CX-101-BG as a current meter on LM723 supply

Discussion in 'General Electronics Chat' started by MaggoT, Dec 15, 2011.

  1. MaggoT

    Thread Starter New Member

    Dec 15, 2011
    13
    0
    Hello!
    I am currently attempting to build a "traditional" bench power supply around a LM723 and four 2N3055 transistors. I have tested parts of the schematics and things seem to work great so far.

    However, I have two CX-101-BG panel meters from http://www.circuitspecialists.com/digital-panel-meter-cx101bg.html
    that I intended to use for voltage and current display. (These are the -BG ones, that should work with a common ground configuration). After a bit of frustration I managed to get one working for straight up voltage measurement using the second example from the datasheet (don't have a schematic for this, sorry), but i cannot get it to measure the voltage across a resistor properly.

    Here is my power supply schematic, R13 is the resistor over which i intended to measure the voltage drop, and display it on the meter.
    The "Current limit board" mentioned in the schematic is just another board that switches in various resistors in series to provide a couple of different steps for setting the current limit for the LM723.

    [​IMG]

    So my question is, how do i wire the CX-101-BG up to measure the voltage drop across R13, if it is at all possible? Are any of the COM, INLO, INHI, REL pins internally connected to ground inside the meter?

    regards
    MaggoT
     
  2. yibeltal

    New Member

    Dec 16, 2011
    1
    0
    Based on the datasheet, I believe the display can work with the following connection.

    1= V+ positive voltage- power supply connection to the meter in this case a +5V
    2= V- negative voltage- power supply connection to the meter in this case GND from 5V supply
    3 = NC not connected
    4 = NC not connected
    5 = COM –common=ground
    6 = INLO input low Negative measuring input.
    7 = INHI input high Positive measuring input.
    8 = RFL reference low maybe Connect to COM= 0V ground
    9 = RFH reference high
    10 = ROH reference out Precision reference voltage output. Connect to RFH for normal operation.

    For the decimal point there are three possibilities each one related to the scale needed for the voltage measured in the circuit.

    11 = D1 connection for dot 1 maybe Connect to V+ to display DP1 (199.9).
    12 = D2 connection for dot 2 maybe Connect to V+ to display DP2 (19.99)
    13 = D3 connection for dot 3 maybe Connect to V+ to display DP3 (1.999)

    I have attached a schematics how to use the display.
    Because the multimeter is set to measure a maximum 200mV at the input, we need to arrange a voltage divider to meet our needs. This is done using two resistors: R1 and R2.
    You didn’t specify what’s the maximum voltage in the circuit to be monitorized.
    For a resistor divider it’s important only the ratio R1/R2, the range of resistors it’s selected only for minimize the current drawn from the tested circuit.
    And because of the tolerance values of the resistors we need to have at least one resistor adjustable to obtain a proper calibration of the circuit. It’s more simple to adjust the R2 and in practice we replace R2 by two other resistors: a fixed one plus a variable one.

    Vin=Vsource R2/(R1+R2)

    You need to provide maximum voltage for the Vsource, we choose R1=10MΩ, Vin =0.2V
    To be determined R2
    Example
    Vsource 20V
    R2=R1/99=101.01KΩ we split R2=R2’ + R2’’= 82kΩ (fixed)+ 25kΩ(variable)

    Example
    Vsource 200V
    R2=R1/999= 10.01KΩ possible solution R2=R2’ + R2’’= 8.2kΩ +2.5kΩ
     
  3. MaggoT

    Thread Starter New Member

    Dec 15, 2011
    13
    0
    Ok, i have done some more testing. The circuit below shows my test setup which is roughly equivalent to what the meter will be measuring inside the PSU.

    The drop across R2 should be 1.66 volts in this case, and with the divide resistors the CX101 should read about 0.150 volts. However the meter reads out of range...

    When measuring across R2 with a multimeter i get 2.5 volts, which confirms that the meter is connecting INLO to GND. Grounding COM/RFL or leaving them floating makes no difference. Measuring across R5 with the multimeter i get 0.225 volts.

    [​IMG]
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The product specifications cover the CX101A and CX101B, but not the CX101BG.

    You would get a proper voltage reading across R3, but not R2. I don't know why you thought you might, as if you are powering it from the supply, and you are using GND for the meter's ground, of course the negative meter lead is going to be the same as ground.

    If you want to be able to measure voltages without worrying about where your ground is, you will need to use a galvanically isolated (not connected electrically to your supply) 5v supply for each meter.
    You can either buy isolated DC-DC converters for around $15/ea, or you could build a couple of them for a buck or two each. You will need to wind a transformer using a toroid.
     
  5. MaggoT

    Thread Starter New Member

    Dec 15, 2011
    13
    0
    Thanks for your reply.
    Since the datasheet had a different configuration for a common ground application, where the supply GND is connected to INLO outside the meter, i just assumed that the measurement inputs where not internally referenced to the supply GND... but then again i don't really understand how this meter works :)

    So if i were to power the meter from an isolated +5v supply i could correctly measure the voltage drop across R2 the way i intended?

    I don't really know about this current measuring, i haven't been able to find any information on how people usually wire output current meters in this type of power supply, is there a more suitable meter, or better way of doing this generally?

    Regarding the DC-DC converter, do you happen to have a schematic and/or information on how i could build something that would work in this situation?

    best regards
    MaggoT
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    What will your input supply voltage range be?
     
  7. MaggoT

    Thread Starter New Member

    Dec 15, 2011
    13
    0
    I have a 2x24VAC transformer which gives roughly 36VDC (without load) after rectifying/filtering, this will be fed into a LM317 which will be adjusted to provide the LM723 with a clean supply voltage of 30VDC.
     
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