adding an analog volt meter to power a supply?

Discussion in 'The Projects Forum' started by dentaku, Sep 1, 2014.

  1. dentaku

    Thread Starter Member

    Jul 29, 2013
    45
    2
    I'm building a little power supply. It's just the standard salvaged transformer, bridge rectifier, LM317 and LM337 kind of thing that can supply a bit over +/- 20V.

    I have an inexpensive 30V analog volt meter and I'm wondering if there's anything I should know about connecting it to this power supply to monitor it's voltage?
    http://www.ibuyla.com/Product/93769...suring_DC_voltage_meter_30V_DC_voltage_meter/
    If I measure the resistance between the + and - terminals of the meter I get 27K so it seems strange to me that I could simply connect this meter across the output of the power supply and not have a negative effect on the circuit.
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,504
    380
    Hi,
    Its about 1K/V, which should not cause a problem with LMxxx Vreg, its loading at 30Vdc will be ~ 1mA.

    I would recommend a Double throw, centre Off switch to your meter, so that you can measure positive or negative outputs.
    E
     
  3. dentaku

    Thread Starter Member

    Jul 29, 2013
    45
    2
    Is there any reason why you mention a Double throw, centre Off switch?
    I was planning to do this?
    [​IMG]
    https://imagizer.imageshack.us/v2/783x585q90/538/TGMLzn.png
     
  4. paulktreg

    Distinguished Member

    Jun 2, 2008
    612
    120
    But your power supply has three output terminals doesn't it? (+Vdc, 0V,-Vdc).
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Yes it does. The DPDT switch will work for that.
     
  6. dentaku

    Thread Starter Member

    Jul 29, 2013
    45
    2
    Yup, I just tried it out for real and it works just like in my simulation.
    You just need to remember when the switch is set to negative because obviously that meter doesn't measure negative.
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Well, if you wanted to get fancy you could use a 3 or 4 PDT switch and wire one of the poles to control one red and one green (or whatever color is your preference) LED to indicate which voltage you are measuring.
     
  8. dentaku

    Thread Starter Member

    Jul 29, 2013
    45
    2
    Well, now you've totally complicated my design! ;)
    I just found a pushbutton toggle switch that is capable of doing just that and I have a red/green LED that changes colour depending on the polarity and a green/amber one that has two anodes to choose from.
    It's one like this (only one of them, not 8 ganged ones of course) and I finally have a mental image of how to wire this up.
    [​IMG]
    The bicolour will take half of the switch and the common cathode would only need one quarter of it.
    [​IMG]
     
    Last edited: Sep 2, 2014
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