Power supply - Page 1
Forum section: electronics / general

author: snipersquad100
author_rank: Frequent Contributor
timestamp: July 25, 2014, 07:28:49 am
content: Hi, I got a 19v 5amp laptop power supply.I need to drop it to 12v to power something else that uses 4.5 amps.I cant find a regulator that will handle that much amps.I was thinking of using diodes to drop the voltage but I would need quite a few,Anyone know of a better way.Don't really want to go down the voltage devider road cos I be using 4.5amps as it is.Thanks

author: rob77
author_rank: Super Contributor
timestamp: July 25, 2014, 07:53:28 am
content: buy one of those adjustable DC-DC converter modules from China - there are some capable of 5A or more.

author: Flump
author_rank: Frequent Contributor
timestamp: July 25, 2014, 08:06:54 am
content: as rob said something like this would dohttp://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=190851387499&ssPageName=ADME:X:AAQ:GB:1123i had one that went a bit mental so keep an eye on itEDITdont use this one, i linked the wrong sort

author: GabYoung92
author_rank: Regular Contributor
timestamp: July 25, 2014, 08:21:18 am
content: Quote from: Flump on July 25, 2014, 08:06:54 amJust got two of those to drive some 100W led's!Works awesome and would do the job easily!Just have to make sure you don't wire it backwards as there is no reverse protection on it

author: Richard Head
author_rank: Frequent Contributor
timestamp: July 25, 2014, 08:49:50 am
content: 1. Open up the unit. ie Smash the case!2. Locate the TL431 voltage regulator. Could be SOT-23, SOT-223 or TO-923. Locate resistor divider network that senses output voltage. Alter the divider ratio to reduce the voltage the 12V.Good luck.

author: rolycat
author_rank: Super Contributor
timestamp: July 25, 2014, 09:12:26 am
content: I would also be inclined to try the solution suggested by rob77, but since you want to drop the voltage I don't think a boost converter such as the one linked by Flump will work. Use a buck converter likethisone.

author: snipersquad100
author_rank: Frequent Contributor
timestamp: July 25, 2014, 10:08:12 am
content: Thank you guys for your help.I think I'll open it up and see if those resister values alter the voltage.Sounds like a challengeIf not I go with the other idea's.cheers

author: snipersquad100
author_rank: Frequent Contributor
timestamp: July 26, 2014, 11:04:34 am
content: Ok I took a dremel to the case to split it open, I went gently but the was no need as the heat shield surrounds right around the inside. After removing most of the gunk I couldn't find a TL431 but I have found a SG5842 which I think does the same job. Datasheet linked below. Could this be the chip to mod the feedback or power out resisters.?

author: snipersquad100
author_rank: Frequent Contributor
timestamp: July 26, 2014, 11:09:08 am
content: hers the chip

author: Psi
author_rank: Super Contributor
timestamp: July 26, 2014, 11:14:27 am
content: http://www.sm0vpo.com/_pdf/SG/SG5842A.pdfyep, find the optoisolater by tracing the SG5842 FB pin. Should be easy to find as it bridges the primary and secondary sides of the PCB.The input side of the opto has the resistor divider that sets the voltage.

author: snipersquad100
author_rank: Frequent Contributor
timestamp: July 26, 2014, 11:59:32 am
content: Thanks for that info.pin 2 FB goes to this opto.Am I right in assuming the input sence are two of these circled?

author: snipersquad100
author_rank: Frequent Contributor
timestamp: July 26, 2014, 12:01:04 pm
content: must remember to put in the pic lol

author: snipersquad100
author_rank: Frequent Contributor
timestamp: July 26, 2014, 12:29:55 pm
content: I think its these two resisters.VOE = 0.2K35C = 3.5 to 4K fluctuatingWould this be correct.

author: snipersquad100
author_rank: Frequent Contributor
timestamp: July 26, 2014, 01:30:17 pm
content: I changed voe to 2.7k but voltage stayed the same, so it not those ones

author: Richard Head
author_rank: Frequent Contributor
timestamp: July 28, 2014, 07:30:41 am
content: I would try to identify the 8 pin IC on the secondary side. Once you have done that then get the datasheet and identify which resistors determine the output voltage.In the photo you attached I couldn't quite make out the part number. It looked like MC3335 but that turned out to be an FM receiver chip! It's probaly just a dual op-amp.If it is just an op-amp as I suspect then the voltage reference may be D3 (Zener). You will the have to draw out the circuit so you can figure out what's going on. That means identifyng the two SOT-23 devices also.