Request: schematic for simple plugpack-style USB charger for MP3 player. - Page 1
Forum section: electronics / beginners

author: nzo
author_rank: Regular Contributor
timestamp: August 26, 2011, 06:32:30 am
content: I have several 230v plugpacks (wall-warts) that are sitting idle, rated between 12v and 9v. I'd like to build a simple USB 5v charger for my MP3 player, for when a computer is not available.Does anyone have a working schematic I could use to build something? I don't (yet) know if any kind of protective circuitry is necessary to prevent the MP3 player from getting blasted.Huge thanksJohn

author: Fraser
author_rank: Super Contributor
timestamp: August 26, 2011, 12:40:22 pm
content: A simple solution is the LM7805 linear regulator. It's not high efficiency but will do the job well & cheaply. The data sheet will tell you all you need to know.Be aware that I managed to find some pretty decent smpsu type USB chargers on ebay for GBP 0.99 (yes GBP 0.99) delivered to the UK. Almost makes DIY too expensive

author: Zero999
author_rank: Super Contributor
timestamp: August 26, 2011, 04:30:46 pm
content: The LM7805 is good and certainly won't let you down.Another method is a switching regulator which is more efficient and requires no heatsinking. Fortunately it's possible to build a switching regulator from a couple of transistors, some resistors, capacitors and an inductor.

author: nzo
author_rank: Regular Contributor
timestamp: August 27, 2011, 01:36:00 am
content: ThanksAuroraandHero999. I've decided to go with a switchmode PSU using an LM2576 chip.You're quite right Aurora. I looked at PSUs on DealExtreme.com and they are very cheap, but I hate the culture of waste that seems to permeate almost every western society. Wherever possible, I try not to add to it

author: Zero999
author_rank: Super Contributor
timestamp: August 27, 2011, 01:41:49 pm
content: I know you've made you decision but for future reference, here's a design which uses cheap components. I meant to post the link before but I forgot.http://www.romanblack.com/smps/smps.htm

author: ejeffrey
author_rank: Super Contributor
timestamp: August 27, 2011, 10:44:42 pm
content: In addition to supplying +5V, your charger needs to connect D+ and D- to each other by a resistor (IIRC 100 ohm is standard) to enable USB charging. If you don't, the devices will expect they are plugged into a computer USB port and power up in low-current mode. With no actual host computer, they will be unable to negotiate to draw 500 milliamp and will not charge.Probably a bit of googling will tell you the right value of the resistor to use.

author: nzo
author_rank: Regular Contributor
timestamp: August 28, 2011, 02:41:19 am
content: ThanksHero999- interesting circuits, worth exploring.Thanks for the tipejeffrey. That info is much appreciated.

author: vtl
author_rank: Regular Contributor
timestamp: August 28, 2011, 03:15:21 am
content: 7805 will work and its very simple to get going. But the main issue would be heat. If you're charging at 500mA (or more if your device supports fast charging from an outlet like a phone) then you'll be getting into a very hot regulator. If your device uses 500mA a 5v and you're stepping it down from 12v its a simple equation to calcuate roughly how much heat the regulator will dissipate to step down the voltage.Power in = 12x0.5 = 6WPower out = 5x.5 = 2.5WPower dissipated = 6-3.5 = 3.5W3.5W doesn't sound like much but i just built up a 7805 reg for a car phone charger and for a 7805 regulator in one of the small packages (small dpak) the thing goes into thermal shutdown within 10 seconds. Using one of the larger TO220s the thing will get up to 70 degrees quite quickly.Clearly if you don't want to stick large heatsinks on your project then this is a bad idea. Most plug packs are switchmode power supplies which are much more efficient, maybe you could look into modifying your plugpack to output 5v? Otherwise you can buy $2 car phone chargers from ebay which contain a switchmode 12v to 5v.

author: nzo
author_rank: Regular Contributor
timestamp: August 28, 2011, 05:08:15 am
content: Thanks for the low down on using a 7805vtland for your timely reminder about heat.I think with everything that has been said in this thread, I'll keep open for a little longer on how to proceed. I have a very heavy little plugpack that was originally used to power an Iomega ZIP drive. It outputs 5v into a barrel plug and might do the job as a front end for a PSU, or perhaps even work without doing anything elseMy MP3 player is an unknown Chinese brand with unknown circuitry. I read somewhere that often protection circuitry is built into the player itself, and that all I need is to feed it 5v. I don't know if that's the case with my player. I want to be on the safe side and have it last a little longer

author: nzo
author_rank: Regular Contributor
timestamp: August 28, 2011, 11:48:21 am
content: Hero999pointed me to someswitchmodePSUs, one of which uses an470uH inductor. Are these usually ready-wound or DIY? What do I ask for when purchasing these?When purchasing smaller ferrite toroids, is there a range of useful sizes to have in my parts cabinet?Different suppliers seem to call some components by different names. For example,RSdon't show any results when searching for 'binocular' inductive cores. What other names might they go by?Thanks very much

author: IanB
author_rank: Super Contributor
timestamp: August 28, 2011, 03:13:07 pm
content: Quote from: nzo on August 28, 2011, 11:48:21 amInductors come ready made like capacitors and resistors. You pick them mainly by their inductance value and by the maximum current they can pass before the core saturates.I have never heard of a binocular inductor. Binocular refers to vision--"bin-ocular = two eyes"--so I'm not sure what you are looking for here?

author: Fraser
author_rank: Super Contributor
timestamp: August 28, 2011, 03:57:23 pm
content: Just for info, Binocular inductors are so called because of their similar appearance to binocularsThey are also known as 'Pig Nose' cores as they have that appearance as wellThey are most often found in RF applications and I do not think RS stock them. You can often see them on ebay or amateur radio component suppliers usually have them. The compound from which they are made will differ depending upon the frequency of operation.Take a look here:http://www.jabdog.com/cores.htmThey are for sale as a set here from a known Dealer:http://www.ebay.co.uk/itm/14PC-FAIR-RITE-BINOCULAR-CORE-SET-AMIDON-PIG-NOSE-/290597630702

author: IanB
author_rank: Super Contributor
timestamp: August 28, 2011, 05:41:12 pm
content: Quote from: Aurora on August 28, 2011, 03:57:23 pmAh, I see, thanks for the explanation. I learn something new.I think that radio and RF applications are quite specialized, where people pick a core material and wind coils for a very particular duty.But in general low frequency electronics the story is different and winding your own inductors is not worth the time. Better and more efficient inductors are purchased ready made in both through hole and SMT varieties.For this application I would have thought a simple buck voltage regulator would do the job? Use an off the shelf buck regulator IC, add an inductor, a capacitor, a resistor or two and you're done. There's almost certainly a preset 5 V IC out there given the prevalence of 5 V and USB power these days?

author: nzo
author_rank: Regular Contributor
timestamp: August 28, 2011, 08:24:30 pm
content: Thanks so much for your help IDing those inductors guys. I'm now happily awash in Ebay sourcesThe price difference between Ebay and traditional distributors like RS continue to astound me. RS is sometimes 5 to 10 times or more. A different business model I guess.

author: Zero999
author_rank: Super Contributor
timestamp: August 29, 2011, 02:15:58 pm
content: Quote from: nzo on August 28, 2011, 11:48:21 amYou can wind your own or buy an off the shelf inductor.The current rating should be equal to the peak current.How much current do you plan to draw?If you look at the graphs the efficiency goes down at higher power levels. Using a lower value higher current inductor may help and you could also replace Q1 with a logic level P-MOSFET but the values of R1 will need changing.