| Author |
Message |
Watson A.Name - \"Watt Su
Guest
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 Posted:
Fri Mar 18, 2005 12:45 am Post subject:
Re: Very low power, power supply |
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"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote in message
news:9+n6JiDzDcOCFwny@jmwa.demon.co.uk...
| Quote: | 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:
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.
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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.
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Rich Grise
Guest
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| Posted:
Fri Mar 18, 2005 2:36 am Post subject:
Re: Very low power, power supply |
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On Thu, 17 Mar 2005 02:15:46 -0600, 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.
|
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 |
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Rich Grise
Guest
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| Posted:
Fri Mar 18, 2005 2:40 am Post subject:
Re: Very low power, power supply |
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On Thu, 17 Mar 2005 02:15:46 -0600, 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.
|
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
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Ken Smith
Guest
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| Posted:
Fri Mar 18, 2005 4:33 am Post subject:
Re: Very low power, power supply |
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In article <Moi_d.4$QH1.349303@news.salzburg-online.at>,
Wolfgang Mahringer <yeti201@gmx.at> wrote:
| Quote: | Hi Ken,
Ken Smith schrieb:
Also: Make 2 banks of series out of the LEDs. Hook the two banks in
reverse parallel.
----/\/\/---!!-----+------------
! !
--- V
^ ---
! !
--- V
^ ---
! !
--- V
^ ---
! !
--------------------+-----------
This way, the current has a path for both directions.
Well, this will work.
But imagine what happens if one the LED fails open...
*BOOM*
|
If one LED fails, the circuit is broken. After the "BOOM" the circuit is
still broken.
Actually there is no "BOOM" the back biased LEDs will fail shorted and the
series RC will still limit the current.
--
--
kensmith@rahul.net forging knowledge |
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Ken Smith
Guest
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| Posted:
Fri Mar 18, 2005 4:37 am Post subject:
Re: Very low power, power supply |
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In article <113jjfdt40lbi1b@corp.supernews.com>,
Watson A.Name - \"Watt Sun, the Dark Remover\" <alondra101@hotmail.com> wrote:
[...]
| Quote: | The overcurrent isn't the problem, it's the flicker. Most white LEDs
are rated for 30mA max, 100mA peak. But the flicker is really
noticeable especially 60 Hz half wave.
|
If you use two strings anti-paralleled, the flicker is very hard to
detect. The frequency is 120Hz and most of the time there is light
produced. The eye detects spikes in light better than dips.
--
--
kensmith@rahul.net forging knowledge |
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Ken Smith
Guest
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| Posted:
Fri Mar 18, 2005 3:06 pm Post subject:
Re: Very low power, power supply |
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In article <e2B_d.15$HE2.585823@news.salzburg-online.at>,
Wolfgang Mahringer <yeti201@gmx.at> wrote:
| Quote: | 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.
| Quote: | 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".
| Quote: |
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 |
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Wolfgang Mahringer
Guest
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| Posted:
Fri Mar 18, 2005 8:13 pm Post subject:
Re: Very low power, power supply |
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Hi Ken,
Ken Smith schrieb:
| Quote: |
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! |
| Quote: | After the "BOOM" the circuit is
still broken.
Using your circuit, there will be no LEDs that are still working. |
| Quote: | Actually there is no "BOOM" the back biased LEDs will fail shorted
How can you tell that? Any sources? |
| Quote: | 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.
Regards,
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:
Fri Mar 18, 2005 9:46 pm Post subject:
Re: Very low power, power supply |
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"Ken Smith" <kensmith@green.rahul.net> wrote in message
news:d1eqq7$c2k$2@blue.rahul.net...
[snip]
| Quote: |
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]
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Michael
Guest
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| Posted:
Fri Mar 18, 2005 10:28 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.
--
Chris W
|
I understand that to wish to use LEDs. You probably have reasons for choosing
that particular light source, and I'm not going to try to convince you
otherwise. But have you considered electrolumenescent strip? They (some of
them?) can connect directly to the mains. They come in a variety of shapes and
sizes (sq. inches/cm), come in several colors including white, draw very little
current, are long lived, and are available on the surplus market at prices that
are competitive with - or less than - a white LED solution. |
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Rich Grise
Guest
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| Posted:
Sat Mar 19, 2005 12:11 am Post subject:
Re: Very low power, power supply |
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On Fri, 18 Mar 2005 15:06:47 +0000, Ken Smith wrote:
| Quote: | 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.
|
That depends on how "broken" your bank account is. :-) Now, if it's only
toasting one _side_ of the bread, then it's time to salvage it. :-)
Cheers!
Rich |
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John Woodgate
Guest
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| Posted:
Sat Mar 19, 2005 2:47 am Post subject:
Re: Very low power, power supply |
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I read in sci.electronics.design that Rich Grise <richgrise@example.net>
wrote (in <pan.2005.03.18.18.11.47.749984@example.net>) about 'Very low
power, power supply', on Fri, 18 Mar 2005:
| Quote: | That depends on how "broken" your bank account is. :-) Now, if it's only
toasting one _side_ of the bread, then it's time to salvage it. :-)
|
Nah! You can always repair the broken heater coil with a bit of Al foil.
(;-)
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk |
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Ken Smith
Guest
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| Posted:
Sat Mar 19, 2005 3:01 am Post subject:
Re: Very low power, power supply |
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In article <113ltpqed6mnm93@corp.supernews.com>,
Watson A.Name - \"Watt Sun, the Dark Remover\" <alondra101@hotmail.com> wrote:
[...]
| Quote: | 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]
|
As it gets hotter, the voltage decreases.
--
--
kensmith@rahul.net forging knowledge |
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Michael A. Terrell
Guest
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| Posted:
Sat Mar 19, 2005 4:15 am Post subject:
Re: Very low power, power supply |
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John Woodgate wrote:
| Quote: |
Nah! You can always repair the broken heater coil with a bit of Al foil.
(;-)
--
Regards, John Woodgate
|
Or toast one slice at a time.
--
?
Michael A. Terrell
Central Florida |
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Chris W
Guest
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| Posted:
Sat Mar 19, 2005 6:10 am Post subject:
Re: Very low power, power supply |
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Michael wrote:
| Quote: | I understand that to wish to use LEDs. You probably have reasons for choosing
that particular light source, and I'm not going to try to convince you
otherwise. But have you considered electrolumenescent strip? They (some of
them?) can connect directly to the mains. They come in a variety of shapes and
sizes (sq. inches/cm), come in several colors including white, draw very little
current, are long lived, and are available on the surplus market at prices that
are competitive with - or less than - a white LED solution.
|
Someone else mentioned that but form the little looking I did, it sounds
like I wouldn't get near as much light as I would from 6 white LEDs.
From what I understand, at the 60hz signal I plan on connecting to,
this stuff isn't very bright. Can you tell me about how much of it I
would need to equal 6 white LEDs? I didn't pick white because of
brightness, I do not want a colored light.
--
Chris W
Gift Giving Made Easy
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