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I have two identical 48 V switched power supplies, both of which can deliver the same current of, say, 4 A.

Can I connect them in series to get 96 V at 4 A? Is this something possible / recommended?

zebonaut
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user1816813
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  • Or this one: http://electronics.stackexchange.com/questions/27832/putting-two-wall-warts-in-series –  Jun 02 '13 at 03:49
  • Maybe this one: http://electronics.stackexchange.com/questions/18350/can-two-atx-smps-be-connected-in-series-to-get-10-and-24v-dc –  Jun 02 '13 at 03:54
  • @DavidKessner Any chance to merge this question (and respective answers) and the three you've mentioned into one? Maybe not including the one about ATX supplies? I am usually against merging because there are subtle differences even among similar questions, but these are really exact duplicates. Can we just merge everything into the oldest question? – zebonaut Jun 02 '13 at 06:43
  • @zebonaut I have no idea, I'm not moderator. I'll leave it up to them to figure out what to do, if anything. –  Jun 02 '13 at 14:25

2 Answers2

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Some gotchas here:

  • This will only work if the outputs of the 48V supplies are 'floating'. Many power supplies earth the low side (negative) output - you cannot put them in series since the 'bottom' supply would have both its positive and negative rails at earth potential (i.e. a short circuit).

  • In the event of overload (excessive current), whichever power supply has the lower threshold will collapse first and bring down the other power supply rail, even if it has a naturally higher overload threshold.

  • The two power supplies won't start up exactly the same way (turn-on time, rise time, etc.) so you won't have a monotonic ramp from 0 to 96V.

Adam Lawrence
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  • Interesting to read all these comments ... Indeed the reply comes from the power supply manufacturer ... that is most of the time in china :-( – user1816813 Jun 03 '13 at 01:19
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This can't be answered in general without knowing the details of your power supplies. Some will accept being connected in series without problems, some might not and start to become unstable. To be sure, you need a manual, ideally with a section like this:

PULS CS10.481, Series Operation

(Source; Note that this same supply will not accept being operated in parallel with another one, which is o.k. for other supplies.)

Two common gotchas for series-connected supplies:

  • If you short the output, the two supplies are connected back-to-back, with a reversed polarity. Think of it this way: One supply is a bit stronger than the other one, and it will force its output voltage onto the other supply, upside down. Even some common conditions, like charging a capacitor connected as a load to the supplies, will look like a temporary short circuit.

  • 48 V is considered a safe, low-voltage. The outputs of your supplies may be designed such that they rely on this fact. Connecting your supplies in series will yield a dangerously high voltage. Aside from posing a danger to the user, there may be components in your supplies that won't accept the increased voltage.

zebonaut
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  • Pretty fair answer I'd say +1. One caveat, if you can technically wire them in series with the centre point at protective earth i.e. -48 0 +48 then this "overcomes" the SELV issue. – Andy aka Jun 01 '13 at 21:07
  • Nevertheless, you said "may be components in your supplies that won't accept the increased voltage." You meant that Inside the supplies there is components that cannot cope 96 V ? How can it be ? they should not see this voltage ? The 96V is for a device that accept it. – user1816813 Jun 01 '13 at 22:25
  • I don't understand the first of the "common gotchas". Both power supplies are driving current in the same direction around the loop, and both will go into current limiting. Nothing is "forced backward". – Dave Tweed Jun 02 '13 at 00:54
  • @user1816813: Think about Y capacitors at the output. Depending on where or how the case or PE of the supplies is connected, these capacitors might not be within their specification any more. Draw a schematic of your setup, including the cases of your supplies. – zebonaut Jun 02 '13 at 06:28
  • @DaveTweed Yes, both will drive the current in the same direction. However, what will happen with the voltages if one supply is just a bit "stronger" than the other? KVL tells you that one supply will have a negative voltage forced upon its output. While this condition may not remain for a long time, both supplies' output voltages may start to oscillate, with a temporary neg. voltage on either supply. – zebonaut Jun 02 '13 at 06:31
  • First, when shorted, the power supplies are acting as current sources, not voltage sources. Second, what do you mean by "stronger"? Higher voltage? Higher current limit? I think you need to write out the KVL equation to demonstrate whatever it is you think you're talking about. – Dave Tweed Jun 02 '13 at 11:07
  • @DaveTweed: This becomes too long for the comments. See here: http://electronics.stackexchange.com/questions/71340/multiple-power-supplies-in-series-negative-voltage-on-one-of-them-possible – zebonaut Jun 02 '13 at 12:43