Cheap 120VAC to 3.3VDC power supply

Dec 01, 2006 3 Replies
N

I'm looking for the cheapest, smallest way to power a 50mA 3.3V circuit from mains power. I'd like the whole circuit, including the power supply, to fit in a matchbox.



I've always used transformers in my designs but they're both too expensive and too big for what I want to do. I recently discovered a number of "off-line" switching regulators from various vendors. Are these the correct solution to the problem? Even these require a fair number of external parts.



I'm sure I'm not the first person who's faced this problem or asked this question. I suspect there's a "standard" answer out there, but searching is difficult given how generic the search terms are.



Thanks,


-Nevo


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T

If you don't need an isolated supply, you could use a capacitor as a lossless series element - by my reckoning you'd need about 1uF to deliver

50mA. Problem is the cap will be very transparent to mains spikes, so you need to do something limit the current surges through the cap and resulting voltage surges at the 3.3V supply. You also need some form of rectification to ensure the cap conducts AC current, otherwise it will just charge up to the peak supply voltage. I have seen commercial circuits using this kind of approach, though, personally, I'd tread very carefully.

Try Googling for "capacitive series elements" or something similar.

T If it\'s not broken, don\'t fix it.
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H

There is no general answer! Look for example here:

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regards - Henry

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"Nevo" schrieb im Newsbeitrag news:APXbh.27$kz1.7@trnddc07...

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J

if you don't need iosolation from the mains use a capacitor to limit the current to about 70ma, a rectifier, and a 3.3V zener.

1.5u .------------+---[33R]-+--+ || / \\ | | 250VAC --[22R]---||----. 1000u === .->Z-' load || \\ / | 10V | | 3V3 c1 `---|--------+----+-------+ ---------------------' c2

use chunky diodes in the bridge, power-on inrush can exceed 15A for a few microseconds. - 1n4001 would probably do, 1n914 probably not a bleed resistor across c1 would probably be a good idea too. c1 should have 'X2' safety rating. and will probably take up most of that matchbox space here's an LTSPICE file for a 4.7V design (no 3.3V zener model) don't use

1n914 diodes in the rectifier of a real one.

--------8

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