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### Author Topic: convert 19v laptop psu to 15v laptop psu  (Read 3088 times)

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#### blackbeard

• Supreme Robot
• Posts: 575
##### convert 19v laptop psu to 15v laptop psu
« on: April 23, 2011, 02:58:36 PM »
so i have this laptop with no psu i'd like to turn on. my laptop has a 19v psu while the other needs a 15v psu. is there any way i could reduce the voltage reliably with a simple circuit. i was looking at using an lm317 but i'm a bit confused about how to find the resistor values. for r1 i was going to use a 2k and a 22k resistor. will i need an outrageous input voltage or will 19v work?
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#### MikeK

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• Posts: 97
##### Re: convert 19v laptop psu to 15v laptop psu
« Reply #1 on: April 23, 2011, 05:01:38 PM »
Is it a regulated supply?  If not you can just use it as is, because regulation would be done in the laptop itself.  But it will generate more heat, because of the higher voltage drop.  Get out your DMM and see how regulated the supply is.

If not, you've calculated the right values for the LM317...Google "LM317 calculator" to make sure.  But the problem is going to be current capacity.  How many amps is the PSU rated for?  The TO-220 case LM317, even with a heatsink, is going to be limited to around 1.5A.

#### Soeren

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##### Re: convert 19v laptop psu to 15v laptop psu
« Reply #2 on: April 23, 2011, 07:33:14 PM »
Hi,

is there any way i could reduce the voltage reliably with a simple circuit.
3..5 diodes of a proper rating in series with the supply would be simple. The exact number depends on the voltage drop of each under the load present.

i was looking at using an lm317
What's the max. current?
LM317 is good for 0.5A, 1.0A or 1.5A depending on their suffix (like LM317T which is 1.5A or eg. LM317H which is 0.5A).

LM338 (the bigger brother of LM317) OTOH is good for 5A (7A peak) and it calculates the same way.

but i'm a bit confused about how to find the resistor values. for r1 i was going to use a 2k and a 22k resistor. will i need an outrageous input voltage or will 19v work?
That's too low an impedance for proper regulation. You need to allow at the very least 100µA to allow that (and that's the bare minimum). It's better to allow at least 1mA of adjust current (like R1=1k and R2=11k).

Don't feed it 19V!  I have yet to see a laptop that doesn't got hot and there's no reason to strain it further, as the electrolytic caps will only die sooner.

Personally, I'd use a string of diodes.
Regards,
Søren

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