| Author |
Message |
Chris W
Guest
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 Posted:
Thu Mar 17, 2005 2:15 pm Post subject:
Very low power, power supply |
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I am looking for a power supply to power a small LED night light. I
want it to run of the 120V AC and I need it to be very small. I don't
want the completed night light to protrude more than 1/2" from the
outlet when it is plugged in. I could probably live with 3/4" if I had
to though. Since this is to power LED's it would be nice if it had a
current limiting feature. It only needs to run 6 20mA white LEDs. So
far the smallest I have found is about a 1" cube. It takes 120V AC to
5V DC.
--
Chris W
Gift Giving Made Easy
Get the gifts you want &
give the gifts they want
http://thewishzone.com
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Don Pearce
Guest
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| Posted:
Thu Mar 17, 2005 2:28 pm Post subject:
Re: Very low power, power supply |
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On Thu, 17 Mar 2005 02:15:46 -0600, Chris W <1qazse4@cox.net> wrote:
| Quote: | I am looking for a power supply to power a small LED night light. I
want it to run of the 120V AC and I need it to be very small. I don't
want the completed night light to protrude more than 1/2" from the
outlet when it is plugged in. I could probably live with 3/4" if I had
to though. Since this is to power LED's it would be nice if it had a
current limiting feature. It only needs to run 6 20mA white LEDs. So
far the smallest I have found is about a 1" cube. It takes 120V AC to
5V DC.
|
You don't need a power supply. Put all the LEDs in series with each
other, and with a 5.6k resistor. Get a resistor with a power rating of
about 5 watts.
d
Pearce Consulting
http://www.pearce.uk.com |
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Ken Smith
Guest
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| Posted:
Thu Mar 17, 2005 3:12 pm Post subject:
Re: Very low power, power supply |
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In article <gU3dD2CxLWOCFwGE@jmwa.demon.co.uk>,
John Woodgate <noone@yuk.yuk> wrote:
| Quote: | I read in sci.electronics.design that Paul Burke <paul@scazon.com> wrote
(in <39t14hF4mtp4kU1@individual.net>) about 'Very low power, power
supply', on Thu, 17 Mar 2005:
Don Pearce wrote:
You don't need a power supply. Put all the LEDs in series with each
other, and with a 5.6k resistor. Get a resistor with a power rating of
about 5 watts.
Or a 250Vdc/160Vac 0.47uF cap. (assuming 60Hz) (about 50p from Farnell),
then it doesn't get warm.
You need some resistance in series to prevent the inrush current zapping
the LEDs and/or the cap. For 120 V mains, 470 ohms 0.5 W may be enough.
|
Also: Make 2 banks of series out of the LEDs. Hook the two banks in
reverse parallel.
----/\/\/---!!-----+------------
! !
--- V
^ ---
! !
--- V
^ ---
! !
--- V
^ ---
! !
--------------------+-----------
This way, the current has a path for both directions.
--
--
kensmith@rahul.net forging knowledge
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Paul Burke
Guest
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| Posted:
Thu Mar 17, 2005 3:23 pm Post subject:
Re: Very low power, power supply |
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Don Pearce wrote:
| Quote: | You don't need a power supply. Put all the LEDs in series with each
other, and with a 5.6k resistor. Get a resistor with a power rating of
about 5 watts.
|
Or a 250Vdc/160Vac 0.47uF cap. (assuming 60Hz) (about 50p from Farnell),
then it doesn't get warm.
But either way I wouldn't be happy unless the LEDs were covered. Blow
one up and there's two little prongs sticking out with mains on them.
Paul Burke |
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Wolfgang Mahringer
Guest
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| Posted:
Thu Mar 17, 2005 3:30 pm Post subject:
Re: Very low power, power supply |
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Hi Don,
Don Pearce schrieb:
| Quote: | On Thu, 17 Mar 2005 02:15:46 -0600, Chris W <1qazse4@cox.net> wrote:
I am looking for a power supply to power a small LED night light. I
want it to run of the 120V AC and I need it to be very small. I don't
want the completed night light to protrude more than 1/2" from the
outlet when it is plugged in. I could probably live with 3/4" if I had
to though. Since this is to power LED's it would be nice if it had a
current limiting feature. It only needs to run 6 20mA white LEDs. So
far the smallest I have found is about a 1" cube. It takes 120V AC to
5V DC.
You don't need a power supply. Put all the LEDs in series with each
other, and with a 5.6k resistor. Get a resistor with a power rating of
about 5 watts.
|
What a bad advice.
Lots of wasted power, lots of wasted LEDs.
LEDs dont like exessive reverse voltage, typically only about 5 volts.
So better use a 470nF capacitor capable of 150V AC, a small bridge
rectifier (needs only to survive the summed LEDs forward voltage) and
a little 100 ohms resistor.
(switch to fixed pitch font to view)
o------.
--- 470 nF 150V
---
|
120V AC |
60Hz .-.
| | 100R 0,25W
| |
'-'
| .-----. the LEDs
-----| ~/ +|- ->|->|->|--|
o-----------|~/ -|-------------|
'-----'
You can vary the LED current with the capacitor.
HTH
Wolfgang
--
From-address is Spam trap
Use: wolfgang (dot) mahringer (at) sbg (dot) at |
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John Woodgate
Guest
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| Posted:
Thu Mar 17, 2005 4:58 pm Post subject:
Re: Very low power, power supply |
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I read in sci.electronics.design that Paul Burke <paul@scazon.com> wrote
(in <39t14hF4mtp4kU1@individual.net>) about 'Very low power, power
supply', on Thu, 17 Mar 2005:
| Quote: | Don Pearce wrote:
You don't need a power supply. Put all the LEDs in series with each
other, and with a 5.6k resistor. Get a resistor with a power rating of
about 5 watts.
Or a 250Vdc/160Vac 0.47uF cap. (assuming 60Hz) (about 50p from Farnell),
then it doesn't get warm.
|
You need some resistance in series to prevent the inrush current zapping
the LEDs and/or the cap. For 120 V mains, 470 ohms 0.5 W may be enough.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk |
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mc
Guest
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| Posted:
Thu Mar 17, 2005 9:30 pm Post subject:
Re: Very low power, power supply |
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Does your night light have to be an LED rather than a neon lamp? An NE-2
neon lamp can run directly from the 120-volt line through a 220k resistor. |
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René
Guest
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| Posted:
Thu Mar 17, 2005 9:34 pm Post subject:
Re: Very low power, power supply |
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On Thu, 17 Mar 2005 10:30:53 +0100, Wolfgang Mahringer
<yeti201@gmx.at> wrote:
| Quote: | So better use a 470nF capacitor capable of 150V AC, a small bridge
rectifier (needs only to survive the summed LEDs forward voltage) and
a little 100 ohms resistor.
|
or use 2 leds anti-parallel, or build de rectifying bridge out of
leds, feeding a resistor or....
(or I would buy a ready made one for a few $ in the local DIY shop:-)
--
- René |
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Wolfgang Mahringer
Guest
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| Posted:
Thu Mar 17, 2005 10:59 pm Post subject:
Re: Very low power, power supply |
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Hi Ken,
Ken Smith schrieb:
| Quote: | Also: Make 2 banks of series out of the LEDs. Hook the two banks in
reverse parallel.
----/\/\/---!!-----+------------
! !
--- V
^ ---
! !
--- V
^ ---
! !
--- V
^ ---
! !
--------------------+-----------
This way, the current has a path for both directions.
|
Well, this will work.
But imagine what happens if one the LED fails open...
*BOOM*
Wolfgang
--
From-address is Spam trap
Use: wolfgang (dot) mahringer (at) sbg (dot) at |
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Watson A.Name - \"Watt Su
Guest
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| Posted:
Thu Mar 17, 2005 11:02 pm Post subject:
Re: Very low power, power supply |
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"Chris W" <1qazse4@cox.net> wrote in message
news:s1b_d.17785$2s.16931@lakeread06...
| Quote: | I am looking for a power supply to power a small LED night light. I
want it to run of the 120V AC and I need it to be very small. I don't
want the completed night light to protrude more than 1/2" from the
outlet when it is plugged in. I could probably live with 3/4" if I
had
to though. Since this is to power LED's it would be nice if it had a
current limiting feature. It only needs to run 6 20mA white LEDs. So
far the smallest I have found is about a 1" cube. It takes 120V AC to
5V DC.
|
Here is the usual way of doing this, with a capacitor to drop the
voltage. You will have to connect all 6 LEDs in series. Make sure the
capacitor is X2 rated for connection across the AC line, and all parts
are insulated to prevent a shock hazard. The typical .47 uF X2 rated
cap is about a half inch thick, but you could put two smaller, thinner
capacitors in parallel.
http://ourworld.compuserve.com/homepages/Bill_Bowden/page10.htm#lineled.
gif
BTW, if you don't use a filter capacitor, the flicker becomes
noticeable.
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Watson A.Name - \"Watt Su
Guest
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| Posted:
Thu Mar 17, 2005 11:05 pm Post subject:
Re: Very low power, power supply |
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"Don Pearce" <donald@pearce.uk.com> wrote in message
news:42393f6b.71356625@news.plus.net...
| Quote: | On Thu, 17 Mar 2005 02:15:46 -0600, Chris W <1qazse4@cox.net> wrote:
I am looking for a power supply to power a small LED night light. I
want it to run of the 120V AC and I need it to be very small. I
don't
want the completed night light to protrude more than 1/2" from the
outlet when it is plugged in. I could probably live with 3/4" if I
had
to though. Since this is to power LED's it would be nice if it had a
current limiting feature. It only needs to run 6 20mA white LEDs.
So
far the smallest I have found is about a 1" cube. It takes 120V AC
to
5V DC.
You don't need a power supply. Put all the LEDs in series with each
other, and with a 5.6k resistor. Get a resistor with a power rating of
about 5 watts.
|
And because you forgot to include a 1N4004 diode, the 6 LEDs will be
subjected to 160V peak in the reverse direction, causing them to
breakdown since they're only rated for 5V reverse x6 = 30V.
I don't think I'll be asking for your consulting services in the near
future..
;-)
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Fred Bloggs
Guest
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| Posted:
Thu Mar 17, 2005 11:09 pm Post subject:
Re: Very low power, power supply |
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Chris W wrote:
| Quote: | I am looking for a power supply to power a small LED night light. I
want it to run of the 120V AC and I need it to be very small. I
don't want the completed night light to protrude more than 1/2" from
the outlet when it is plugged in. I could probably live with 3/4" if
I had to though. Since this is to power LED's it would be nice if it
had a current limiting feature. It only needs to run 6 20mA white
LEDs. So far the smallest I have found is about a 1" cube. It takes
120V AC to 5V DC.
|
You have to run the LEDs off DC or you will never be able to get 20mA
average current through them at 60Hz sinusoidal without significant
over-current. |
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John Woodgate
Guest
|
| Posted:
Thu Mar 17, 2005 11:40 pm Post subject:
Re: Very low power, power supply |
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I read in sci.electronics.design that Watson A.Name - "Watt Sun, the
Dark Remover" <NOSPAM@dslextreme.com> wrote (in <113je1b4n5n3c45@corp.su
pernews.com>) about 'Very low power, power supply', on Thu, 17 Mar 2005:
| Quote: | And because you forgot to include a 1N4004 diode, the 6 LEDs will be
subjected to 160V peak in the reverse direction, causing them to
breakdown since they're only rated for 5V reverse x6 = 30V.
|
The reverse current is limited by the series resistor or capacitor. For
the resistor, it's 90 V/5600 ohms = 16 mA. Whether that's acceptable or
not can be found on the data sheet.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk |
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Jim Gregory
Guest
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| Posted:
Thu Mar 17, 2005 11:45 pm Post subject:
Re: Very low power, power supply |
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Red/amber low-current LEDs can glow at 1mA, but I'm not sure about the
"white" ones.
What's wrong with using a 120V neon indicator and integral dropper? It will
last for up to ten years.
Or go for a self-indicating, internally soft-lit translucent plug (used as
nursery/corridor courtesy light)? |
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Watson A.Name - \"Watt Su
Guest
|
| Posted:
Fri Mar 18, 2005 12:38 am Post subject:
Re: Very low power, power supply |
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|
"Fred Bloggs" <nospam@nospam.com> wrote in message
news:4239B9C3.40104@nospam.com...
| Quote: |
Chris W wrote:
I am looking for a power supply to power a small LED night light.
I
want it to run of the 120V AC and I need it to be very small. I
don't want the completed night light to protrude more than 1/2"
from
the outlet when it is plugged in. I could probably live with 3/4"
if
I had to though. Since this is to power LED's it would be nice if
it
had a current limiting feature. It only needs to run 6 20mA white
LEDs. So far the smallest I have found is about a 1" cube. It
takes
120V AC to 5V DC.
You have to run the LEDs off DC or you will never be able to get 20mA
average current through them at 60Hz sinusoidal without significant
over-current.
|
The overcurrent isn't the problem, it's the flicker. Most white LEDs
are rated for 30mA max, 100mA peak. But the flicker is really
noticeable especially 60 Hz half wave. |
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