What to read to make a simple SMPS (12v to 5v) - Page 1
Forum section: electronics / beginners

author: TheBorg
author_rank: Frequent Contributor
timestamp: August 28, 2014, 09:31:09 pm
content: I'm designing a circuit to run an arduino at 5v from a source at 12v that will also be used to run a few steppers. I'm looking to make it as efficient as possible because I would like to run it off of a battery at some point.The problem is, I've realized I know basically naught about electronics. What I know could probably have been learned in a 3 day course.I could build a 7805 circuit really easily, but that gets really hot and the efficiency sucks. Not a good thing!I'm not wanting you guys to design an SMPS for me... just maybe point me to somewhere where I can learn to build my own. That would be great!Thomas

author: Paul Price
author_rank: Super Contributor
timestamp: August 28, 2014, 09:37:39 pm
content: Murata make a direct plug-in highly efficient replacement switcher, just three leads, same size, no heat sink. 78xxSR series, cheap.Ok, so I've designed the circuit for you: Unplug the 7805, plug in the 7805SR.

author: Whales
author_rank: Super Contributor
timestamp: August 28, 2014, 09:55:18 pm
content: Easier than you think. You friend is the 34063 IC, which is easy to obtain online. All of the maths is laid out for you in step-order in the datasheet and the external parts are not that complicated.

author: Technicial
author_rank: Newbie
timestamp: August 28, 2014, 11:05:51 pm
content: I agree with Whales, i designed my own buck converter using the MC34063 and with the help of Dave's video. Pretty fun and not too difficult

author: tautech
author_rank: Super Contributor
timestamp: August 28, 2014, 11:27:15 pm
content: QuoteThomas, give us some more info, particularly current requirement?Will you want to over-build it for any future project?There is lots of stuff likePaul Pricementions, but you should give more info of your requirements.

author: TheBorg
author_rank: Frequent Contributor
timestamp: August 28, 2014, 11:43:29 pm
content: Quote from: tautech on August 28, 2014, 11:27:15 pmTypical current for everything running from the 5v rail should be less than 500ma total. Anything thats able to source 750ma continuously would be overkill for my application.It's a pan/tilt/slide camera rig so I am running at least 3 stepper motors off the 12v rail and then all the control circuitry is run off an ATMega328. I've got several shift registers and will have 3-4 items running on the I2C bus. Potentially bluetooth connected in the future. That's pretty much it!

author: TheBorg
author_rank: Frequent Contributor
timestamp: August 29, 2014, 05:35:28 am
content: Quote from: Whales on August 28, 2014, 09:55:18 pmThanks, that's perfect! I watched the video and it looks pretty simple, the math is super easy. The hard part will be figuring out a good diode and determining my Vin minimum. I was planning on running it off of a generic wall-wart, I'm not entirely sure what the tolerance would be on that. I'll just assume +- 10% for now, I've measured a similar wan-hung-lo power brick that measured 12.68V (no load), so I think a +-1.2v range will be fine. I need to get a dummy load soon if I want to keep expanding my hobbyI'll see about getting everything laid out tomorrow morning!

author: ludzinc
author_rank: Unknown
timestamp: August 29, 2014, 06:46:40 am
content: IMHO I'd grab an LM2596 module off of eBay and reverse engineer that.You start with a known working designYou can follow the design guide in the data sheet and grind the maths to see if you come up with the same answersYou get insight to availability of cheap and plentiful parts.It's a massive drag to get something like an inductor choice wrong and need to wait for new parts before you can try your next iteration of a design. This way you can still learn, but as your starting point already works, you gain some early satisfaction!

author: TheBorg
author_rank: Frequent Contributor
timestamp: September 01, 2014, 08:44:41 pm
content: Quote from: ludzinc on August 29, 2014, 06:46:40 amWell my mind was changed yet again, as I ran out of time this weekend to do the math as I went on a family camping trip and needed some other parts in my order asap. I threw some 34063's into my cart and ordered an LM2596 module from eBay. It should be within a couple dollars to implement either option, and for a hobby one-off it will be fine.

author: mariush
author_rank: Super Contributor
timestamp: September 01, 2014, 09:33:57 pm
content: There is a calculator software on Sourceforge for MC34063 and similar chips :http://sourceforge.net/projects/mc34063uc/... but even without software, you can use formulas in datasheet to figure out the parts.You need to use Schottky diodes and a suitable inductor (pay attention to peak currents and other parameters)LM2596 boards on eBay are cheap and pretty reliable but you don't learn much about it that way... they don't "optimize" those circuits so the components often won't match with what the datasheet formulas say. Some also use some fake chips or some clones of LM2596 or LM25**.You can get the parts from Digikey or Mouser, they have good filtering mechanisms on their sites.

author: TheBorg
author_rank: Frequent Contributor
timestamp: September 02, 2014, 04:49:44 am
content: Quote from: mariush on September 01, 2014, 09:33:57 pmOr anonline calculatorthat even references Dave Jones' videoIt's pretty handy, here's what I put in (for the sake of anyone else with the same problem later on).Code:12v input10% tolerance5v output1A max output current50mv ripple0.4v diode voltage drop (based on 1N5822 diode @2a peak current)1.1v step down saturation voltage75kHz operating frequency10k ohm R1Which gets me the results below (some odd 16uH inductor and 0.15 ohm resistor).Which after some quick searching puts me with these 2 parts.0.15 ohm resistor15uH inductor, 3A ratedThe rest of the parts I have offhand.Edit: the circuit should run better theoretically if I use a 250pf cap instead of a 270pf one, the circuit will run a bit faster (~79kHz), but the inductor value will be more in-spec.

author: Syntax_Error
author_rank: Regular Contributor
timestamp: September 06, 2014, 03:54:16 pm
content: You should check outhttp://www.onsemi.com/pub_link/Collateral/HB206-D.PDFandhttp://www.linear.com/designtools/app_notes.php(click on Power). These are chock full of useful information.Good luck on your power supply design.