Kerry Wong Turns Old Laptop Power Bricks Into a Flexible 1.25-32V Lab Power Supply
Rather than leave old abandoned PSUs in a drawer, Wong has put them to use driving a flexible and powerful bench-top power supply.
Any bits box worth its name will soon gather a collection of unused DC power adapters which typically sit unloved — but engineer Kerry D. Wong decided to do something about the problem, turning a collection of 12V power bricks into a flexible 1.25V-32V lab power supply.
"Over the years, I have accumulated many used computer power bricks. Although I could just use them by themselves to power other electronics with similar voltage and current requirements," Wong explains, "I thought I would combine a few of them together as the input to a linear regulator so that I can make a powerful lab power supply."
"The power bricks I have in mind are these LITEON 12V/2.5A ones as I have quite a few of them lying around. I wanted to use a linear regulator as the output stage as linear regulators have superior power supply rejections and therefore can reduce the ripples introduced by the switching power supplies. The linear regulator I wanted to use is a [Texas Instruments] LM338, which is is perfect as I can use three of these 12V power bricks in series to achieve an 1.25V to 32V output range."
Each individual power supply in the system is capable of supplying 2.5A of current, while the LM338 can handle up to 5A of continuous output current: Two parallel power supplies per LM338, then, is easily handled — but three in series are required for reaching a full 36V input voltage. Rather than using three banks of two parallel power supplies for a total of six bricks, Wong opted to use two parallel bricks when supplying under 10V - the times when higher current is required, with higher-voltage experiments operating at a lower current.
The project required some modification to the power bricks themselves, and comes with a warning: "By removing the earth ground, we also removed one of the safety protection mechanisms," Wong admits. "In the event that the primary and secondary of the pulse transformer becomes shorted for example, the output would become live and creating a very dangerous situation. A GFCI [Ground Fault Circuit Interrupter] outlet should be used with these modifications to reduce the risk of electrical shock."
The full build log can be found on Wong's website.