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Dilemma

Discussion in 'General Electronics Discussion' started by GeoffC, May 31, 2015.

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  1. GeoffC

    GeoffC

    39
    4
    Mar 11, 2013
    I have been looking around on E-bay/Amazon for a dual bench regulated power supply. They are all rather expensive. (£150 + Postage £25 and more)

    What I have in mind is to buy two signal power supplies, (£50 each) and use side by side by connecting the two together.

    http://www.ebay.co.uk/itm/5A-0-30V-Adjustable-DC-Power-Supply-Precision-Variable-Digital-Lab-w-clip-CE/301531393178?_trksid=p2045573.c100034.m2102&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D20140107092241%26meid%3Df72fff1084604a16ad49e951d48163ff%26pid%3D100034%26rk%3D1%26rkt%3D4%26mehot%3Dpp%26sd%3D151123354933

    ie:

    Taking the positive terminal power from one

    Connecting the zero terminal to the positive terminal to the other/second

    Hoping to take negative volts from the zero terminal of the second power supply

    (Similar as connecting batteries together)

    If I were to do this - would it work? - might there be conflict between the two? This idea of mine could save me £50 or more, crazy as it seems.

    I know I could buy two voltage regulators - LM317 & LM337 – but by the time I’ve hurried them together, not forgetting further expense, transformers, Photo Resist board, cases, connectors, and so on.

    Any help would be appreciated

    Regards, Geoff. (GeoffC)
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,588
    2,360
    Nov 17, 2011
    Indeed. This works perfectly fine if the outputs of the two power supplies are completely insulated (i.e. not connected to protective earth). In fact, this is how dual supplies sometimes are constructed.

    You may need to consider a double pole double pole switch at ´the output of your new "dual supply" to be able to simultaneously turn on and off both voltages.
     
  3. GeoffC

    GeoffC

    39
    4
    Mar 11, 2013
    Hello Harald,
    Thank you for your reply, it is appreciated. Your remark regarding the two power supplies being insulated from the protective earth. This was the kind of conflict that I feared. BUT there are three out-put terminals being Positive / Earth/Ground / Negative (This negative in my book is zero volts)
    So, am I right in keeping clear of the centre terminal earth/ground’. I assume the plug’s earth pin would be connected to the casing.
    Regards, Geoff. - (GeoffC)
     
  4. davenn

    davenn Moderator

    13,671
    1,892
    Sep 5, 2009
    NO, the Earth/Ground is the Zero volts rail its the common connection between the 2 supplies ( where the + of one and the - of the other is joined)

    lets take a ± 12V supply

    the black negative is -12V, green ground, is 0V and the red positive is +12V

    Dave
     
    Harald Kapp likes this.
  5. GeoffC

    GeoffC

    39
    4
    Mar 11, 2013
    Hello Dave,

    Yes, you are right, IF it were a dual voltage supply, BUT it is a singlel voltage supply (0 – 30v @ 3amp). There are three O/P terminals one being positive and one calling itself Negative. The third between the two happens to be called ‘EARTH’, which I assume is connected to the earth pin of the plug. There are no negative volts from the negative pin, this is why I call it ‘zero volts’ Anyway,Thanks for the interest

    Geoff
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    10,588
    2,360
    Nov 17, 2011
    Geoff,
    simply measure the DC resistance from "+" and "-" of your supply to the EARTH connection (with the supply powered off, of course). Having the EARTH terminal does not necessarily mean that the supply is internally connected to this socket. It may be there for convenience only.
    If you measure a noticeable DC resistance (< 1MegOhm), you should not connect two of these supplies in series as this will create current flow between the two supplies via the EARTH terminal.
    If the Dc resistance is rather high (> 1MegOhm) a series connection should be possible.

    Note that the value 1MegOhm is arbitrary. The important thing is negligible current flow through the EARTH terminal(s).
     
  7. GeoffC

    GeoffC

    39
    4
    Mar 11, 2013
    Hello Harald,

    Thank you for your last ideas of taking resistance readings between “+” & “-“ terminals, but have chickened out (Cold Feet) and sent it back to exchange for a real Dual Power Supply. A little more money, but I feel this way I won’t be caught between the devil & the deep blue sea. It is way over the top for what I need, just 0 – 20v (x2) at 1amp would be ample. Anyway, I have learnt something regarding the resistance flow between the terminals that you mentioned. (Needs to be >1MegOhm) - Got-It

    Many thanks, Geoff.
     
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