Can you connect laptop power supplies in series? - Page 1
Forum section: electronics / projects

author: zirconx
author_rank: Contributor
timestamp: March 26, 2019, 11:31:35 am
content: I have some unused laptop power supplies like this: 19.5v 3.3A :https://d68my205fyswa.cloudfront.net/lcf/image/data/sku/A-DEL-04-G/USA/1.pngI know that it's possible hook ATX computer power supplies in series to get more voltage, I believe they have to unhook the negative output from the case when they do that.Would it work to connect these laptop power supplies in series? I'm thinking about using them to feed an adjustable buck converter:Any caveats? Would it make any difference if I use laptop power supplies with or without the 3rd wire ground pin on the 120v AC side?

author: Cliff Matthews
author_rank: Unknown
timestamp: March 26, 2019, 11:50:56 am
content: I would open them first to clip the Y-cap between HV and secondary sides and then series wire them with a fuse and a 6-amp rectifier between them to ensure one won't back-feed into the other on start-up or shutdown. A 2-watt 470-ohm bleeder across each may not be a bad idea either. - The usual precautions here to make sure the negative output is not connected the mains ground, stay safe if you've not done this before or don't understand it.. maybe read a few LT adapter schematics - YMMV

author: Zero999
author_rank: Super Contributor
timestamp: March 26, 2019, 12:47:37 pm
content: Quote from: Cliff Matthews on March 26, 2019, 11:50:56 amI'm not sure what you're saying.I wouldn't recommend modifying it, without doing some reverse engineering first. If the output of the power supplies is connected to earth, then there might not be a Y capacitor. Another possibility is the Y capacitor is connected between the output and earth, not the primary. Finally, removing the Y capacitor can cause EMC problems.Quote from: zirconx on March 26, 2019, 11:31:35 amIt depends on whether the outputs are isolated from earth. If not, then connect a diode in reverse parallel with each power supply, before connecting them in series.If the outputs are not isolated from earth, then you could in theory remove the earth connection from one of them and do the above, but it's risky.

author: Cliff Matthews
author_rank: Unknown
timestamp: March 26, 2019, 01:53:52 pm
content: Since this isn't the beginner's section, I guess I threw some of the usual "mains-panic" to the wind..

author: RoGeorge
author_rank: Super Contributor
timestamp: March 26, 2019, 02:02:54 pm
content: Connecting power supplies in series is not a good idea.Sometimes it can be done, but only if the sources were designed to support series connection. A laptop power supply is not designed to work like that. It can work or it can start a fire, you will never know.Connecting series power supplies is not the same as connecting series batteries.Apart from the Y capacitor, a lot of very ugly things can happen at startup. If one power source starts faster while the load is connected, then the other power source will see a reverse polarity at its output. Some power sources can cope with this, some other might be destroyed. Same if one power source over-current protection is triggered while the other one is not.

author: Zero999
author_rank: Super Contributor
timestamp: March 26, 2019, 02:11:20 pm
content: Quote from: RoGeorge on March 26, 2019, 02:02:54 pmThat's why I recommended connecting a reverse parallel diode to each power supply. Then if one starts first, the maximum reverse polarity the other one will see is a diode drop, which will be harmless. If the outputs are isolated, there's no need to panic. It will work. The main issue is the increased leakage current, but that can be minimised by connecting the output to earth, either directly or via a 100nF ceramic capacitor.

author: james_s
author_rank: Super Contributor
timestamp: March 28, 2019, 04:41:51 am
content: In my own experience, usually it works just fine. Is it a good idea? Probably not. Would I incorporate it into a commercial product for sale? No way. But for a hobby project? Give it a try, understanding that there is a possibility of damaging the power supplies, I'd give it at least a 90% chance of working fine.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 28, 2019, 04:48:32 am
content: It usually works just fine if the PSUs do not have grounded outputs.