[SI-LIST] Re: DC Power supplies Connected in parallel is not an SI topic
- From: "Tom Dagostino" <tom@xxxxxxxxxxxxx>
- To: <olaney@xxxxxxxxx>, <pakbazf@xxxxxxxxxx>, <a.ingraham@xxxxxxxx>
- Date: Mon, 13 Aug 2012 10:39:54 -0700
There is a big difference between hot swappable supplies designed for that
purpose and trying to parallel two different supplies of vastly different
voltages.
The question as originally stated is unanswerable, this is not enough
information to give a specific answer to the question. Most of the responses
to the question have hit on specific issues that can occur when applied to
specific power supply topology and specifications.
Putting diodes on the output of both a 5V and a 12V supply will only guarantee
the 5 volt supply will be disconnected from the load and the 12V supply will
over voltage the circuit in question if the 12V supply can handle the load.
There is a whole series of answers depending on the designs and specifications
of the two supplies.
And yes, I do recall a question like this in college...
Tom Dagostino
Teraspeed Labs
9999 SW Wilshire St.
Suite 102
Portland, OR 97225
USA
971-279-5325 Office
971-279-5326 FAX
503-430-1065 Cell
tom@xxxxxxxxxxxxx
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of O. Laney
Sent: Monday, August 13, 2012 10:01 AM
To: pakbazf@xxxxxxxxxx; a.ingraham@xxxxxxxx
Cc: si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: DC Power supplies Connected in parallel is not an SI
topic
Power supply paralleling is done all the time, either with supplies designed
for the purpose, or by using ORing diodes or other means. The freshman advice
is for the naive, which freshmen are, and simulators are. What you missed is
that real power supplies are not like theoretical sources. Hot-swappable
redundant supplies are foundational for high reliability power. You can look it
up.
------ Original Message ------
From: "Faraydon Pakbaz" <pakbazf@xxxxxxxxxx>
To: a.ingraham@xxxxxxxx
Cc: si-list@xxxxxxxxxxxxx;si-list-bounce@xxxxxxxxxxxxx
Sent: 8/13/2012 9:34:27 AM
Subject: [SI-LIST] Re: DC Power supplies Connected in parallel
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>Is this a trick question? I am seeing a chain of email trying to
>answer=
>
>this. I think the answer is in freshman year introductory circuit
>analysis...Never connect two voltage sources in parallel. I thi= nk any
>simulator produces error if one try to do that. In practice something
>will blow...Don't mean to discredit anybody's commen= ts or questions
>but did I miss something...Thanks.
>
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>>
>>What happens when 2 DC-power supplies are connected in parallel which=
>>
>
>has
>2
>
>>
>>different voltages let say one has 12V and other has 5V.
>>
>
>
>Because this sounds like a question from a beginner (one who is just
>learning about electronics) ...
>
>What happens depends entirely on the design of the two power supplies.
>There are many possibilities.
>
>They could design a power supply like an audio power amplifier, meaning
>that it tries to make 5V (or 12V) no matter which way the current needs
>to go. Then you would have two power supplies "fighting" with each
>other and Ohm's law predicts very large currents from one supply into
>the other, until something blows or melts.
>
>But most power supplies are not made this way. Usually they need to
>supply current to the load only (for positive voltages) because the
>load's voltage normally doesn't get higher than the power supply that
>is feeding it. So you can think of them like an ideal Thevenin voltage
>source in series with a diode: current goes only one way. In that
>case, the power supply with the higher voltage "wins" and the lower one
>is effectively "not there" and has no current.
>
>Real power supplies could be either of these, and more. As already
>noted, some come with over-voltage protection which adds another factor
>into the picture. And all real power supplies have voltages at which
>they "break" if you connect them wrong.
>
>Paralleling power supplies with the same voltage is tricky enough, let
>alone trying to do it with two intentionally different voltages. One
>would never use two supplies with distinctly different voltages, in an
>attempt to get more current, which is what I think you asked.
>
>In a nutshell, there is no one answer, no one result to your question.
>
>By the way, this has nothing to do with Signal Integrity. While there
>are many experts here, this is not the right place to ask that.
>
>Andy
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