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Cheap 120VAC to 3.3VDC power supply

N

Nevo

Jan 1, 1970
0
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
 
T

TuT

Jan 1, 1970
0
Nevo said:
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

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.
 
J

jasen

Jan 1, 1970
0
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.

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]---||---<BR >-. 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<----- cut here save as filename.asc
Version 4
SHEET 1 880 680
WIRE 64 96 -16 96
WIRE 176 96 144 96
WIRE 368 96 240 96
WIRE -16 144 -16 96
WIRE 304 144 272 144
WIRE 368 144 368 96
WIRE 464 144 432 144
WIRE 272 272 272 144
WIRE 304 272 272 272
WIRE 464 272 464 144
WIRE 464 272 432 272
WIRE 512 272 464 272
WIRE 656 272 592 272
WIRE 768 272 656 272
WIRE 464 304 464 272
WIRE 768 304 768 272
WIRE 656 320 656 272
WIRE -16 368 -16 224
WIRE 368 368 368 272
WIRE 368 368 -16 368
WIRE 272 416 272 272
WIRE 464 416 464 368
WIRE 464 416 272 416
WIRE 656 416 656 384
WIRE 656 416 464 416
WIRE 768 416 768 384
WIRE 768 416 656 416
WIRE 272 448 272 416
FLAG 272 448 0
SYMBOL voltage -16 128 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value SINE(0 315 50)
SYMBOL res 160 80 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R1
SYMATTR Value 22R
SYMBOL cap 240 80 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 1.5µ
SYMBOL diode 304 160 R270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName D1
SYMATTR Value 1N914
SYMBOL diode 368 160 R270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName D2
SYMATTR Value 1N914
SYMBOL diode 304 288 R270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName D3
SYMATTR Value 1N914
SYMBOL diode 368 288 R270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName D4
SYMATTR Value 1N914
SYMBOL cap 448 304 R0
WINDOW 0 -43 36 Left 0
WINDOW 3 -33 63 Left 0
SYMATTR InstName C2
SYMATTR Value 1000µ
SYMBOL zener 672 384 R180
WINDOW 0 24 72 Left 0
WINDOW 3 -76 69 Left 0
SYMATTR InstName D5
SYMATTR Value 1N750
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL res 752 288 R0
SYMATTR InstName R2
SYMATTR Value 80R
SYMBOL res 608 256 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R3
SYMATTR Value 33r
TEXT -50 506 Left 0 !.tran 0 2 0 .001 startup
-------------------8<------ cut here

Bye.
Jasen
 
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