring the voltage drop across the shunt with the heavy red and black wires..
    Volt Current Meter Shunt Setup.png
     
    Last edited: Apr 17, 2018
  16. arishy

    Thread Starter Member

    Apr 26, 2014
    82
    0
    Thank you for you followup.
    From the attachment you can LCDshunt.jpg the device has it OWN shunt.
     
  17. arishy

    Thread Starter Member

    Apr 26, 2014
    82
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    Now according to your wiring and some common sense here what I am going to do ( Remember Think black and Thick black are connected internally) :

    1.Thick Black and Thick Red will go to the NEGATIVE of the PS
    2. Thin Yellow and Thin RED will go to the POSITIVE of PS
    3. I will leave the thin Black floating.

    I will try to take a picture that matches the quality of yours
     
  18. arishy

    Thread Starter Member

    Apr 26, 2014
    82
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    Mind boggling result: !!!

    1. I got the V correct
    2. NO CURRENT reading......BUT

    When I connect the THIN Black wire ( suppose to be floating) to the PS chasse I get a reading !!!!!!!!
    35.7 ??????; I connected my current meter; it measures 0.36 AMP

    The V is 4.97 and the load is 10 ohm 50 W resistor

    It seems I will have to live with fact that my display has to be divided by 100 to get the true current.
     
  19. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I think that the black wires are connected to each other through the internal shunt in the meter. Because the internal shunt is a VERY low resistance, they seem to be electrically equivalent, but actually differ in potential by a tiny fraction of a volt. That differential is amplified and then converted by the internal microcontroller to the current reading. It could be that the programming in the uC is incorrect for the value of the shunt which would result in an incorrect current reading.
     
  20. whitehaired novice

    Member

    Jul 15, 2017
    267
    34
    I have come to this thread late and admit that I have not read it all--so I could be just wasting your time. These kind of meters require a separate battery supply, not connected to the circuit you are measuring. As you say, there is an internal connection but you can't have any other connection. The meter can't get power from the circuit it measures.
     
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