Custom DC power supply from used laptop power brick

Thread Starter


Joined Jul 29, 2013
Hi all. I have a laptop power supply giving 12V, 5A out. I wish to split this into 4 tunable outputs with these specs:

out1 & out2 : 9-12 V, with a bypass switch on both to get the 12V 5A out directly. Shared current available 2,5 A, when not bypassed.

out3 & out4 : 3-5 V. Shared current available 2,5 A.

I know how I could build this with LM317 or similar components. But I'm not sure if it's the best way to do it. How would I do this with the least amount of components and being reasonably power efficient so I dont need active cooling?

Thread Starter


Joined Jul 29, 2013
Edit: Oh and yeah I know the power consumption doesn't add up. But I don't plan on maxing out all the outputs simultaneously.


Joined Jun 22, 2012
There are two ways to do it, you can use LM317 regulators to give upto 1.5A each on heatsinks, or a LM2596, or you can alter the brick output voltage if it uses a TL431 variable zener,..
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Joined Mar 14, 2008
For best efficiency you should use a switching regulator for the 3-5V output, as that will dissipate up to 22.5W with a linear regulator.

The 9-12V outputs will dissipate up to 7.5W with a linear regulator which is doable with a reasonable heatsink in ambient air.
But a linear regulator will not give a full 12V out with 12V in, as they always have some forward drop voltage.
An LM317 has a drop of about 2.5-3V at maximum current.
There are low dropout devices, such as the LM1086IT, but they still drop about 1.5V at maximum current.

If you want to get close to the full 12V, you will likely need a discrete design with a power P-MOSFET, and a few other active and passive parts.
The LTspice simulation of an example circuit is below:
The voltage drop at 12V output is determined by the on-resistance of the MOSFET.
Q3 provides a current-limit function to protect the MOSFET from a short circuit.
(Note that I don't have a model for a P-MOSFET in a TO-220 case for mounting on a heatsink, so one must be selected.)

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Joined Feb 20, 2016

One of these, or similar could be a good start. This version has fixed and variable options. Wire an external pot, from the fixed min voltage to the strip.

Or these, and remove the pot and put an external on in its place.
3A buck converter.jpg
Ebay or similar is a good place to start.

Just be careful as most switching of voltage on these sort of reg will feed a full volts pulse to the output as the switch changes over.
Have 2 resistors in series, (or a resistor and a pot), and short one out as required. Do not wire them to have the voltage feedback go momentarily open.


Thread Starter


Joined Jul 29, 2013
Thanks for your replies, I realized I don't have time to look deeper into this with the holidays coming now. But later. Merry christmas!