Create ~17v-18v from a PC AT powersupply

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Jon
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Create ~17v-18v from a PC AT powersupply

Is it going to harm a PC AT powersupply to tie a 12v and a 5v line together to generate ~17v? I need 18v DC @ 2.6A to test an old Compaq LTE 5380, and befoer I go shopping for an AC adapter I'd like to test and see if the beast still works. 12" of 1024x768 sounds great (like my iBook...) but it's only a P-133. The label on the bottom of the Compaq says "18VDC MAX 2.6 A MAX" and has a graphic showing that the center pin is positive. If I won't kill the PC supply to test this, it should be really simple to get it to run.

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"Tie to together"???

What do you mean by "tie together?" You need either a -5 and +12 VDC line or a +5 & -12 VDC line to make this work. You will get 17 V between these two lines (e.g., attach the -5 VDC line to the negative outer terminal and the +12 VDC line to the center pin. Note than both lines must be capable of handling 2.6 A (or what ever that laptop really needs). Be careful as many ATX supplies only put out the required current on the +5 and +12 lines and you can't use those because they will only give you 7 volts.

Good luck and have a fire extinguisher ready. Wink

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On a whim I tried just using

Running the 5v and 12v together on one pin and a ground on the other was my idea. Maybe a couple diodes for safety. Running the voltagess together wouldn't add them up? I'm quite rusty on my electical theory, but you can stack two 1.5v AA batteries to get 3v, right? Couldn't one do the same with power supply voltages, or would that cause trouble? That's what I was thinking about.

On a whim I tried just using the same plug setup that I've got for the G-Vision LCD: 12v of the loaded AT supply, center positive, plenty of amps. To my suprise it booted right up to BIOS. Flat CMOS batttery, I guess. At least I now know that it powers up with 32mb RAM, 2GB drive, and a great XGA screen. If it keeps looking so nice I'll have to retire my ailing Toshiba 410CDT as my war-driving rig.

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The display regulates down to

The display regulates down to a lower voltage internally, which is why it works. However, you are going to heat up the regulator much more than you should and will probably shorten it's life.

As for making 17V, it doesn't work on the power supply because power will flow from the 12V rail to try to bring up the 5V rail.

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Putting two AA batteries in S

Putting two AA batteries in SERIES will get you 3V. Putting them in parallel will give you 1.5V, but with more current. As stated in the 2nd post, you need a -5V and a +12, or -12V and +5V to make 17V.

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Ah, parallel vs series. Ok.

Ah, parallel vs series. Ok. I guess this lappy is going to run fine on 12v vs 18v. I'm not sure if being under-voltage is trouble because the label says "18 VDC MAX". I'm assuming it is running well on 12v because Compaq was smart enough to figgure on making the power supply work easier with 12v auto and air adapters? So is it really just sucking more amps and doing it's own conversion?

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There will most likly be pots

There will most likly be pots inside one of the atx psus's which could allow you to adjust the voltage. The most ive ever get is around 16V...

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My guess would be that 12V is

My guess would be that 12V is enough to run the laptop, but not to charge the batteries at the same time. If you've got a dead battery anyway, who cares? If not, it might still charge while the lappie is off/asleep.

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I doupt it will work at all..

I doupt it will work at all...

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I dont see it runing on 12V a

I dont see it runing on 12V at all either. Even 16V would mot likely be cutting it close. Just get DC to DC coverter and change the 12V to 18V.

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[quote]On a whim I tried just

On a whim I tried just using the same plug setup that I’ve got for the G-Vision LCD: 12v of the loaded AT supply, center positive, plenty of amps. To my suprise it booted right up to BIOS. Flat CMOS batttery, I guess. At least I now know that it powers up with 32mb RAM, 2GB drive, and a great XGA screen. If it keeps looking so nice I’ll have to retire my ailing Toshiba 410CDT as my war-driving rig.

Er, it's running on 12v just fine. :macos: As for charging the battery, well, it kinda charged some the first couple of times but now it does nothing much. I'm not sure if it's because the battery IS dead or if there isn't enought power to charge it and the lappy. Sadly I don't have a CD module for it, so I've had to put the HDD into the main PC to load Win98 and am now loading a dual boot with FreeBSD. It's been kinda sucky, because the FreeBSD boot floppies won't recognize my 3Com 3C589C. No FTP install. >:( Of course 98 liked it just fine to do ICS with my iBook's modem...

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And this post is coming from

And this post is coming from the LTE 5380. It's using the Dell TrueMobile 1150 WiFi card, Links in textmode, and ICS to the iBook. Woo.

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Re: Putting two AA batteries in S

Putting two AA batteries in SERIES will get you 3V. Putting them in parallel will give you 1.5V, but with more current. As stated in the 2nd post, you need a -5V and a +12, or -12V and +5V to make 17V.

hahahaha, sorry, a power supply is not a battery. All the outputs are parallel. You can't wire them in series. In fact if you didn't diode protect the 5V line, you'd probably fry the supply.

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The parrallel/series comment

The parrallel/series comment was in regards to him asking about batteries. Not power supply voltages. And yes, a diode would most deffinitely be needed. I didnt say to just twist the wires together, I just said those voltages would be needed if he wanted 17V, not how to wire them up.

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Even then you'd still be left

Even then you'd still be left with a floating ground and you wouldn't be able to draw very much current. Also if you had a good ground you'd have -7 volts, not +17 volts (assuming you used -12 and +5 available on most supplies).

So unless his device can accept a differential input power supply, it's not going to work. He's much better off just using a transformer than trying to make a power supply do something it wasn't meant to do.

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I stated as much in my first

I stated as much in my first post. A DC/DC power supply would work with a lot less hassel.

EDIT: oh yeah. I was wrong about the -12V and +5V. But as far as I know, a +12V and -5V could give you 17V if done properly. But would be more work then its worth in most cases when a DC/DC power supply would do everything that is needed in one little box.

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I was digging through the pow

I was digging through the power adapter bins at Jerry's shop ( http://www.computerswapandshop.com/ ) and amid the vaious Compaq 18V @ 1A units, I found a Compaq 2872, which is for the Armada line. The LTE 5000 series and the same vintage of Armada are related, and this adapter puts out 18.75V DC @ 3.15A. Trouble is that it takes one of the 3-prong V shaped AC cables. I don't have any, none of the used shops have any (even Surplus Exchange, YIKES!) and to buy one new is $20. Geez, that $20 is more than I've spent for the lappy itself, and the AC/DC brick combined. So, I just opened the brick, found that the nice Compaq engineers used standard ground/blue/brown wires just like the desktop power cable adpater I cut up to splice in. Now it works great, and even charged the battery. I have yet to try the run times on a charged battery, but at least it took a charge.

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Re: I stated as much in my first

EDIT: oh yeah. I was wrong about the -12V and +5V. But as far as I know, a +12V and -5V could give you 17V if done properly.

It would only work if his device supported differential voltage input. Otherwise, tying them together with no diode would short a huge amount of current from +12V to -5V and using a protection diode would just give you +12V. Most supplies don't have a -5V output either.

You can't just use another voltage rail as your ground reference. There are significant consequences to doing that.

There is no way to do as you suggest "properly". The proper way is to use a DC/DC buck converter or a transformer.

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OK, I give up. I have stated

OK, I give up. I have stated twice that I would use a DC/DC converter. As thats what I said would be easiest, I never went into detail as to how to make a -5 and +12 work, I just said it was possible. But whatever.

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Wow. A couple run tests show

Wow. A couple run tests show that I'm getting the better side of 2 hours. The latest run test I did ran a cron job every 5 mins to 'touch' a file so I would have a down-to-5-mins accurate log of when the system went into the ~3% remaning sleep mode. I ran a 'touch' on a file just before I pulled the power plug, and the difference between the two shows about 2.5 hrs run time. Wow. And this is a 3300mAH NiMH battery. The Li-ON in the Toshiba gets nearly 3 hrs from 3600mAH. The two get some nice runs times for being 8-9 year old systems.

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Re: OK, I give up. I have stated

OK, I give up. I have stated twice that I would use a DC/DC converter. As thats what I said would be easiest, I never went into detail as to how to make a -5 and +12 work, I just said it was possible. But whatever.

It's because you are giving someone hope that it's possible, when it really isn't.

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Dr Bob, could you explain how

Dr Bob, could you explain how on earth this works???

http://www.pureoc.com/Extremecpuoc_3.asp

Even when they don't share a comman ground, I still dont understand how it would work...

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FWIK it used two power suppli

FWIK it used two power supplies, and ran them in series. That is different than what I thought I could do. Bad things would happen if you tried to do that with the Molex plugs on a single supply.

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Ran a tird test, got 2.5 hrs.

Ran a tird test, got 2.5 hrs. Cool. :coolmac: Today I went and happened to find a shop that had a 5380 for sale. They had it plugged into the wall with an AC/DC brick that puts out 18.5VDC and only about 2A. Luckily they had about 3 more of the bricks down in the random power brick bin. Woo! $5 and trip home later, I can use a basic 2-prong AC cable and I've got at least 4 of those... Acute

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Re: Dr Bob, could you explain how

Dr Bob, could you explain how on earth this works???

http://www.pureoc.com/Extremecpuoc_3.asp

Even when they don't share a comman ground, I still dont understand how it would work...

The simple fact that they don't share common grounds is the only reason it works. If you connected the chassis together both power supplies would blow. Why? CUZ THEY AREN"T DESIGNED TO WORK THAT WAY

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