using PC PSU to power a laptop.

[dariob]

Hi,

I have an old laptop without a PS. However, I have a jack which will fit into its power socket. Can I use an AT PSU to power the laptop? It takes 15v 2.0A. Just curious if there is any way to do this without spending 50 bucks on a new PS for an old crappy laptop.

 

[FreiDOg]

Hi,

I have an old laptop without a PS. However, I have a jack which will fit into its power socket. Can I use an AT PSU to power the laptop? It takes 15v 2.0A. Just curious if there is any way to do this without spending 50 bucks on a new PS for an old crappy laptop.

straight out of the PSU, no.
You need to step the desktop PSU up to 15V (probably using the 12V rail).
You'd need a switchmode power supply to do that, use something like the LT1370 boost converter (If gee pops his head in here, he may have a better / simpler product, this is his area)
You can download Linear's SwitcherCad to run some quick and dirty sims of how that would work. (use the switch selector guide under the File menu).
It's not a terribly complicated circuit, but to get good characteristics (especially with respect to ripple noise) is well, not as simple as it might seem.

 

[BillR]

Hi,

I have an old laptop without a PS. However, I have a jack which will fit into its power socket. Can I use an AT PSU to power the laptop? It takes 15v 2.0A. Just curious if there is any way to do this without spending 50 bucks on a new PS for an old crappy laptop.

15v at 2.0 AMPS is just not a lot. It’s possible Radio Shack has one with that much current. Some computer speaker systems even use a psu like that.

I’d do some shopping

 

[dariob]

15v at 2.0 AMPS is just not a lot. It’s possible Radio Shack has one with that much current. Some computer speaker systems even use a psu like that.

I’d do some shopping

Thanks for the tip, but as I am an electonics component pack-rat I have tons of DC power supplies and related junk. I couldn't find one at that power range. Radioshack also didn't have anything for me. They also charge about $20+ for a 12v 1a PS. Can you tell me though, if I found a 15v PS for more amperage that the laptop needs and plug it in, it won't fry the lappy will it?

 

[BillR]

Thanks for the tip, but as I am an electonics component pack-rat I have tons of DC power supplies and related junk. I couldn't find one at that power range. Radioshack also didn't have anything for me. They also charge about $20+ for a 12v 1a PS. Can you tell me though, if I found a 15v PS for more amperage that the laptop needs and plug it in, it won't fry the lappy will it?

Nope, extra amps are “potential”, they won’t be used if not needed. Your fine with that.

 

[dariob]

straight out of the PSU, no.
You need to step the desktop PSU up to 15V (probably using the 12V rail).
You'd need a switchmode power supply to do that, use something like the LT1370 boost converter (If gee pops his head in here, he may have a better / simpler product, this is his area)
You can download Linear's SwitcherCad to run some quick and dirty sims of how that would work. (use the switch selector guide under the File menu).
It's not a terribly complicated circuit, but to get good characteristics (especially with respect to ripple noise) is well, not as simple as it might seem.

Pardon for being uninformed, but if I tried without any mods to the PSU, whats the chance that it would work? 0%?

 

[BillR]

Pardon for being uninformed, but if I tried without any mods to the PSU, whats the chance that it would work? 0%?

Let us look at this from another point of view. What is the voltage of the battery in that laptop?

 

[dariob]

10.8v 4000mAh

 

[BillR]

10.8v 4000mAh

Assuming it's only a single tap battery (only a + and - connection) it would be easier to drop the 12V line from a computer PSU to say 10.5 to 11 volts and use those taps instead, no?

Just a thought

 

[FreiDOg]

Pardon for being uninformed, but if I tried without any mods to the PSU, whats the chance that it would work? 0%?

yea, about 0.
devices are designed to work within a certain voltage range, when it says 15V, it's probably 15+/-5%, maybe even +/-10%,
but not many electronics are going to tolerate an input that's 20% low. (well maybe from AC, but not a regulated DC input).

anyway as BillR said, a power supply rated to more amps than you require is fine.
devices only draw what they need. Anything beyond that is just gravy, headroom, extra, ect.

 

[dariob]

yea, about 0.
devices are designed to work within a certain voltage range, when it says 15V, it's probably 15+/-5%, maybe even +/-10%,

So why is the batt only 10.8v? I am guessing to charge the battery. As per BillR, could I tap a 12v into the batt connections and power it that way, or will it fry?

 

[BillR]

So why is the batt only 10.8v? I am guessing to charge the battery. As per BillR, could I tap a 12v into the batt connections and power it that way, or will it fry?

I would drop the voltage as I mentioned, but making voltage lower is much easier and less expensive then making it higher

 

[dariob]

I would drop the voltage as I mentioned, but making voltage lower is much easier and less expensive then making it higher

Ok, so, I would have to create a circuit which lowers the voltage. Since I have little knowledge of this type of engineering, I am thinking that it would be relatively simple and involve a resitister of some kind. I am relatively adequate with a soldering iron and have no fear of buidling ciruicts, but I would hate to have you guys walk me though every little step since I am inexperienced. Would you recommend I undertake this project?

 

[BillR]

Ok, so, I would have to create a circuit which lowers the voltage. Since I have little knowledge of this type of engineering, I am thinking that it would be relatively simple and involve a resitister of some kind. I am relatively adequate with a soldering iron and have no fear of buidling ciruicts, but I would hate to have you guys walk me though every little step since I am inexperienced. Would you recommend I undertake this project?

The no math method, down and dirty.

One potentiometer about 0 to 300 ohms or more, 2 amps capable.

One volt meter

One 12-volt automotive bulb that draws about 2 amps or so. (Think stoplight)

Put the potentiometer in line with the positive lead between your 12v supply and the bulb.

Hook the ground from the PSU to the ground on the bulb. Turn it all on.

Read the voltage as you turn the pot up or down till you get about 10.5 to 10.8.

Turn it all off, measure the resistance of the potentiometer. What ever that reading is go buy a wire wound resistor of similar value. They even make adjustable ones.

Now, you could do all that crap and probably have some fun.

Or

Do the math and find out you need a Zener Diode rated at about 10.7 volts at about 30 watts of heat dissipation or a tad more. That part would go in series with the positive lead of the 12-volt line.

Your choice.

 

[dariob]

Sounds like a new hobby! I'll give it a try. However, don;t expect to get away scott free. Anticipate a few PMs in the next few days