Low cost 2W mains supply - Page 1
Forum section: electronics / projects

author: splin
author_rank: Frequent Contributor
timestamp: November 28, 2014, 10:51:36 pm
content: What would be a realistic budget for a 240V mains supply providing 5V (ish), 2W max, preferably PCB mount but definately not a wall-wart type supply? There are dirt cheap supplies on Aliexpress such as these at less than 50 US cents:http://www.aliexpress.com/item/Factory-whosale-price-230V-240V-110V-220V-1W-2W-3W-led-driver-adapter-3-1W-led/32211085572.htmlHowever, I wouldn't touch them with a bargepole (well perhaps with a rubber bargepole!). Are there any safe, reliable but reasonably low cost supplies (less than $2 for 1K off unit pricing) that would meet European safety and emissions regulations? Modules from the usual distributers, Digikey, Farnell etc. are relatively expensive.Power factor correction wouldn't be required and the load would be in the range 300mW to 2W.There must be low cost, safe and CE mark compliant supplies available from China but how do you find a supplier you can trust? Any good UK suppliers?Rolling my own is an option but as I've not done it before I don't know how difficult it would be to get right and get it through EMI/EMC compliance etc. Also I guess it wouldn't be a trivial to determine that its long term reliability would be acceptable.Using a 50Hz transformer is a fairly cheap option but they aren't particularly cheap and take a lot of space.TIA for any suggestions,Tony H

author: electr_peter
author_rank: Unknown
timestamp: November 28, 2014, 11:02:51 pm
content: If you want to build a cheap low power supply, have a look at transformerless resistive and capacitive supplies. These are easy and safe if build properlyMicrochip appnote

author: tom66
author_rank: Super Contributor
timestamp: November 28, 2014, 11:11:01 pm
content: You can get something like this from Farnell. It's not necessarily low cost though, at £11 per unit:http://uk.farnell.com/tracopower/tmpm-04105/ac-dc-5v-0-8a/dp/2080706Perhaps similar units are available from China?

author: splin
author_rank: Frequent Contributor
timestamp: November 28, 2014, 11:41:00 pm
content: Quote from: electr_peter on November 28, 2014, 11:02:51 pmUnfortunately they don't provide any isolation (which I forgot to mention is required) and 2W is quite a lot from a capacitive supply. I did consider the possibility of using a capacitive supply followed by a DC-DC convertor to provide the isolation. I'd be more comfortable designing that than a mains supply, but I'm not sure it would be cheaper than a bought in mains to DC switcher.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 28, 2014, 11:53:54 pm
content: http://uk.farnell.com/vigortronix/vtx-214-003-105/ac-dc-conv-fixed-1-o-p-3w-5v/dp/2401030?CMP=GRHB-FINDCHIPS-1007992

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 28, 2014, 11:55:11 pm
content: Does it need isolation?If not, look at the Power Integrations Linkswitch 304/5/6 series

author: splin
author_rank: Frequent Contributor
timestamp: November 29, 2014, 12:04:58 am
content: Quote from: tom66 on November 28, 2014, 11:11:01 pmWay too expensive unfortunately. I was hoping to find something for less than $3 (£2), and preferably less than $2. It might not be possible of course.QuoteUndoubtably, but how do you find ones that you can be certain meet the relevant safety standards? Just because the supplier puts a CE mark on a product means very little. Even if the module is a recognised brand how do you know you're not getting a counterfeit? Testing a few samples is all well and good but how can you be sure that the ones you buy subsequently are the same product rather than a cheaper substitute that looks the same?Buying from a reputable distributer should be the answer but the problem is finding something cheap enough. I guess the best way would be to find something that's used in very large numbers, such as usb phone chargers. Unfortunately they all seem to be the mains plug type supplies.

author: splin
author_rank: Frequent Contributor
timestamp: November 29, 2014, 12:28:29 am
content: Quote from: mikeselectricstuff on November 28, 2014, 11:53:54 pmBut still a lot more expensive at £4.11 (100+) than this:http://uk.farnell.com/pro-power/ctfcs6-5/transformer-6va-2-x-5v/dp/1780867at £2.18 plus 30 or 40 pence for a diode bridge, electrolytic cap and a linear regulator. I don't require dual voltage operation - 230V only. A lot more compact though and cheaper to mount and assemble.Do those small switchers obviate the need for fusing, input protection (eg. MOV) and EMI filtering?

author: IanB
author_rank: Super Contributor
timestamp: November 29, 2014, 12:30:42 am
content: Reputable, safe, tested, compliant with regulations, meets specifications. All of that sounds like it costs money.From what I have read and understand, getting good quality products from China requires close involvement with the factory and careful monitoring every step of the way. This also costs money.Pound stores and dollar stores are full of USB mains adapters selling for the price you want to pay. Yes, they come in a mains plug package, but the circuit board inside them simply has two wires for mains input and two wires for 5 V output. I dare say those kinds of boards could be bought standalone in volume. But it comes back to your question: could you trust them?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 29, 2014, 12:32:51 am
content: Quote from: splin on November 29, 2014, 12:04:58 amQuoteBy paying more than $2 for it.What sort of quantity ? unless you're into 1000s it will probably be cheaper to buy a known approved unit than roll & approve your own.QuoteQuoteYou can't have it both ways (Unless you're someone like Apple) - cheap, reputable. Pick one.

author: splin
author_rank: Frequent Contributor
timestamp: November 29, 2014, 12:43:23 am
content: Quote from: IanB on November 29, 2014, 12:30:42 amTrue, but for very high volume products the overhead will be very low per unit.QuoteNo! Especially after seeing consumer programmes demonstrating them bursting into flames etc.I'm sure that the circuit board in the charger that Apple, Nokia etc. supply costs them peanuts. I doubt that I could persuade them to sell me any though. Well perhaps if I wanted 20 million units they might.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 29, 2014, 12:43:58 am
content: If you're talking 100+ qtys, you should definitely be talking to specialist PSU distributors and not the likes of Farnell etc.e.g. Stontronics, Ecopac, Powesolve, PowerPax etc.This might be worth a look, probably still not $2 though. I don't think you stand any chance of getting near that safely.http://www.powerpax.co.uk/5w-miniature-low-cost-pcb-mount-series-ac-dc-power-supplies..just found what looks like the same range at RShttp://uk.rs-online.com/web/p/embedded-switch-mode-power-supplies-smps/7719379P/?3W £4.90 50x at RS - wouldn't surprise me if it got towards £3 at higher qtys elsewhere

author: ludzinc
author_rank: Unknown
timestamp: November 29, 2014, 04:55:38 am
content: Quote from: electr_peter on November 28, 2014, 11:02:51 pmEasy? Sort of. Safe? No freaking way!Your DC rail is floating at mains potential.Works if the electronics are burried inside your washing machine, but probably lethal for hand held devices ....

author: electr_peter
author_rank: Unknown
timestamp: November 29, 2014, 07:35:10 am
content: Quote from: ludzinc on November 29, 2014, 04:55:38 amIt is safe as in "does not explode in your face". But without transformer it can cause other safety issues for personnel/equipment.Original question did not mention isolation.

author: electronic_eel
author_rank: Regular Contributor
timestamp: November 29, 2014, 03:35:32 pm
content: http://www.soselectronic.com/?str=371&artnum=128759&name=myrra-47132-2-5w5v-471323.12 EUR @ 100pcs.Has true isolation and meets the common safety standards.

author: splin
author_rank: Frequent Contributor
timestamp: November 29, 2014, 06:08:04 pm
content: Quote from: electronic_eel on November 29, 2014, 03:35:32 pmThat's a good find - thanks. The website claims discounts up to 20% for registered users so in larger volumes, say 1k+ the price might get down to 2.5 EUR or $3.1