Small 5V power supply - Page 1
Forum section: electronics / projects

author: Mikey
author_rank: Regular Contributor
timestamp: January 18, 2013, 08:19:08 pm
content: I am going to do a project where I need to power an atmega328p, a rf receiver module, and 4 relays.It is going to be a home automation node that will simply receive a signal from the sender and then turn the relays on/off depending on the message. It will have 3x 230V out and 1x open/closed output so it will be easy to add lights to it, and also be able to just send a trigger signal (going to use that for my automated garage door).To keep installation simple it would be easiest to have the 230V to 5V power supply on the pcb, but looking at bit on cost and such I come down to it would be easiest and cheapest to take one of those cheap 5V usb power supplies.But, I also remember how they looked when Dave took two apart, so what would people here suggest? Are there any known decent of them out there? Or can I do the power supply myself very easy? (only done a few simple pcb's before)

author: SeanB
author_rank: Super Contributor
timestamp: January 18, 2013, 08:29:33 pm
content: Find a genuine phone charger ( does not matter what manufacturer as long as it is a 5v unit) and use the board out of that. I recycled an old Nokia one as a power supply for a LED lamp, just had to change the input capacitors as they were not looking too well after 5 years or so of being powered.

author: Mikey
author_rank: Regular Contributor
timestamp: January 18, 2013, 08:31:41 pm
content: Quote from: SeanB on January 18, 2013, 08:29:33 pmProblem is they are like $15-20 if I buy them here from a reliable source, isnt really cheap any longer then.

author: SeanB
author_rank: Super Contributor
timestamp: January 18, 2013, 09:18:07 pm
content: Who said new? Car boot sale, dumpster dive a little or look in a cupboard or ask friends and family if they have an old one no longer in use.

author: retiredcaps
author_rank: Super Contributor
timestamp: January 18, 2013, 09:20:39 pm
content: With so many people getting new phones almost annually, unused/old phone chargers are plentiful supply.

author: mariush
author_rank: Super Contributor
timestamp: January 18, 2013, 09:29:29 pm
content: The cheap usb chargers/adapters often have very bad regulation. I've tested one recently and it was outputting 5.7v up to about 200mA, with half a volt or so ripple... above 300mA it started to drop down in voltage to as low as 4v at 360mA.Some phone chargers can have as much as 7-9v output when there's little load, like 10-50mA. You'll still have to get an adjustable low drop linear regulator or something to protect your chips from such regulators. You set the regulator to 4.7v or something like that, which is still enough for the controller to still run properly, and low enough to have room for the 0.2-0.3v minimum drop on the regulator.I don't say you shouldn't use one, search for some chargers but TEST them first.Or you could just get some cheap 5-9v power adapter and use the regulator to get 4.7-5v you need. If you get one of those that has 3v-4.5v-6v-7.5v-9v-12v then you could just set it to 7.5v and use a 30-50 cents 7805 to get 5v.

author: muvideo
author_rank: Frequent Contributor
timestamp: January 18, 2013, 09:35:33 pm
content: Follow Sean's advice, you just have to search for them.Last year I bought a new/old stock of 9.4V 700mA chargers,good for CV or CC application (leds) less than 2€ each.I'm using them for everything, once learned the circuitinside it's easy to mod the feedback circuitry to changethe output voltage/current (by a reasonable amount).Now I simply take the calibre, and open an hole in the plasticcase, in the point where I know I will expose the resistorsthat set current and voltage setpoints.Another alternative is use plain old transformers, checkyour power requirements, the low power ones are prettysmall and low cost. Two relays and a micro, a 3VA onecould be enough.Fabio.

author: Praxis
author_rank: Contributor
timestamp: January 19, 2013, 10:37:18 am
content: If you need good regulation, look for a 9v wall-wart and run it through a linear regulator circuit. Older, cheaper transformer-based supplies will generally give cleaner power at low load than a cheap switching supply. Actually, in any case where you're powering circuitry that doesn't have it's own regulator on it with a line-derived source, it's a good idea to stick one on.If you don't know how to tell the difference between a transformer and a switching wall-wart, hold it in your hand and think: "could I concuss someone with this?" If the answer is yes, it's got a transformer.

author: kripton2035
author_rank: Super Contributor
timestamp: January 19, 2013, 10:42:45 am
content: this one has some good regulation, may be too much current for you but it's cheap !http://dx.com/p/5v-2a-regulated-switching-power-supply-110-220v-94518

author: Psi
author_rank: Super Contributor
timestamp: January 19, 2013, 12:09:58 pm
content: Since this device only interacts with the outside world through relays and an rf receiver it's a perfect fit for a capacitor based mains powersupply since its completely isolated.http://ww1.microchip.com/downloads/en/appnotes/00954A.pdfCapacitor based powersupplies are often used for modular timers/dimmers and other small devices that fit behind lightswitches and wall plates where 5V/12V is needed from the 230V mains.I recommend reading about them before attempting to build one. There's safety issues to consider when working on anything that's not mains isolated.The designs ive seen produce 24V and use a 7805 to get mcu vcc.They also use 12v relays with resistors in series to run from the 24V. I forget why that's done, i think its to do with running the powersupply close to its rated current limit and still being able to pull in relays.