Multiple DC supply question

Discussion in 'The Projects Forum' started by TWRackers, Apr 3, 2012.

  1. TWRackers

    Thread Starter Active Member

    Dec 29, 2008
    41
    2
    This is the first project I've attacked where this situation has arisen, so I need to ask a question here.

    My current project requires two, maybe three, DC voltages for different portions. So far I know I'll need +12 vDC and +6 vDC, both well under 1 A current draw. I have some PS kits which use an LM317T voltage regulator to provide an adjustable DC output from 3 volts up to 37. Each board has its own FW bridge rectifier diodes and filter cap on the input side of the voltage regulator. Output is the +V and Common pair. Okay, simple enough.

    Here's the burning question. Can two such supplies share their Common connections IF (big IF) they're also sharing the same transformer secondary winding on the input? Or should each be given its own secondary, which would most likely imply two transformers. Using two transformers is not a problem, I'd just rather use only one for simplicity sake if it'll work. I doubt I can easily find a transformer with dual secondaries at the local shop, so I'm not considering that option.

    I'm sure this has come up before for others, so an answer would be much appreciated.

    TWR
     
  2. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    A common ground is practical under such circumstances--the lower two diodes in each bridge all get connected in parallel and that will not hurt provided that the individual diode current rating is sufficient for all loads added together. The low voltage regulators must soak up the excess power from the high input voltage--to reduce this, series resistors may be added--calculate such at the highest load current to make sure that the regulator does not run out of headroom voltage.

    The hooker comes in when a negative voltage is required--and it appears that you do not need a negative voltage too--when that is the case, full wave center-tapped configuration is the best way to go.
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,126
    3,048
    You might be able to use a single transformer, bridge and filter cap arrangement, followed by your regulators providing separate voltages from that single source. But pay attention to current loads and power dissipation. The transformer and bridge must be able to supply the combined current of all your regulators.

    You might also consider just pulling the PSU from an old (ie. free) computer. It'll have ±12V, 5V and maybe other voltages, lots of current, built in switch and cord, heat and over-current protections and so on.
     
  4. TWRackers

    Thread Starter Active Member

    Dec 29, 2008
    41
    2
    I don't have to worry about sharing the bridge rectifiers or the filter caps, each PS kit has its own and just needs the addition of a transformer. Only the transformer secondary will possibly be shared. I just want to make sure that tying the DC commons together downstream of the bridges will be okay if the same secondary winding is upstream for both.
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,126
    3,048
    Got it. Should be fine, as was noted. Just be aware that the different supplies may influence the others if the total loading approaches the capacity of the single secondary. Don't turn your transformer into a fuse. ;)
     
  6. TWRackers

    Thread Starter Active Member

    Dec 29, 2008
    41
    2
    No danger of that, I'll be using a transformer with reserve current capacity over what my DC supplies will require all together. In fact, the +6 vDC supply will be powering only a 4PDT relay (150 mA continuous ON current), while the +12 vDC will drive some LEDs (ULN2803A) and two small DC motors (Parallax Motor Mind B boards). I could even power the 6V supply from the 12V one by bypassing the smaller supply's input diodes (the kit gives you that option), but then the 6V load does add into the 12V supply's load, and I'd prefer to keep them separated.
     
  7. TWRackers

    Thread Starter Active Member

    Dec 29, 2008
    41
    2
    So I now have three of these adjustable DC supplies built, but I left out the four diodes on each which form the input bridge rectifiers. The intent was to power all three from a common not-quite-so-regulated DC supply bus, maybe +18 vDC to provide overall power to the separate +12, +6, and +5 regulated supplies. None of the regulated supplies will be supplying even close to 1 amp (the max rating for each). Unless I'm seriously wrong, I should be able to let all three DC supplies have their negative sides tied together with the negative side of the supply bus, establishing a common ground for everybody. Obviously the supply bus will be rated for at least 3 amps just to allow some margin.

    Now the new question:

    Each of the regulated supplies has a filter cap just inboard of where the bridge diodes would've been. If I were simply to wire all the inputs (without diodes) to the supply bus (now maintaining proper polarity between them), that's effective putting those input filters in parallel with each other AND the output filter of the supply bus. Would it be a good practice to put a single forward-biased diode between the supply bus and the input filter of each regulated supply (one per supply)? I believe that would prevent any regulated supply from drawing current from the input caps of its brethren.

    I've tried researching this on the web, but all I find are references to redundant supplies providing power to a shared load; I'm trying to share the supply bus to my regulated supplies.

    Is my analysis correct, and would this be a good plan? Did I miss anything obvious?

    TWR
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,126
    3,048
    I think you have nothing to lose from this. Diodes are cheap and will give the benefit you mentioned. They'll also take a little heat (literally) off the regulators - not a lot but every little bit helps - and provide reverse voltage protection against hooking something to one supply and ruining another. Hmmm...actually you should have a reverse voltage diode on each regulator already.
     
  9. TWRackers

    Thread Starter Active Member

    Dec 29, 2008
    41
    2
    In this case, diodes are free... specifically, the unused diodes from the input side of each supply, which are still in the parts boxes of the three kits. Basically, I choose the input polarity in advance (since I'm bringing in DC and not AC), and I can use two pairs of existing holes, one for the blocking diode described above on the new positive side, and I can jumper the holes which would've held the other diode for that particular input polarity, but on the negative side. That side also becomes my common across the three regulated supplies.
     
Loading...