Two 12v 2a power supplies in series

Discussion in 'The Projects Forum' started by odm4286, Apr 24, 2013.

  1. odm4286

    Thread Starter Active Member

    Sep 20, 2009
    155
    5
    Hello everyone first let me start by saying I am new to electronics so if my explanation seems a bit off please forgive me. So basically I wanted to create my own 24volt power supply for bread boarding projects. I found two old identical power supplies I had that were rated at 12v 2a.

    I then googled a circuit that showed how to wire them in series here
    http://www.rcgroups.com/forums/showthread.php?t=1170241

    *NOTE my power supplies are not grounded (plastic case)

    Now everything works great, but I'm having trouble wrapping my head around the concept. How exactly does the neutral of power supply two handle the increased voltage? Lastly, is this method safe?
     
    Last edited: Apr 24, 2013
  2. aws505

    Member

    Mar 11, 2013
    59
    7
    Sometimes you don't even need to break the earth-ground connection on the power supply. Think of how these power supplies are likely to be made: The mains voltage goes through a step-down transformer, then through a D-Bridge, then is likely smoothed by caps and regulated by a transistor network. The first step, the transformer, removes the output's reference to the AC line's neutral. Of course, I'm speculating. You should really look up the datasheet for your power supplies and see if they have a reference to the AC line's neutral. Alternatively, you can use a DMM to measure continuity from the minus output to the ground pin on the back. If it reads open (greater than 1MOhm), it should be safe to *try* hooking the supplies together without breaking the earth-ground.

    Generally, if you can avoid breaking the earth-ground, it'll be better. Breaking the earth-ground connection can cause the case of your supply to float up to high(er) voltages and can either zap you when you touch it or it can arc to the other case if you put them close together.

    Many supplies can be hooked in series without breaking the earth-ground connection, but some supplies do not have good isolation; some supplies have NO isolation, so make sure to check continuity before you try this with a new supply.

    Hope that helps. Let us know if you're still confused.
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,447
    3,363
    Bench power supplies are normally floating, that is, there is no connection to the earth safety ground. If that is the case then it is ok to connect the positive terminal of one supply to the negative terminal of the other supply in order to create a split dual supply. You should always check to make sure the supply is floating before attempting to do this.

    If your supply is in a plastic case and does not have a grounding pin in the power plug (such as wall adapter) then it is very likely that the supply is floating. Always double check first to be certain.

    Don't do this with a transformerless laptop charger.
     
  4. odm4286

    Thread Starter Active Member

    Sep 20, 2009
    155
    5
    Hmm trying my best to follow so excuse the layman's, both power supplies were ungrounded by default. The connection to ac is two pronged there is no ground prong so I just skipped that step
     
  5. aws505

    Member

    Mar 11, 2013
    59
    7
    Excellent. You're in luck, then. Go ahead and try to connect the negative output of one supply (call it Supply A) to the positive output of the other supply (call it Supply B). Turn it on and make sure nothing explodes. Now, you can use the negative output of Supply B as the negative voltage supply and the positive output of Supply A as the positive voltage supply. The common output is now "ground."
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,447
    3,363
    Think of two 9V batteries in the palm your hand. The 9V is measured between the +ve and -ve terminals of the battery and is not with reference to any absolute ground. The battery could very well be sitting at 100V for all we know. We say that the batteries are "floating" with respect to ground, i.e. they are not grounded.

    You can connect the +ve terminal of one battery to the -ve terminal of the second battery resulting in a net voltage difference of 18V between the unconnected terminals.

    If on the other hand, both -ve terminals were connected to earth ground, when you attempt to connect the +ve terminal of the first battery to the -ve terminal of another battery (which is therefore already grounded) you will create a short circuit on the first battery with disastrous results.
     
  7. odm4286

    Thread Starter Active Member

    Sep 20, 2009
    155
    5
    Thanks a lot MrChips that helps me understand this. Looks like I have the following

    [​IMG]

    Uploaded with ImageShack.us

    24v from 1 to 3
    12v from 1 to 2
    12v from 2 to 3

    Now that everything works correctly I do have one other question. Aside from looking up the manufactures spec for this PS, which I doubt I could even find its a cheap Chinese ps for an external Hard-drive, how do I know this is safe? Should I worry about it "burning out" the supplies and becoming dangerous? Thanks again for the help everyone, next step ill be adding some diodes for protection and a POT to make it a variable supply :)
     
  8. MrChips

    Moderator

    Oct 2, 2009
    12,447
    3,363
    According to your diagram, PS1 is grounded.
    PS2 was also grounded but you have broken the connection to ground. That is why you have not blown up PS2.

    You cannot use a pot to turn it into a variable supply. Your pot will burn out.
    You can reduce the 12V if you use an adjustable voltage regulator chip or circuit.
     
  9. strantor

    AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    How do you see that working? You can't just throw a pot across the 24V and use the output as a "variable supply." Variable signal maybe, but not variable supply. The pot will severely limit the amps you can deliver, and any load at all will pull the voltage down to nothing. If you want a variable supply, you are going to need something more sophisticated than just a pot.
     
  10. odm4286

    Thread Starter Active Member

    Sep 20, 2009
    155
    5
    ^^ looks like I have more to learn but that is my goal, and sorry about the diagram my power supplies are not grounded at all that circuit is from another forum.
     
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