DC 17V from PC power supply?

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! :)

-- @~@ Remain silent. Nothing from soldiers and magicians is real! / v \ Simplicity is Beauty! May the Force and farces be with you! /( _ )\ (Fedora 18 i686) Linux 3.9.6-200.fc18.i686 ^ ^ 21:12:01 up 6:41 0 users load average: 0.00 0.01 0.05 ???! ???! ???! ???! ???! ???! ????? (CSSA):

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?

-- @~@ Remain silent. Nothing from soldiers and magicians is real! / v \ Simplicity is Beauty! May the Force and farces be with you! /( _ )\ (Fedora 18 i686) Linux 3.9.6-200.fc18.i686 ^ ^ 21:18:01 up 6:47 0 users load average: 0.00 0.01 0.05 ???! ???! ???! ???! ???! ???! ????? (CSSA):

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

lower

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.

Another possibility is to change the feedback resistors so that you get

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

No.

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...

You can probably use a surplus or used notebook supply of similar specs.

Here's one with 19V and 2.65A for just $4:

And if you need 4.74A and can spare $15:

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

No, because probably one side of each is already grounded inside the pwoer supply.

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

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

No, it doesn't. The -5 V supply has it's positive end connected to common, so they add. +5 V & +12 V supplies would have 7 V between the outputs.

The correct term is: "let the magic smoke out".

On older PSUs, the -12V had a 7912 regulator (1A max).

Your best bet is an open frame 5 & 12V switcher that can be modified.

You'll probably find a TL431 programmable zener with sampling resistors connected to the 5V rail, with suitable modification of the resistors, you can sample the 12V rail instead - if you adjust the resistor values to get

If you find a surplus Aztec open frame switcher - they use a house coded TL431, but its obvious when you see it.

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 f***ed up by someone messing with the DIP switches!

In the UK; Maplin sells a universal laptop PSU with a voltage selector switch and an assortment of power jack attachments.

This device depends on the laptop having its own internal charging circuit, its not intended for charging any battery directly.

Most developed countries probably have a better choice than our just Maplin.

If its a lithium battery - charging with anything other than properly designed circuitry can be very dangerous!

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

The early PC motherboards were shipped with four banks of 16K RAM.

No Jamie

Guess this project is too dangerous for a beginner! Thank you all! :)

-- @~@ Remain silent. Nothing from soldiers and magicians is real! / v \ Simplicity is Beauty! May the Force and farces be with you! /( _ )\ (Fedora 18 i686) Linux 3.9.6-200.fc18.i686 ^ ^ 12:30:01 up 21:59 0 users load average: 0.04 0.04 0.05 ???! ???! ???! ???! ???! ???! ????? (CSSA):