Modifying PSU to provice 5V output - Page 1
Forum section: electronics / beginners

author: ullbeking
author_rank: Newbie
timestamp: November 25, 2017, 05:26:08 am
content: Hello! I want to build my own power supplies to provide clean and safe power to the SBCs I am experimenting with. This is so I can power multiple devices, each with 5V DC input, from a single AC power source. As a beginner in this sort of thing, I will be working on the PSU’s DC output side, for example by modifying the output of a laptop charging brick or ATX PSU. Each SBC draws about 0.2A, peaking at about 0.5A.I learned basic circuit theory many years ago but I have since forgotten details. In addition to the theoretical relationships I know there are voltage drops to consider; conductor length, thickness, and flexibility; ensuring that the voltage drop is taken into account; and that everything is set up stuitably for DC-DC buck converters (LM2596) that I will be using.An introductory guide to power electronics would be great, if anybody could please point me to one. I've already got a copy of Horowitz and Hill, and it seems pretty encyclopaedic and a bit unywieldy. Can anybody please recommend altnernative resources, which I would like also to to check out? Thanks!!

author: ogden
author_rank: Super Contributor
timestamp: November 25, 2017, 06:53:52 am
content: You can get DC-DC converter modules based on LM2596 assembled, tested and shipped for $0.78 on eBay. Worth to reconsider building such. Better make some really exciting stuff.

author: ullbeking
author_rank: Newbie
timestamp: November 25, 2017, 09:21:40 am
content: Quote from: ogden on November 25, 2017, 06:53:52 amLike this?https://www.aliexpress.com/item/10PCS-mini-DC-DC-Converter-Step-Down-buck-Power-Supply-Module-for-Flight-Control-Car/2038556861.htmlI've got a bunch of them waiting to be used. My question really concerned what to use to the source PSU, which I would like for it to have good, clean 20V DC output so I can lower the voltage easily and produce a device capable of powering several devices where the basic current drawn is 0.2A, with 0.5A peaks.My point is that, due to my relative inexperience, I need the following from you:Recommendations for what to use for the main PSU: 240V AC to 20V DC. I only really want to mess around on the low-power DC end as I don't want to have to modify a high-powered ATX PSU until I get a bit more experience.When it does finally come to take the leap and use an ATX PSU, I need to know how to use these things:https://uk.rs-online.com/web/p/distribution-blocks/1383483/andhttps://www.mouser.co.uk/Connectors/Terminal-Blocks/Fixed-Terminal-Blocks/_/N-7rqd7. Got any tips or links?Can you please recommend a basic book about electronics that specifically covers power supply fundamentals?Thank you!!

author: newbrain
author_rank: Super Contributor
timestamp: November 25, 2017, 09:34:45 am
content: Quote from: ogden on November 25, 2017, 06:53:52 amUnfortunately, many of those module will sport an LM2576 despite the 2596 marking.That said, they are cheap and work in pinch, but have high ripple even on moderate loads.

author: ullbeking
author_rank: Newbie
timestamp: November 25, 2017, 09:50:18 am
content: Quote from: newbrain on November 25, 2017, 09:34:45 amDo you have a recommendation for a supplier of genuine LM2596's? I bought a few "LM2596" buck DC-DC converters from Aliexpress, but of course I have no idea if they are really that or fakes.And what has the high ripple? LM2596, LM2576, or both?

author: ogden
author_rank: Super Contributor
timestamp: November 25, 2017, 09:56:40 am
content: Quote from: newbrain on November 25, 2017, 09:34:45 amEven then it is economically feasible to buy those modules, desolder fake chips and put genuine in place.Other option to consider - read datasheet of the chip:http://www.ti.com/lit/ds/symlink/lm2596.pdf, read "typical applications" chapter. There's circuit for 5V regulator with all the components specified and layout examples. Just copy it and you are done.Why struggle designing your supplies if there's so many easy solutions around. Better invest your time into something else that's really worth your time.

author: Avacee
author_rank: Unknown
timestamp: November 25, 2017, 10:01:42 am
content: They'll all have some rippleIf you want a smooth 5V then you could have the LM2596 drop the laptop's power brick (~18V?) output down to about 8V and then use a 7805 linear regulator.

author: ogden
author_rank: Super Contributor
timestamp: November 25, 2017, 10:12:36 am
content: Quote from: ullbeking on November 25, 2017, 09:21:40 amTo feed 5V DC-DC converters 12V is optimum "popular" voltage. Get some 12V LED constantvoltagesupply and you are done. Obviously any other powerful enough 12V DC supply you can find in your junk box will be good, including some old laptop supply. Do not pick higher voltage than 20..24V.QuoteActually ATX supply have 5V output you are looking for. Only modification would be simple jumper wire in the ATX power connector to power it on. Thou most likely it is too big and too powerful for you, does not have enclosure as well.QuoteNo need to become expert in power supplies. Manufacturers already assembled ready to use designs with extensive comments. You shall read datasheets and application notes of the ICs, copy reference designs. Usually this is enough to get your supply running. Apart from just buying ready to use modules..

author: newbrain
author_rank: Super Contributor
timestamp: November 25, 2017, 10:23:05 am
content: Quote from: ogden on November 25, 2017, 09:56:40 amGood point, though designing "trivial" things is always a learning experience.But switching converter are not trivial, and can be the cause of a lot of frustration.Quote from: ullbeking on November 25, 2017, 09:50:18 amUnfortunately not, I only ever got the fake onesSo, the high ripple was measured with a (probable) LM2576, see attachments for 0.75A, fed from a laptop brick.Note the switching frequency, an LM2596 should be at ~150kHz.

author: ogden
author_rank: Super Contributor
timestamp: November 25, 2017, 10:24:52 am
content: Quote from: Avacee on November 25, 2017, 10:01:42 amDigital part of SBC's are happy with ripple mentioned chips are putting out. Of course, you can always find application which needs clean supply, but as OP did not tell much about intended application.QuoteWell.... 7805 have very poor PSRR at high frequencies, many datasheets do not even specify PSRR above 100KHz. It could be so that your advice to use 7805 does not help much regarding hi-freq rippleIf you need to clean-up switching noise using linear regulator w/o LC filter, then you have better chances withhttp://www.linear.com/product/LT3042or similar. As I said - no need for most applications.

author: ogden
author_rank: Super Contributor
timestamp: November 25, 2017, 10:31:36 am
content: Quote from: newbrain on November 25, 2017, 10:23:05 am700mV ripple is unacceptable, especially in case of 5V. Could be so that output bulk capacitor is not big enough. Also agree that switching frequency is way too low.

author: ullbeking
author_rank: Newbie
timestamp: November 25, 2017, 11:07:24 am
content: Quote from: ogden on November 25, 2017, 09:56:40 amThis is gold. Thank you!Quote from: ogden on November 25, 2017, 09:56:40 amWhat do you mean by struggling to design my own supplies? I haven't found an easy solution to my problem yet. I want a single PSU that plugs into the 240V AC mains, converts that to 12V or 20V DC at the other side, and then to split that into a sufficient number of 5V DC conductors to power SBCs, each of which draws a peak of 0.5A.If I treat the "LM2596 Fixed Output Series Buck Regulator" (Section 9.2.1, Figure 32, in the PDF data sheet you linked me to) as ablack boxwhose output is going to be wired to a connector of the appropriate size, then could I put several of them in parallel? I intend to split the 12V or 20V DC PSU output and multiplex that across the inputs of these black boxes, provided that there's enough power.If you know of an easier, ready made solution for my problem, I'd love to hear it because buying something like this off the shelf would save me a lot of time because there are so many other things I need to work on.Quote from: ogden on November 25, 2017, 10:12:36 amReally? I have one of these, which is genuine:Brand: EDACPOWERModel: EA10681U-120AC in: 100-240V 2A 50-60 HzDC out: 12V 6ASomebody else told me that the 12V 6A DC output is strange and I would need very thick conductors to handle that sort of current. They said a 20V output (which would then have half the current), would be preferable because the voltage drop would be less, and less energy would be wasted by turning into heat, etc. What do you think?Quote from: ogden on November 25, 2017, 10:12:36 amI don't know what this is. Can you point me to an example please?Quote from: ogden on November 25, 2017, 10:12:36 amI don't have much of a junk box because I have only just recently regained my interest in electronics, and we got rid of all our old e-waste because we move around a lot, have recently started a family and were focused on having clean(er) house (that's a discussion for another thread). At the time I didn't foresee a use for all these old laptop charging bricks and similar such things.Quote from: ogden on November 25, 2017, 10:12:36 amIndeed. The ATX PSU would be IDEAL. I even have a spare, nice one that I'm not even using at the moment:https://www.bequiet.com/en/powersupply/935But it has no enclosure, we have two curious cats and an 18-month daughter who loves to look at what I'm working on and poke around and press thing, and it's a high-power device. I don't know how safe it would be to work with this, or maybe I'm just being paranoid.The extra nice thing about an ATX PSU is that it also has a 3.3V output, which I'm also going to need to power SPI flash chips that I'll be repogramming (this is one of my applications of working with PSUs and SBCs). In other words, I'm working with SBCs for two reasons:To use them to flash new BIOSes onto flash chips of old computers that I'm experimenting with. This will be done by running the flashing software on the SBC, wiring a SOIC-8 or SOIC-16 clip to the SBC's GPIO, and running the appopriate flashing software.I am building a cluster of SBCs because I want to learn about clustering, about the SBC's architecture, and about ARM.BTW, the SBC I am referring to is the Orange Pi.Quote from: ogden on November 25, 2017, 10:12:36 amA big reason for this is education. Indeed, I would prefer to purchase as much of this off the shelf as possible, as my main goal is to get these SBCs running, start flashing some BIOSes, and build that cluster I mentioned earlier. The PSU issue is a bit of a distraction, but an interesting one, and I want to keep it going on in the background over time as I am missing an understanding of power electronics, which would be useful for many other things too.I have a copy Horowitz and Hill, though. Is there anything else "classic" like that? I need something for when I get stuck on basic concepts! I want something that the basics in very nice way, or is an enjoyably readable, standard reference (like H&H).

author: Avacee
author_rank: Unknown
timestamp: November 25, 2017, 11:20:15 am
content: Quote from: ullbeking on November 25, 2017, 11:07:24 amTo get you up and running you can get USB chargers with multiple outputs - search amazon for "multi-port usb charger"You'd need to check the specs that it can delivery .5A to each port simultaneously - though iirc 2A is recommended for a PIhttps://www.amazon.co.uk/NEXGADGET-Charger-Station-Multi-Ports-Charging-White/dp/B06ZXW4ZMYclaims to deliver 1A to 8 ports.https://www.amazon.co.uk/Anker-PowerPort-Family-Sized-Technology-Smartphones-Black/dp/B00PK1IIJYclaims 2.4A to each port.Not much of a learning experience though :p

author: iainwhite
author_rank: Unknown
timestamp: November 25, 2017, 04:17:19 pm
content: Quote from: ullbeking on November 25, 2017, 11:07:24 amI suggest looking at the sticky thread on the Beginners section:https://www.eevblog.com/forum/beginners/electronics-primers-course-material-and-books/As well as Horowitz & Hill, other popular options are:MIT 6.002 (Circuits and Electronics) courseware & book by Anant AgarwalPractical Electronics for Inventors by Paul Scherz.Electronic Principles by Albert Malvinobut you should really read the thread...

author: mariush
author_rank: Super Contributor
timestamp: November 25, 2017, 04:44:02 pm
content: Since you're a beginner, you should really consider buying a simple 12v AC .. 24v AC toroidal transformer, as big or as small as you can afford it. They're from around 10$ for a 20VA one all the way to hundreds of dollars.You'll have isolation and very little noise (no switching noise from atx power supply), and you just need to use a bridge rectifier to convert the output of the transformer to DC and then use a big capacitor to smooth out the voltage. Then you can use dc-dc converters (one or multiple) to convert that [ ~ 1.414 x Vac - 1.5v ] Volts DC into 5v