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YB27VA Dual Digital voltmeter/Ammeter in Dual Rail PSU?

Discussion in 'Power Electronics' started by Simonbond, Apr 17, 2014.

  1. Simonbond

    Simonbond

    4
    0
    Feb 25, 2010
    Hi Forum,

    I have a similar dual rail PSU query to the chap asking about metering his negative voltage rail. I had posted to AllAboutCircuits also.

    I bought a pair of YB27VA volt/amp meters, hoping to use one for the +ve rail & the other for the -ve rail, but I'm beginning to see that I maybe cannot use it on the -ve.

    My PSU is very similar to the Visio-Jameco unit:
    ( http://www.jameco.com/Jameco/Products/ProdDS/20626 Manual Revision 3.pdf ).

    Does anyone have any suggestions? Maybe I could get the -ve voltmeter working, but not the -ve ammeter? These meters seem to monitor the current flowing in the return connection rather than the supply wire.

    I've pasted my original post below for info.

    Many thanks & sorry for any duplication.

    Simon B.

    --------------------------------------

    Ive made myself a nice adjustable voltage dual rail lab style Power Supply Unit (PSU); just a standard design using adjustable Linear Voltage Regulators (LM317 & LM 337 for the positive (+ve) & negative (-ve) voltage rails).

    These regulators are rated at up to about an amp & will give me about +1.4v to +30v for the +ve rail, & -1.4v to -30v for the -ve. No adjustable current limit control, apart from the built in short circuit & over temperature protection.

    Obviously it's not as good as something I could buy, but will be adequate for my circuits. Maybe a few extra decoupling capacitors would improve it further.

    My question is about fitting volt meters & ammeters to it. It's easy to fit an analogue voltmeter & an ammeter to the +ve & -ve supply rails, but these 4 meters will take a lot of space.
    I found these twin-colour digital modules (YB27VA seems to be the part), each showing voltage in red & current in blue.

    I can see how easy it would be to put one in my +ve supply rail, but am not sure if I can ever put one in the -ve supply rail, as wouldn't I need to connect it 'backwards' to work? But would it work or get damaged?

    Does anyone have any experience in these modules? It took a while to work out what to do with the red, black & yellow wires & the badly translated specification & weird trio of connection diagrams!!!. It seems the module derives its power form the test voltage, but only if its between 4.5v & 30v, & who knows how the one in the -ve rail derives its power from!

    If anyone has any advice or suggestions, please let me know.

    If I get it working properly, I shall put it in the Projects Section of the website.

    Many thanks,

    Simon Bond.



    -----------------------------------------------------------------
    Sent from my iPad2:-
    Simon.D.Bond MEng, BSc(Hons), IEng, MRAeS.
    -----------------------------------------------------------------.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,192
    2,694
    Jan 21, 2010
    Essentially you place the voltmeters across the output, and the ammeters in series with it.

    In each case, it is preferable to measure the current from the regulated rail (i.e. not the 0V rail.

    However these modules can only measure current in the most +ve rail, so while this will work for your +ve regulator, it will not work for the -ve side.
     
  3. Simonbond

    Simonbond

    4
    0
    Feb 25, 2010
    Thanks for your help.

    That's a shame, as I bought a pair of +ve ones, rather than a matched pair.

    Maybe I can use the +ve module as planned & the voltmeter part of the 2nd one for the -ve rail? Or back to analogue ammeters?

    Thanks again,

    Simon B.
     
  4. Simonbond

    Simonbond

    4
    0
    Feb 25, 2010
    Hi again,

    As suggested, I have abandoned using the ammeter for my negative rail.

    I was trying to get the positive side working, & the voltmeter was straight forward, but I could not get a reading on the ammeter. I was using a decade resistance box as a load, & used the yellow wire which is the ammeter supply labelled IN+.

    But according to the spec, these modules have a resolution of 0.1A or 100mA, so neither is suitable for use in my lab PSU as current drawn will me minimal.

    So definitely back to analogue ammeters!

    Thanks again for your help.

    Simon B.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,192
    2,694
    Jan 21, 2010
    What you should look at is a digital meter capable of measuring 0 to 199mV (often called 200mV) from a source that cam be floating with respect to the power for the meter. With a 0.1 ohm resistor as a current shunt, you can measure 0 to 1.99 amps (so a resolution of 10mA).

    You need to look very closely at the specs to ensure that this can be done, and preferably to place the current shunt just before the 317 or 337 so the voltage is regulated. This does mean that you'll read the LM3x7's quiescent current (and the current through the voltage divider), but that's a fairly low and constant value and also may be under the resolution of the meter.

    You may also be able to control the decimal point so your meter reads 0.00 to 1.99 rather than 0 to 199, but that may involve hacking them a little.

    Here is an example. The fact that it's a DC meter and can be powered from AC tells you that the sense terminals are very likely to be isolated from the power connections. Note that the power supply for these is 9 to 12V. I would recommend that you create a -12V rail relative to your +30 rail, and a +12V rail with respect to your -30V rail to power these units.

    If you don't understand exactly what I mean, ask and I'll draw you a schematic.
     
  6. Simonbond

    Simonbond

    4
    0
    Feb 25, 2010
    Thanks Steve,

    That's a concept I didn't think of. I assume the better the precision of the 0.1Ohm current sense resistor the better.

    The circuit I am basing my lab PSU on is:

    http://orca.st.usm.edu/~jmneal/dual_psu/
    http://orca.st.usm.edu/~jmneal/dual_psu/psu2.pdf

    Mine has extra diodes between the Vout & Adj pins of the LM317 & LM337 Voltage Regulators.

    It would be very useful if I could see your circuit diagram, so I can work out where to place the current sense resistor. That would be very kind of you & I would be very grateful.

    I found an old analogue ammeter in my spare parts/junk drawer. I can easily shunt it to read f.s.d at 1000mA, which is the max I would ever drawer form these small regulators. Somebody suggested that I could build this meter into a nice gray box with standard 4mm banana sockets, & use it in series with my PSU when I want to monitor current. Most of my work is with dual-rail op amps, & other analogue electronics, & I rarely ever monitor the current drawn.

    However I'm building some high-power laser diode circuits, & will need to carefully monitor things more closely! So I will be going for the complete dual ammeter solution. Ultimately I'd like to build a more capable PSU over the next year or so; with maybe 5A capacity, with current limit & 5v & 3.3v rails for my MicroControllers/logic etc.

    Thank you very much (from a rather cold & damp UK) for your help, & look forward to maybe seeing your circuit diagram.

    Many thanks,

    Simon B.
     
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