Very low power, power supply

[Chris W]

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

 

[Don Pearce]

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.

d

Pearce Consulting
http://www.pearce.uk.com

 

[Paul Burke]

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.

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

 

[Wolfgang Mahringer]

Hi Don,

Don Pearce schrieb:

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

 

[John Woodgate]

I read in sci.electronics.design that Paul Burke <paul@scazon.com> wrote
(in <39t14hF4mtp4kU1@individual.net&gt 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.
--
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

 

[Ken Smith]

In article <gU3dD2CxLWOCFwGE@jmwa.demon.co.uk>,
John Woodgate <noone@yuk.yuk> wrote:

I read in sci.electronics.design that Paul Burke <paul@scazon.com> wrote
(in <39t14hF4mtp4kU1@individual.net&gt 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

 

[René]

On Thu, 17 Mar 2005 10:30:53 +0100, Wolfgang Mahringer
<yeti201@gmx.at> wrote:

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

 

[Wolfgang Mahringer]

Hi Ken,

Ken Smith schrieb:

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

 

[Watson A.Name - \"Watt Su]

"Don Pearce" <donald@pearce.uk.com> wrote in message
news:42393f6b.71356625@news.plus.net...

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..
;-)

d

Pearce Consulting
http://www.pearce.uk.com

 

[Fred Bloggs]

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.

 

[John Woodgate]

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&gt about 'Very low power, power supply', on Thu, 17 Mar 2005:

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

 

[Jim Gregory]

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)?

 

[Watson A.Name - \"Watt Su]

"Fred Bloggs" <nospam@nospam.com> wrote in message
news:4239B9C3.40104@nospam.com...

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.

 

[Watson A.Name - \"Watt Su]

"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote in message
news:9+n6JiDzDcOCFwny@jmwa.demon.co.uk...

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&gt about 'Very low power, power supply', on Thu, 17 Mar
2005:

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.

The usual white LED is rated at 5V max in the reverse direction, this
arrangement far exceeds that voltage. As for power, the max dissipation
is about 3.3V * 30mA or 100mW. 5V at 20mA would be 100mW, but the
breakdown voltage is usually much higher than that, some say the LEDs
will break down in the tens of volts range. In that case, the
dissipation would far exceed the 100mW max. That's plainly not
acceptable.

--
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

 

[Rich Grise]

On Thu, 17 Mar 2005 02:15:46 -0600, 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.

A 5K, 2W resistor, and an antiparallel 1N4004. Put the LEDs in series.
(and the 1N4004 in parallel with the series string, but opposite
polarity.) 5.1K might be easier to find - it's not terribly critical.
But it _will_ dissipate two watts, so you might want to use a 3W R.

Cheers!
Rich

 

[Rich Grise]

On Thu, 17 Mar 2005 02:15:46 -0600, 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.

Or, just get a low-profile night light:
http://froogle.google.com/froogle?q=electroluminescent+night+light&num=50&hl=en&lr=lang_en&ie=UTF-8&safe=off&sa=N&tab=ff&oi=froogler

Cheers!
Rich

 

[Ken Smith]

In article <Moi_d.4$QH1.349303@news.salzburg-online.at>,
Wolfgang Mahringer <yeti201@gmx.at> wrote:

Hi Ken,

Ken Smith schrieb:

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*

If one LED fails, the circuit is broken. After the "BOOM" the circuit is
still broken.

Actually there is no "BOOM" the back biased LEDs will fail shorted and the
series RC will still limit the current.


--
--
kensmith@rahul.net forging knowledge

 

[Ken Smith]

In article <113jjfdt40lbi1b@corp.supernews.com>,
Watson A.Name - \"Watt Sun, the Dark Remover\" <alondra101@hotmail.com> wrote:
[...]

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.

If you use two strings anti-paralleled, the flicker is very hard to
detect. The frequency is 120Hz and most of the time there is light
produced. The eye detects spikes in light better than dips.

--
--
kensmith@rahul.net forging knowledge

 

[Ken Smith]

In article <e2B_d.15$HE2.585823@news.salzburg-online.at>,
Wolfgang Mahringer <yeti201@gmx.at> wrote:

Hi Ken,

Ken Smith schrieb:

Well, this will work.
But imagine what happens if one the LED fails open...
*BOOM*


If one LED fails, the circuit is broken.
It isn't entirely. 3 LEDs are still working!

Not for long enough to matter. Besides, at the first failure the circuit
will no longer meet its specifications for total light output making the
whole thing delared broken. People rarely keep a toaster that will only
toast one of the two slices.

After the "BOOM" the circuit is
still broken.
Using your circuit, there will be no LEDs that are still working.

Actually there is no "BOOM" the back biased LEDs will fail shorted
How can you tell that? Any sources?

I've broken a LED or two in my life. They tend to turn into low value
resistors at low amounts of destruction. You have to do something fairly
major to make the part go open. Merely running some 20-40mA through them
backwards is not nearly major enough.

They are typically rated for 5V backwards but may not break down until
perhaps as much as 10V. 40mA times 10V is only 0.4W. That will not cause
any sort of "BOOM".

and the
series RC will still limit the current.
Yes, thats true.

Also, using a bridge rectifier gives you the advantage of less flicker
(120Hz) for all 6 LEDs. Your suggestion will flicker with 60Hz
alternatively.

It gives you less flicker at the cost of a bridge rectifier. The bridge
costs money to install and also has a failure rate so it increases the
chance of failure.

If you really care about the flicker, adding another capacitor can reduce
it.

--
--
kensmith@rahul.net forging knowledge

 

[Watson A.Name - \"Watt Su]

"Ken Smith" <kensmith@green.rahul.net> wrote in message
news:d1eqq7$c2k$2@blue.rahul.net...
[snip]

I've broken a LED or two in my life. They tend to turn into low value
resistors at low amounts of destruction. You have to do something
fairly
major to make the part go open. Merely running some 20-40mA through
them
backwards is not nearly major enough.

They are typically rated for 5V backwards but may not break down until
perhaps as much as 10V. 40mA times 10V is only 0.4W. That will not
cause
any sort of "BOOM".

The typical LED will handle 100mW with noticeable heating. That's
because most of the heat has to be conducted thru the two leads, which
are not very heavy. I'd say that if you made it dissipate "only" 0.4W,
it would get mighty hot and melt.

[snip]

--
--
kensmith@rahul.net forging knowledge