Cheap simple isolated AC -> 5V 50ma supply ? - Page 1
Forum section: electronics / projects

author: tnt
author_rank: Regular Contributor
timestamp: August 04, 2012, 03:51:03 pm
content: Hi,I'm looking for a way to takes mains (240V AC) and get 5V 50ma regulated out of it.Now the conditions are:- Cheaper than 13 eur : There are dedicated modules that do 5V 600ma for 13 eur at farnell but I don't nee 600ma ...- Isolated : What its powering will have touch sensors ... I don't want to touch something that is not isolated from mains- Small : I don't want a big linear supply with a large transformer. The farnell module is ~ 30mm x 30mm x 20 mm- Integrable on the PCB: I know I could take appart a random wall wart but I want something I can just design in my PCB. Parts of the PCB already have mains access (for SSR / current sensing / ...).- Using off the shelf component : No widing my own transformer or stuff like that.- As simple as possibleAnybody got ideas ?Cheers,Sylvain

author: poorchava
author_rank: Super Contributor
timestamp: August 04, 2012, 05:09:51 pm
content: You can search ebay/DealExtreme/Aliexpress/etc for Mains->USB adapters meant for charging phones. I've seen those go as low as $2 so they're very cheap. What i don't know though, is to what degree can those be stripped off the plastic enclosure in order to fit pcb. Whole adapter is a bit bigger than 30x30mm, but I don't know how big the functional internals are (or how they are designed for that matter). IMO worth trying for that kind of money.

author: T4P
author_rank: Super Contributor
timestamp: August 04, 2012, 05:19:24 pm
content: They work and that's all we need to knowWell i can only guarantee they work best to 300mA but anything above that ... all bets are off (above 300mA you get starved current output + lots of noise)

author: tnt
author_rank: Regular Contributor
timestamp: August 04, 2012, 05:56:32 pm
content: As I said, I don't really want to buy wall wart and have to tear them appart.

author: T4P
author_rank: Super Contributor
timestamp: August 04, 2012, 06:05:53 pm
content: Quote from: tnt on August 04, 2012, 05:56:32 pmDo you have to? Just get those that you can open with a screw

author: muvideo
author_rank: Frequent Contributor
timestamp: August 04, 2012, 06:28:22 pm
content: I think an EE20 transformer fits the requirements,http://www.nordtek.com/index.php?m=products&id=81unless you find an off the shelf switcher that fits your needs.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 05, 2012, 06:53:02 am
content: These look suitable - 1W PCB mounting SMPS modules :http://www.mornsun-power.com/cn/product_inf.aspx?typeID=17#May be hard to find in small qtys though

author: amyk
author_rank: Super Contributor
timestamp: August 05, 2012, 08:45:02 am
content: Quote from: T4P on August 04, 2012, 05:19:24 pmOP only needs 50mA. Even the cheapest Chineseshould be able to manage that easily.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 05, 2012, 08:50:35 pm
content: Quote from: amyk on August 05, 2012, 08:45:02 am..just better hope the isolation holds up and it doesn't catch fire the first time it sees a mains transient.

author: mariush
author_rank: Super Contributor
timestamp: August 05, 2012, 09:41:18 pm
content: 4$http://uk.farnell.com/myrra/44087/transformer-1-5va-12v/dp/1689049230v -> 12v 1.5VA ... 27 mm x 32mm x 25mm ...1$http://www.monoprice.com/products/product.asp?c_id=102&cp_id=10228&cs_id=1022801&p_id=5277&seq=1&format=2(cut the end of the cable, solder it to the pcb and the transformer pins)~1$ pcb on the bottom with a small bit on the outside, maybe a fuse , 4 diodes, a ldo and 2 capacitors and you're done.1-2$ plastic box to protect the insides.

author: tom66
author_rank: Super Contributor
timestamp: August 09, 2012, 06:19:07 am
content: Ringing choke converter. Capable of 100mA at 5V.Make sure to use a low ESR cap on output.

author: tnt
author_rank: Regular Contributor
timestamp: August 09, 2012, 11:32:45 am
content: @tom66: Nice, thanks a lot, that's exactly what I was looking for.Do you know if there is any off the shelf transformer that would be suitable for L1/L2/L3 ?

author: tom66
author_rank: Super Contributor
timestamp: August 09, 2012, 11:49:11 am
content: Quote from: tnt on August 09, 2012, 11:32:45 amI think you'd need to wind your own. It's not too difficult - you'd need to buy the magnet wire and transformer cores, but they are standard.If you were making enough you could get a quote from a manufacturer of transformers.

author: arekm
author_rank: Unknown
timestamp: January 14, 2015, 09:02:17 pm
content: Are there any good, cheap (so likely chinese) power supplies these days? Few pcs only. Looking for 5V, 0.5A, 1A ones, enclosure not needed. The goal is to safely power arduinos (that are connected to user touchable buttons etc) from 230V.Founds somehttp://www.ebay.com/itm/3-3V-5V-9V-12V-AC-DC-DC-DC-Power-Supply-Buck-Converter-Step-Down-Module-1A-600mA-/400761809788but I assume aren't that great (if someone else could look and confirm)

author: LukeW
author_rank: Frequent Contributor
timestamp: January 15, 2015, 03:16:17 am
content: But the above ringing choke converter schematic doesn't have any galvanic isolation from the mains!Since you only want 50 mA, how about something like a transformerless RC dropper style power supply, followed by a voltage regulator, followed by something like an Analog Devices iCoupler IC to give you the isolation?

author: poorchava
author_rank: Super Contributor
timestamp: January 15, 2015, 06:29:35 am
content: Quote from: LukeW on January 15, 2015, 03:16:17 amIt does give isolation, as long as C6 and C9 are properl rated.

author: dave_j_fan
author_rank: Regular Contributor
timestamp: January 15, 2015, 05:07:55 pm
content: here i get chargers for 0.5$ (india )...lastime i was looking for even lessshop keeper got angry

author: Kalvin
author_rank: Super Contributor
timestamp: January 15, 2015, 05:53:29 pm
content: How about an USB charger: 5V @ 100mA with isolation. Just remove the covers and place it on the PCB inside the enclosure, if necessary.

author: arekm
author_rank: Unknown
timestamp: January 15, 2015, 06:00:37 pm
content: Find good, cheap, tested, isolated usb charger first. (bought some goobay branded after readinghttp://www.lygte-info.dk/info/usbPowerSupplyTest%20UK.htmlto see how they look inside)