Adjustable CC/CV PSU using laptop AC adapter? - Page 1
Forum section: electronics / projects

author: nbritton
author_rank: Frequent Contributor
timestamp: October 03, 2015, 03:53:05 am
content: I have a 230W +19.5VDC laptop AC adapter that I'd like to adapt into a triple output variable constant current / constant voltage power supply. I'm sure something like this has already been done, any pointers where I can find more information about this? How would you accomplish this without using grossly inefficient linear regulators?

author: ez24
author_rank: Super Contributor
timestamp: October 03, 2015, 05:30:07 am
content: Quote from: nbritton on October 03, 2015, 03:53:05 am230W +19.5VDC laptop AC adapter ==> what brand ?I'm sure something like this has already been done ==> good luck

author: Kleinstein
author_rank: Super Contributor
timestamp: October 03, 2015, 07:50:54 am
content: If you don't want a pure linear regulator, it will be a switched mode regulator. Except for the input voltage there will be no big difference to starting from rectified mains voltage, except that one channel might use a step down without isolation.

author: Fungus
author_rank: Super Contributor
timestamp: October 03, 2015, 11:41:15 am
content: Quote from: nbritton on October 03, 2015, 03:53:05 amBrowse eBay, there's dozens of things like this:http://www.ebay.com/itm/351276332291

author: rjeberhardt
author_rank: Regular Contributor
timestamp: October 03, 2015, 02:12:21 pm
content: I've done exactly that using one of these:http://www.ebay.co.uk/itm/5A-0-8V-29V-Constant-Current-Voltage-LED-Driver-Step-Down-Module-Charger-Module-/131584272548?hash=item1ea308a8a4and an old Dell 20 V 3.5 A laptop power supply. It works O.K. but when tested at 1.5 A load it gave 50 mV triangular wave ripple at about 300 kHz with 100 mV switching spikes with a fundamental frequency of about 10 MHz. So, if you want to use it to power anything sensitive you need to be careful with the filtering.Russell.

author: ez24
author_rank: Super Contributor
timestamp: October 03, 2015, 07:09:41 pm
content: Quote from: rjeberhardt on October 03, 2015, 02:12:21 pmDid you have to do anything with the center pin to get it to work? (Dell PS use 3 wires)

author: ez24
author_rank: Super Contributor
timestamp: October 03, 2015, 07:59:29 pm
content: There is no 3 output buck off the shelf item.Here is a +- 12v buck (rare item), using this and one of the ones you have picked, would give you a triple outputhttp://www.ebay.com/itm/251352773668?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AITThere are a lot of dual bucks that put out 12v and 5v

author: Fungus
author_rank: Super Contributor
timestamp: October 03, 2015, 08:03:23 pm
content: Quote from: ez24 on October 03, 2015, 07:59:29 pmNo, but you have have 3 single output bucks.

author: rjeberhardt
author_rank: Regular Contributor
timestamp: October 04, 2015, 09:02:48 am
content: Quote from: ez24 on October 03, 2015, 07:09:41 pmNo. The one I had just had a two wire output.Russell.

author: jay
author_rank: Regular Contributor
timestamp: October 05, 2015, 03:05:03 pm
content: Quote from: rjeberhardt on October 03, 2015, 02:12:21 pmSo did the ripple come from the module you got from that Ebay seller or from the Dell laptop power brick?I've used Lenovo & IBM laptop PSUs for many projects including a bench power supply. The ripple is surprisingly low even at high currents (IIRC, I tested up to 4A). There was a bit of voltage drop at high currents, but that probably comes from the long cable and replacing the cable with something thicker would probably fix that. There might be a clear quality difference between brands.. At least I had several old power bricks from other brands and many of them didn't work at all or had serious issues with regulation or ripple.. Well, many were 10 years old so the output caps had died. Good to check them carefully before using.

author: Fungus
author_rank: Super Contributor
timestamp: October 05, 2015, 03:25:01 pm
content: Quote from: jay on October 05, 2015, 03:05:03 pmI always thought the little center pin was some sort of voltage sense feedback to adjust for that. I might be wrong though, I'm just guessing.

author: ajb
author_rank: Super Contributor
timestamp: October 05, 2015, 04:03:17 pm
content: Quote from: Fungus on October 05, 2015, 03:25:01 pmDell at least uses the center pin to read an identifier from a little one-wire device in the adapter. That way the laptop can identify whether a 65W or 90W adapter is connected and nag at you if you connect the wrong one.

author: rjeberhardt
author_rank: Regular Contributor
timestamp: October 05, 2015, 06:34:30 pm
content: Quote from: jay on October 05, 2015, 03:05:03 pmThe Dell PSU gives very little ripple on no load but when loaded the ripple shoots up to about 100 mV PP. However it is switching at 200 kHz. The ripple I'm getting on the Ebay module output is 300 kHz so is coming from that module.Thinking about it, a switching regulator feeds a pulse waveform into an LC tank circuit which converts the pulse waveform into a triangular waveform (the ripple). The feedback is taken from the capacitor. There must always be that triangular ripple on the output unless further filtering is added. The Ebay module has the output connected directly to the tank circuit so external filtering is needed if low ripple is required.Russell.

author: ez24
author_rank: Super Contributor
timestamp: October 05, 2015, 07:54:59 pm
content: Quote from: rjeberhardt on October 05, 2015, 06:34:30 pmJust curious - on the output of the buck is there any 200 KHz from the Dell? Also what is the PP of the ripple from the buck, you say it is 100 mV PP on the input to the buck?

author: rjeberhardt
author_rank: Regular Contributor
timestamp: October 06, 2015, 08:50:54 am
content: Quote from: ez24 on October 05, 2015, 07:54:59 pmO.K. here is the output of the buck converter on 10 V output with a 10 ohm load and with no additional filtering:The triangular wave is 40 mV pp at 277 kHz. (Ignore the frequency counter it is triggering on the switching spikes). There is no sign of the ripple from the Dell PSU getting through. The ripple changes very little with the load.Russell

author: ez24
author_rank: Super Contributor
timestamp: October 06, 2015, 08:57:58 pm
content: Quote from: rjeberhardt on October 06, 2015, 08:50:54 amthanks

author: Siwastaja
author_rank: Super Contributor
timestamp: October 07, 2015, 06:13:56 am
content: Do the three outputs need to be isolated from each other, or can they share common ground?