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Series Power Supply Safety Circuit

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by jocanon, Apr 7, 2014.

  1. jocanon

    jocanon Member

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    So this is just in the idea stage right now, but I wanted to create this post anyway so that I can get some input in formulating my idea. The problem I need to solve is I have two 12v power supplies connected in series (DC isolated on upper one) in order to create one 24v power supply. Everything works fine unless/until one of the PSUs goes out but the other keeps going. Then you have a situation where the fan is not running on the one that is off but the one that is working is still pulling current through the one that is turned off with no fan to cool it. What happens is it heats up and eventually starts to smoke if the one that is working is not turned off. I want to build something that will prevent this from happening.

    My idea so far is a circuit that is in-line with the mains AC power cord so that it will just turn off power once it detects voltage has dropped below say, 18v. I already have an on/off switch in-line with the AC power cord routed through a Hammond ABS plastic enclosure, so it would be easy to integrate other logic circuitry at that enclosure. I am not an EE, but I think I could do that with a comparator circuit and a relay pretty simply, but I will need some help building the actual circuit. Or, if there is an even more simple, safer, or just better idea to solve the problem that I have not thought of, please chime in and let me know.

    Safety is the keep issue here. I need some out of the box thinking to solve this problem. I want it to be a plug and play thing that I can give to anyone that has such a 24v supply that they can just use this for their AC power cord & on/off switch and run the DC output through it as well in order to detect when voltage has dropped. I am picturing it all fitting nicely in an ABS enclosure that Velcros to the top of the series PSUs with AC power coming in one side, on/off switch mounted on top, and DC output coming out the front...maybe for extra bells and whistles, if I want to, even putting a little volt meter and/or current read out on there as well (but that's for another thread...). We are running high currents at 24v, about 50 amps. Is my idea a good one? Is it possible? Thanks!
     
  2. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    Why are you not using one 24 volt supply?
     
  3. jocanon

    jocanon Member

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    Cost, I can get two 12v power supplies for less than 1 24v supply. I am talking about high end server power supplies.
     
  4. dave

    Dave New Member

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  5. ChrisP58

    ChrisP58 Well-Known Member

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    Two 12V relays. Connect the coils individually to each power supply. Connect the contacts of both relays in series to the load.

    Both relays need to be energized for load current to flow.
     
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  6. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    Are you saving enough to make a supervisory circuit?
    How much are you willing to spend?
     
  7. jocanon

    jocanon Member

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    I was hoping to be able to make the circuit fairly inexpensively, around $10-$50 max. If it cannot be done for that much then it might not be worth it. I buy used power supplies in bulk and get them for around $10 a supply, cannot find a 24v supply capable of 75 amps for that cheap.
     
  8. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    No wonder you are worried about them dieing.
    You use these for high end servers?
     
  9. jocanon

    jocanon Member

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    Lol, no, they come from high end servers. They are actually quite reliable, believe it or not. In 2 years of selling them on the secondary market I have only had 9 total returns due to failure and I sell them with a 1 year warranty. I offer the warranty because people (similar to above) have doubts about them since they are used and I am taking the risk...call me crazy, BUT the track record has proven it to be a well calculated risk on my part.

    Anyway, back on topic...
     
  10. ronv

    ronv Well-Known Member Most Helpful Member

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    The relay idea would work, but a 50 volt 50 amp DC relay probably won't be cheap or small. What does the voltage measure across the bad one when it fails? Does turning it off (by itself good one still on) solve the problem or does it still smoke?
     
  11. jocanon

    jocanon Member

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    It measures zero. What I suspect happens in most instances is the fan goes bad then once the PSU is no longer getting a tach signal from the fan, as a safety feature it just shuts down. So all is well with just a 12v PSU, but when they are connected in series and the fan goes out on one but not the other and you have a load connected to the positive of the good one and negative of the bad one what you get is a situation where you are running current through the bad PSU that is technically off, but just acting as a conduit now and heating up more and more as time goes by with no fan to cool it off. So I need some way for the good PSU to detect the bad PSU is shut down and to also shut off once that happens. There is actually a way that somebody in one of the other forums figured out how to hack into the logic circuitry of the particular PSU I have to make this happen, but I only have 10 of that model left so what I was hoping for is a solution external from the PSU that could be more or less universal to any model. Maybe it will be too expensive. But it seems like there would be a simple way to turn a switch off and on if voltage falls below a given value.
     
  12. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    Use the relay to switch the AC input side of the power supplys and it would only need 15 amp contacts.
    Use double pole relays and run the AC supply thru both so if ether fails power is cut to both.
    This way you would have two 15 amp AC circuits.
    This would work with any power supply.
     
  13. jocanon

    jocanon Member

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    I like that it would work with any power supply (versus an internal mod)...then people that already have 2 PSUs connected in series can use it too for added safety.
     
  14. jocanon

    jocanon Member

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    Just had another thought that I like about that, I think you would only need one relay, right? Once the voltage drops you just shut it down completely on the AC side and there is only one AC cord coming into a Y with two going out to each PSU, you shut it off before it splits to the Y.
     
  15. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    Yes you can use one circuit but you still need two relays. One powered from each supply.
     
  16. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I often stack power supplies. I have a bunch of 0-100V 2.5A supplies and I need 400 volts. Don't ask.

    If one of the supplies goes into current limit the other supplies continue to supply power and reverse the first supply. Then things go wrong.
    I have diodes across each supply. So worst case there is only -1 volt across a supply.

    I don't know if this is the same kind of problem.
     
  17. jocanon

    jocanon Member

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    Not doubting you, because I am sure you know more than I do, but why would I need two? I mean, couldn't I just switch off the one main power line if voltage falls?
     
  18. jocanon

    jocanon Member

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    That's a lot of volts! :) I think my case is different because I am running relatively low voltage, 24v, at high current, about 50 amps. But I don't know, maybe you are on to something.
     
  19. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Power supplies don't like power being push through from a different supply.
    The diode routs the current around the supply.
     
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  20. ronv

    ronv Well-Known Member Most Helpful Member

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    I think it gets a little more complicated. If it shuts off when the voltage is absent. How does it turn on in the first place?
    It's good that you still have voltage from the good supply, so something can be worked out without adding a 3rd supply.
    But Ron's fix might be easiest if that is the problem. I think we can work something out either with 2 switches, a momentary on and off and some logic. I'll have a look at some parts or some more ideas will come up. The diode would be easy to try, but will need a bit of a heat sink at 50 amps, so might be kinda big.
     
  21. jocanon

    jocanon Member

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    Gotchya...thanks ronv for the explanation, that makes sense.
     

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