Cheapest way to get non isolated 3.3VDC from 230VAC - Page 1
Forum section: electronics / beginners

author: bitshift
author_rank: Regular Contributor
timestamp: October 09, 2015, 05:36:21 pm
content: Hey guys,I need to get 3.3VDC (+- 10% at most) from 230VAC. I would like to avoid a transformer for size and cost reasons. The current will, for the most part, be under 100mA but may be as high as 350mA for brief periods.I've looked into a capacitive dropper supply but it's not too practical when the 350mA peak current requirement is considered.Is there a cheap and hopefully relative efficient way of meeting these requirements?

author: SeanB
author_rank: Super Contributor
timestamp: October 09, 2015, 05:49:35 pm
content: How long do you need the 350mA for? How about using a capacitive dropper to get 5V1 and charge up an ultracapacitor ( 0.047uF 5V5 is quite cheap, and will do the 350mA for a few seconds depending on the internal resistance of the one you use) to 5V then use a LDO 3V3 regulator for the MCU rail. Cheap, simple and you can fit the parts into a small area, aside from the clearance needed at the one end of the mains capacitor.

author: Gyro
author_rank: Super Contributor
timestamp: October 09, 2015, 06:19:40 pm
content: IF you're pulling 250mA for any significant period of time then you should really be looking at some sort of down-conversion, transformer, small smps adapter or whatever. 350mA via any sort of dropper is going to be pulling something like 80W from the mains supply, not very Eco friendly!Edit: Just re-read the post, even at 100mA you're talking over 20W loss.

author: Joule Thief
author_rank: Regular Contributor
timestamp: October 09, 2015, 06:29:45 pm
content: http://www.ebay.com/itm/AC-DC-220V-to-12V-3-3V-250mA-200mA-Isolated-Step-Down-Buck-Power-Supply-Module-/231640983362?hash=item35eee0e742

author: Jeroen3
author_rank: Super Contributor
timestamp: October 09, 2015, 06:35:07 pm
content: You'll either need a (pulse) transformer, coil or a heatsink. Just buy something like this and don't spend any more development hours.http://www.farnell.com/datasheets/1425307.pdf

author: BravoV
author_rank: Super Contributor
timestamp: October 09, 2015, 06:48:41 pm
content: Transformerless Power Supplies: Resistive and Capacitive ->http://ww1.microchip.com/downloads/en/AppNotes/00954A.pdf

author: Gyro
author_rank: Super Contributor
timestamp: October 09, 2015, 07:30:47 pm
content: QuoteYes if you're talking 10-20mA max, but not for 100 - 350mA.

author: kripton2035
author_rank: Super Contributor
timestamp: October 09, 2015, 07:45:28 pm
content: isolated power supplies are in fact quite small - and smps are not that smaller compared to isolated supplies :see example below :left board is 5 x 7 cm, contains a 220v-6v transformer with 2 varistors, a 3.3v regulator, an esp8266, and 2 sensors for temp humidity and pressureright board is a 3.3v smps supply at low price from ebay

author: mariush
author_rank: Super Contributor
timestamp: October 09, 2015, 08:51:26 pm
content: Not exactly cheapest, but should take very small amount of space... bridge rectifier or single diode for rectification, capacitor to keep voltage to a minimum of at least 40-50v, zener diode to keep maximum voltage below reasonable amount, then use a switching regulator running at high frequency to keep the inductor size small .. likeLMR16006just to pick something on a quick search on Digikey, which works with up to 60v. With 50v in, for 3.3v and 350mA we're talking of about 25mA at 50v which is small enough.

author: nbritton
author_rank: Frequent Contributor
timestamp: October 10, 2015, 01:24:24 am
content: A diode plugged directly into the mains will get you half wave rectification, then I would imagine you could run that through a buck converter.

author: Kean
author_rank: Unknown
timestamp: October 10, 2015, 02:30:12 am
content: Quote from: AcHmed99 on October 09, 2015, 09:39:18 pmYep, LNK306 or similar is probably suitable for your needs

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 10, 2015, 08:27:54 am
content: Quote from: Kean on October 10, 2015, 02:30:12 am+1 - the Power Integrations LNK30x devices are designed for exactly this, and can be cheaper, definitely smaller than capacitive dropppers at higher currents.

author: bitshift
author_rank: Regular Contributor
timestamp: October 12, 2015, 08:03:04 am
content: Apologies for the delay in response.The Power Integrations LNK30x devices look perfect, thank you all for your suggestions