when can power supplies be connected in series, and when can

[Michael Noone]

Hi - I remember hearing that some DC power supplies cannot be connected in
series, while others can. Can anybody tell me what
features/specifications/etc. to look for in a power supply that can be
connected in series with other power supplies? Thanks!

-Micahel Noone

 

[John Fields]

On Tue, 29 Mar 2005 20:05:06 GMT, Michael Noone
<mnoone.uiuc.edu@127.0.0.1> wrote:

Hi - I remember hearing that some DC power supplies cannot be connected in
series, while others can. Can anybody tell me what
features/specifications/etc. to look for in a power supply that can be
connected in series with other power supplies? Thanks!

---
Power supplies that can be connected in series have floating outputs.

--
John Fields
Professional Circuit Designer

 

[Rikard Bosnjakovic]

John Fields wrote:

Power supplies that can be connected in series have floating outputs.

This would be batteries I presume. What power supplies does now have
floating outputs?


--
Rikard Bosnjakovic http://bos.hack.org/cv/

Anyone sending unwanted advertising e-mail to my address will be
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[John Fields]

On Thu, 31 Mar 2005 16:41:31 GMT, Rikard Bosnjakovic
<bos@REMOVETHIShack.org> wrote:

John Fields wrote:

Power supplies that can be connected in series have floating outputs.

This would be batteries I presume. What power supplies does now have
floating outputs?

---
Damn near every wall-wart in the world, pretty much any lab supply,
and all the HP stuff I've got around here. If you want something more
definitive, Google is your friend.

--
John Fields
Professional Circuit Designer

 

[mike]

Michael Noone wrote:

Hi - I remember hearing that some DC power supplies cannot be connected in
series, while others can. Can anybody tell me what
features/specifications/etc. to look for in a power supply that can be
connected in series with other power supplies? Thanks!

-Micahel Noone

You can hook power supplies in series if they're designed to be hooked
in series.
Make sure the outputs are designed to float as far as they will in series.
Make sure they can withstand reverse current, negative voltage at rated
current.

Sounds like any old supply will do as long as the load is happy.
But what happens when you short the output. One will limit first and be
ripped negative by the other one. I've seen power supplies, even with
reverse protection diodes, latch up when the output was pulled negative.
You can get some interesting turn-on transients, especially if you have
to throw two switches.
Some VERY interesting things can happen when the supplies have foldback
current limiting.
If your supplies are mismatched, your problems multiply. You really
don't want your 40A supply trying to reverse your 4A supply.

Safest is to tell the vendor what you intend to do and get their blessing.
mike

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[Chris]

Michael Noone wrote:

Hi - I remember hearing that some DC power supplies cannot be
connected in
series, while others can. Can anybody tell me what
features/specifications/etc. to look for in a power supply that can
be
connected in series with other power supplies? Thanks!

-Micahel Noone

Hi, Michael. In addition to Mr. Fields' note, both power supplies
should be capable of carrying the current requirement of the load. For
instance, if PS 1 is 5V @ 3A, and PS2 is 5V @ 1A, you'll get 10V if you
put them in series, but you can't draw more than 1 amp from your 10V
supply.

If you're dealing with high voltage, you also have to look at the hipot
between the transformer primary and secondary, and the hipot between
the secondary and the chassis, if that's at another potential.

If your power supplies are regulated, the combined supply will have
different (and usually somewhat worse) specifications for load
transient response. That's because the output capacitors of the
supplies are in series, too. Also take a look at load regulation.
It's best to do some experimentation if these are critical.

Good luck
Chris