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DC 17V from PC power supply?

M

Mr. Man-wai Chang

Jan 1, 1970
0
1. If I shorted the +5V and +12V in the molex connector,
would I get +17V? What's the max. current?

2. Could I use 17V to charge a notebook battery rated 16V and 3.8A?
Would it hurt the battery?

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M

Mr. Man-wai Chang

Jan 1, 1970
0
Mr. Man-wai Chang submitted this idea :
NO!
Shorted is the correct description.
They are both referenced to ground so MAGIC SMOKE will result. :-?

Thanks! Expected answer! :)

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M

Mr. Man-wai Chang

Jan 1, 1970
0
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?

--
@[email protected] Remain silent. Nothing from soldiers and magicians is real!
/ v \ Simplicity is Beauty! May the Force and farces be with you!
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T

Tom Biasi

Jan 1, 1970
0
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?
If you manage to find 17 volts the current rating will that of the lower
rated source.Look at the supply label. It will say what the current
rating of each supply is. The 5 volt supply can supply much more than
the 12 volts but you will need to use the lesser rating.

Tom
 
P

P E Schoen

Jan 1, 1970
0
"Tom Biasi" wrote in message
If you manage to find 17 volts the current rating will that of the lower
rated source.Look at the supply label. It will say what the current rating
of each supply is. The 5 volt supply can supply much more than the 12
volts but you will need to use the lesser rating.

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. The outputs are referenced to ground and
may be tied to chassis, so you may have to float it. You can probably find a
nominal 16-20 VDC laptop supply capable of 4 amps for about $10 and it would
be much safer, smaller, and more convenient. But if you want to play with an
ATX or similar power supply, there are schematics available that give a
general idea of how they are made.
http://www.pavouk.org/hw/en_atxps.html

Another possibility is to change the feedback resistors so that you get 17V
from the 12V supply (while the 5V output will go up to about 7 volts). Just
make sure the PSU doesn't have a crowbar on the 5V supply.

There are other web pages showing ways to hack these PSUs to make battery
chargers and get various voltages and even make DC-DC converters out of
them.

Paul
 
D

Daniel Pitts

Jan 1, 1970
0
1. If I shorted the +5V and +12V in the molex connector,
would I get +17V? What's the max. current? No.

2. Could I use 17V to charge a notebook battery rated 16V and 3.8A?
Would it hurt the battery?

Potentially. Also, battery charging circuits aren't just applying the
rated voltage. There is a lot more to it than that.

Based on your recent posts here, I suggest finding a good book about DC
electricity. Maybe taking a class or two. Or three. I just finished the
third (DC, then AC, then Linear Circuits). There are a lot of things
that are just intuitive with electronics, but a lot that isn't. Just
guessing and trying will end up in a lot of magic smoke, and potentially
loud bangs and fires.

I'm a huge fan of HyperPhysics, though that isn't where I learned
electronics from. It might be a good starting place though...

<http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/cktetrcon.html#c1>
 
P

P E Schoen

Jan 1, 1970
0
"Mr. Man-wai Chang" wrote in message
2. Could I use 17V to charge a notebook battery rated 16V and 3.8A?
Would it hurt the battery?

You can probably use a surplus or used notebook supply of similar specs.
Here's one with 19V and 2.65A for just $4:
http://www.mpja.com/19-Volt-Desktop-Power-Supply-265A-EIAJ-Plug/productinfo/19078 PS/

And if you need 4.74A and can spare $15:
http://www.mpja.com/19V-474A-Desktop-Supply-Delta/productinfo/30336 PS/

You can always add one or more diodes in series with the output to lower the
voltage. And these units may also have built-in current limiters and battery
charging profile circuits (although those would more likely be built into
the laptop computer - at least that's how I would design it - I wouldn't
trust an external device that could be swapped with something else).

Paul
 
M

Michael Black

Jan 1, 1970
0
1. If I shorted the +5V and +12V in the molex connector,
would I get +17V? What's the max. current?
No, because probably one side of each is already grounded inside the pwoer
supply.
2. Could I use 17V to charge a notebook battery rated 16V and 3.8A?
Would it hurt the battery?
NO, because these likely are fancy batteries and you don't just push
current through them, it has to be controlled based on the state of the
battery.

Leave the batteries in the notebook, and charge it that way. The charger
already came with the notebook, it also powers the notebook. If the
charger/power supply is missing, then find one that will do the job. I
see them endlessly no longer big because they are switching supplies. I
pulled an XBOX 360 supply out of a pile of junk when the students moved
out at the end of April. Then a few weeks ago, I found another one,
except the actual XBOX 360 was with it.

Michael
 
M

Michael Black

Jan 1, 1970
0
OTOH, should I attempt -5V and +12V pins at the power connector? Or should I
try +5V and -12V?
The minus supplies, if they are even still there, have minimal current,
were way in the beginning because RAM might have needed some negative
voltage (I can't remember if those RAM were still in existence in 1981
whtn the IBM PC came out, but they were lower density RAM), and of course,
RS232 interfaces need -12. FOr much of the time, those neagative supplies
have eitehr been underused or not used at all.

Plus, the negative supplies will also have one side ground, just like the
positive supplies. And even worse, -5 plus +12 is 7volts, the minus
subtracts from the positive.


Michael
 
M

Michael Black

Jan 1, 1970
0
The original 5150 PC had provision for 3 rail RAM, selected by the
configuration DIP switches.

I have yet to aquire a 5150 board that hadn't been fucked up by someone
messing with the DIP switches!
I'd forgotten about the dip switches on those.

But how many were actually shipped with low density RAM? I thought it was
the 4K RAM that needed negative voltage, and I thought by the time the
IBM came out, it would use 64K RAM, which I thought did away with the need
for negative voltage.

Michael
 
J

Jamie

Jan 1, 1970
0
Mr. Man-wai Chang said:
1. If I shorted the +5V and +12V in the molex connector,
would I get +17V? What's the max. current?

2. Could I use 17V to charge a notebook battery rated 16V and 3.8A?
Would it hurt the battery?
No
Jamie
 
M

Mr. Man-wai Chang

Jan 1, 1970
0
Guess this project is too dangerous for a beginner! Thank you all! :)

--
@[email protected] Remain silent. Nothing from soldiers and magicians is real!
/ v \ Simplicity is Beauty! May the Force and farces be with you!
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J

JW

Jan 1, 1970
0
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!
 
J

Jasen Betts

Jan 1, 1970
0
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.
 
M

Michael Black

Jan 1, 1970
0
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
 
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