DC 17V from PC power supply?

[JW]

On Tue, 25 Jun 2013 12:17:35 -0400 "P E Schoen" <paul@peschoen.com> wrote
in Message id: <kqcfh9$qon$1@dont-email.me>:

Usually the +5V and +12V supplies are fairly high current. And in many cases
only the +5V supply is directly regulated. It might be possible to reverse
the diodes and output capacitor on the +12V supply to make it -12V, and then
you should get 17V between them.

IF it's possible, and I have a feeling that it's not, you'd better flip
the polarity on the electrolytic caps as well!

 

[Jasen Betts]

On 2013-06-25, Mr. Man-wai Chang <toylet.toylet@gmail.com> wrote:

On 25/06/2013 8:01 PM, John G wrote:
NO!
Shorted is the correct description.
They are both referenced to ground so MAGIC SMOKE will result. :-?

OTOH, should I attempt -5V and +12V pins at the power connector? Or
should I try +5V and -12V?

Which one is safer and could supplies 4A?

neither, both -5 and -12 are typically capable of less than 1A (0.2A
on a device at hand)

-5 has been discontinued in new PCs

If you need 17V and 4A you could try an aftermarket adjustable laptop
power brick, It might be easier to find one at 18V or 16V though, but
there are 17V ones out there.

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[Michael Black]

On Thu, 27 Jun 2013, Jasen Betts wrote:

On 2013-06-25, Mr. Man-wai Chang <toylet.toylet@gmail.com> wrote:
On 25/06/2013 8:01 PM, John G wrote:
NO!
Shorted is the correct description.
They are both referenced to ground so MAGIC SMOKE will result. :-?

OTOH, should I attempt -5V and +12V pins at the power connector? Or
should I try +5V and -12V?

Which one is safer and could supplies 4A?

neither, both -5 and -12 are typically capable of less than 1A (0.2A
on a device at hand)

-5 has been discontinued in new PCs

If you need 17V and 4A you could try an aftermarket adjustable laptop
power brick, It might be easier to find one at 18V or 16V though, but
there are 17V ones out there.

Which I find all the time in the garbage. Or that time I needed a power

supply for a Powerbook 1400, I opened up an inkjet printer I'd dragged
home from the garbage, and it had the right voltage (those tend to have
higher voltages than +12, but still decent current). The external power
supplies have the advantage that they are already cased. The inkjet
supplied a standalone power supply board, making it easy to extract, but
not casing.

But, the problem is that applying voltage to a battery isn't a good thing,
if the battery is some more recent kind, it needs a proper charger.

Michael