[SI-LIST] Re: DC Power supplies Connected in parallel is not an SI topic
- From: Faraydon Pakbaz <pakbazf@xxxxxxxxxx>
- To: <tom@xxxxxxxxxxxxx>
- Date: Mon, 13 Aug 2012 15:36:32 -0400
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Well said Tom!! That is what I was wondering and said but I almost got
yelled at....
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|RE: [SI-LIST] Re: DC Power supplies Connected in parallel is not an =
SI topic =
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There is a big difference between hot swappable supplies designed for t=
hat
purpose and trying to parallel two different supplies of vastly differe=
nt
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 wh=
en
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
www.teraspeed.com
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827
-----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 a=
n 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 ar=
e.
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=3D
>
>this. I think the answer is in freshman year introductory circuit
>analysis...Never connect two voltage sources in parallel. I thi=3D nk =
any
>simulator produces error if one try to do that. In practice something
>will blow...Don't mean to discredit anybody's commen=3D ts or question=
s
>but did I miss something...Thanks.
>
>
>
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>>
>>What happens when 2 DC-power supplies are connected in parallel which=
=3D
>>
>
>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, meanin=
g
>that it tries to make 5V (or 12V) no matter which way the current need=
s
>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 voltag=
e
>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 on=
e
>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 facto=
r
>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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