Building a simple power supply... - Page 1
Forum section: electronics / beginners

author: blakethepatton
author_rank: Newbie
timestamp: August 01, 2016, 05:59:52 am
content: I recently replaced my old laptop with a new one. While recycling the components out of the old laptop I have decided to recycle the LCD from it. I've now gathered the components and it all works great but I want to make a mobile power supply. I need to be able to deliver 12 volts at 4 amps. I thought the best way to do this would be to use a boost converter and some 18650 cells (From the old laptops battery) to power it.I boughtthis boost converterbecause it can handle supply voltages of as little as 3 volts and can boost it easily to my required 12 volts. However their maximum input is 4 amps. With two sets of two 18650's in series I should be able to have ~6 volts at somewhere up to 20 amps (not sure about that but I only need 8 amps). But if I run this through the boost converter I'll only get 12v @ 2A. So I have an idea but I don't know if it'll blow up in my face so I figured I'd ask you guys before I gave it a try.You might wonder why I don't just put all 4 18650's in series and that's because the voltage would not be 12 volts all the time, they could output up to 16 volts on full charge and if one cell was discharged(they do have protection), then I'd get no more power.TL;DR: Will this work?Edit: Removed short circuit in the preview image

author: Signal32
author_rank: Frequent Contributor
timestamp: August 01, 2016, 06:26:02 am
content: Well the way you have it wired up in the image will certainly not work since you have the inputs of the boost converters in series and the output in parallel.This means that the input of the right boost converter is short circuited.If you put both the boost converter inputs in parallel, same as the outputs are in parallel, this might work, not 100% sure about how nicely boost converters play when in parallel.For the specific boost converter you have the max input current is I think 3A, not 4A.What solution I would go with is putting all 4 cells in series and then using a buck converter to step down to 12v. Buck converters generally have better efficiency.

author: blakethepatton
author_rank: Newbie
timestamp: August 01, 2016, 06:36:37 am
content: Alright thanks! I'd still love to know if my boost converter would work.

author: Signal32
author_rank: Frequent Contributor
timestamp: August 01, 2016, 07:34:11 am
content: Quote from: blakethepatton on August 01, 2016, 06:36:37 amI actually have two of those boost converters so I was able to try it out.Turns out they're rather good at voltage regulation which is actually not that good for putting them in parallel.This is because you will never be able to set them to the exact same voltage or even if you do they will drift.Which means that the one set to the higher voltage will be doing all the work, while the other one will be idle.So no, putting them in parallel won't work.

author: blakethepatton
author_rank: Newbie
timestamp: August 01, 2016, 07:45:45 am
content: Quote from: Signal32 on August 01, 2016, 07:34:11 amBummer! Well thanks for trying it out for me. I hadn't thought about how they wouldn't ever be at the same voltage. I guess it's time to get a buck converter!

author: Signal32
author_rank: Frequent Contributor
timestamp: August 01, 2016, 07:55:58 am
content: Quote from: blakethepatton on August 01, 2016, 07:45:45 amWell not necessarily. If you Really really really want to do it you could use (2 cells + buck converter) x2 to get 6V + 6V. Then put the buck converters in series and get 12v.There would be no load sharing issues then, but uneven charge of the batteries would lead to one buck converter shutting down, giving only 6v to the output, etc ...