Stuck when using 2 LCD panel meters in parallel

Discussion in 'The Projects Forum' started by Efron, Feb 24, 2014.

  1. Efron

    Thread Starter Member

    Oct 10, 2010
    81
    15
    Hello,

    I'm working on a DIY power supply project and I'm stuck with a problem since long time ago now.

    The issue is that I want to display the output voltage and current of the power supply. To do that, I use 2 LCD panel meters.

    Currently I'm using these ones:

    http://www.circuitspecialists.com/pm-128e-backlit.html

    These panel meters can be configured in 5Vdc with common ground. This is the option I chose, although I also tested them with 9Vdc isolated power supply.

    In any case I use isolated power supply, so in practice there is should be no problem with any common ground.

    Both panels are powered in parallel and connected following the schematic below.

    In this configuration, LCD2 is not displaying correct measurement. However, if I disconnect LCD1, LCD2 works fine (when standalone).

    From this schematic, it can be seen the Vin- of LCD1 is directly connected to the Vin+ of LCD2. My best guess is that LCD2 does not appretiate this for some reason.

    How should I proceed, keeping in mind that I would like to avoid to power both panel meters with 2 separate power supply?

    I understood that, when using panel meters that don't accept the comm on ground, the Vin- is somehow linked to the V+ of the power supply. Is there something similar in a common ground configuration?

    Any suggestion will be appretiated.
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi,
    You have the Common [0v] of the two meters connected, so you will get incorrect operation.

    I would have the 0.1R shunt in the high side , powered by the isolated supply.

    The voltage meter in the low side, powered from 5V derived from the power supply.

    This will give you the true reading of current thru the load and the voltage across the load, your present configuration will not do that.
    E
     
    Last edited: Feb 24, 2014
    Efron likes this.
  3. Efron

    Thread Starter Member

    Oct 10, 2010
    81
    15
    ericgibbs, can you be more explicit with your statement? Why it is not good idea having the Common [0v] of the two meters connected?

    And do I correctly understand you, when you say common [0V] do you mean the Vin- in my schematic? or the negative power supply pin of both LCD?
     
    Last edited: Feb 24, 2014
  4. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi,
    If you do a continuity test of a LED module between -IN and what you call Gnd I expect you will measure a short circuit, as one would expect.

    Could you please check that and let me know what you measure.?

    E
     
  5. Efron

    Thread Starter Member

    Oct 10, 2010
    81
    15
    You're right. The continuity test was positive, meaning that the Vin- is internally attached to the power 'ground' (or almost).

    Which means that in practice, I'm connecting the Vin+ of the ammeter to the power 'ground' (see pic).

    Well, in this situation I can understand that the ammeter is not displaying the correct value.

    One could expect displaying almost 0 because Vin+ = 'power ground' ~ Vin-. In reality, it is displaying about 13mV instead of 50mV (when output current is about 500mA). I suppose there somehow a competition between the low value of 0.1 Ohm and the internal low equivalent resistor value inside the PM.

    PROPOSAL FOR FINAL DESIGN
    Would the proposed final design work?
     
    Last edited: Feb 24, 2014
  6. BillB3857

    Senior Member

    Feb 28, 2009
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  7. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi Efron,
    I would do the same as Bill has done, ie: use two 9V PP3 batteries, one for each LCD panel, the current draw is minimal, the batteries should last for months.

    You could if required add a double pole single throw toggle/slide switch to save battery power.

    Eric

    EDIT:
    Looking at your 'final' circuit.
    I assume that your PSU, 0V line is ground free.? I would still recommend swapping the Voltage and Current LED panels over.
    ie: measure the high side current and have the low side voltage LED, across the Load referenced to 0V.
     
    Last edited: Feb 25, 2014
  8. BillB3857

    Senior Member

    Feb 28, 2009
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    348
    If you look at the circuit I linked to, the opto isolators use power from the source I had available so that when my test device was turned on, the meters would turn on also. You could simply use power from the supply you want to measure, before the regulator to drive the opto inputs. No switch needed for turning the meters on / off.
     
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