Searching \ for '[EE] White LEDs on mains' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/displays.htm?key=leds
Search entire site for: 'White LEDs on mains'.

Exact match. Not showing close matches.
PICList Thread
'[EE] White LEDs on mains'
2008\08\23@201333 by Jinx

face picon face
A few weeks ago someone mentioned seeing white LEDs powered
by mains. I'd seen it too but couldn't remember where

Found this in one of my pdf stashes when looking for something else

http://home.clear.net.nz/pages/joecolquitt/acwhiteleds.pdf

11kB

2008\08\24@074854 by Rolf

face picon face
Jinx wrote:
> A few weeks ago someone mentioned seeing white LEDs powered
> by mains. I'd seen it too but couldn't remember where
>
> Found this in one of my pdf stashes when looking for something else
>
> http://home.clear.net.nz/pages/joecolquitt/acwhiteleds.pdf
>
> 11kB
>
>  
Why have the 'bridges' between the parallel strings of LED's? I'm
talking about how the nodes between the LED's that conduct on one cycle
of AC are connected to the nodes that conduct on the other cycle...

What purpose do they serve. I initially thought it could serve as a
mechanism to keep the system running in the event of a 'fail open' LED,
but no, that would not help...

Rolf

2008\08\24@084035 by Jinx

face picon face
> Why have the 'bridges' between the parallel strings of LED's?

Don't know. The LEDs won't be conducting half the time. It would
help with assembly a bit I guess, as you'd solder two LEDs in anti-
parallel and then join them to the next pair. Maybe ?

Irms = 6fCV for the 0.47uF cap gives 15mA. 20mA would be more
like 0.62uF for 120V/60Hz and 29uF for 230V/50Hz

2008\08\24@085112 by Jinx

face picon face
> 29uF for 230V/50Hz

How many ? 0.29uF


2008\08\24@114526 by Tony Smith
flavicon
face
> > Why have the 'bridges' between the parallel strings of LED's?
>
> Don't know. The LEDs won't be conducting half the time. It
> would help with assembly a bit I guess, as you'd solder two
> LEDs in anti- parallel and then join them to the next pair. Maybe ?


Has any ever made a bridge out of LEDs?  It rectifies and lights up!

I've seen LEDs run off mains simply by using a resistor.  The last one I saw
was in one of those cheap main filter things.  Not the best for a lamp, but
it seemed to work ok as an indicator, if the layer of dust was any guide to
its age.

Usually it's a physically large resistor, either because dropping 240v
produces a few watts or to give a large gap for isolation in case something
goes wrong (sure!), but sometimes 2 or 3 small wattage ones in series.  The
series method is for the really cheap stuff.

Tony

2008\08\24@144904 by McReynolds, Alan A

picon face
Jinx wrote:
> A few weeks ago someone mentioned seeing white LEDs powered
> by mains. I'd seen it too but couldn't remember where
>
> Found this in one of my pdf stashes when looking for something else
>
> http://home.clear.net.nz/pages/joecolquitt/acwhiteleds.pdf
>
> 11kB
>
>

RE the PDF design:

Why use back-to-back LEDs? One can simply use a full wave rectifier and then use only one direction of LEDs.

Second, since each LED has a significant voltage drop, the voltage drop of the string could be quite large.  This reduces the drop across the resistor.  A 1K resistor can't be optimal for a single LED *and* a string of 12?

I've built some "fireflies in a bottle" decorations.  These operate on 120V using a full wave rectifier, fuse and ~500ohm resistor.  The resulting rectified DC current feeds a series of 55 yellow LED's.  These have proved to be very durable.

...Alan

2008\08\24@145350 by Sean Breheny

face picon face
When running LEDs from the mains, I would be wary of the LED's
peak-reverse-voltage rating (typically only 5 or 10V). I *think* that
if you exceed that you simply cause avalanche breakdown (which, as
long as the resistor is there to limit current is probably OK) but
technically you are greatly exceeding the ratings of the LED. You
should use a silicon diode (like 1N4004) in series with the LED to
prevent it from seeing a couple hundred volts across it backwards.

Also, I'd want to use more than one current limiting resistor in
series. That way, in the unlikely event that one fails shorted, at
least the others are there to limit the current.

Sean


On Sun, Aug 24, 2008 at 11:44 AM, Tony Smith <spam_OUTajsmithTakeThisOuTspambeagle.com.au> wrote:
{Quote hidden}

> -

2008\08\24@153228 by Mike Harrison

flavicon
face
On Sun, 24 Aug 2008 14:53:25 -0400, you wrote:

>When running LEDs from the mains, I would be wary of the LED's
>peak-reverse-voltage rating (typically only 5 or 10V). I *think* that
>if you exceed that you simply cause avalanche breakdown (which, as
>long as the resistor is there to limit current is probably OK) but
>technically you are greatly exceeding the ratings of the LED. You
>should use a silicon diode (like 1N4004) in series with the LED to
>prevent it from seeing a couple hundred volts across it backwards.
>
>Also, I'd want to use more than one current limiting resistor in
>series. That way, in the unlikely event that one fails shorted, at
>least the others are there to limit the current.
>

Resistors don't fail short - even the standards for Intrinsically safe equipment for use in
flammable atmospheres state that resistors can be assumed to be infallible as regards failure to
lower values.

however using multiples in series helps spread heat load.



{Quote hidden}

>> --

2008\08\24@171310 by Wouter van Ooijen

face picon face
Sean Breheny wrote:
> You
> should use a silicon diode (like 1N4004) in series with the LED to
> prevent it from seeing a couple hundred volts across it backwards.

That would work only if the leakage of the LED is (much) higher than
that of the diode. To avoid that (potential?) problem I would put the
diode across the LED, not in series.

--

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu

2008\08\24@172046 by Jinx

face picon face
> When running LEDs from the mains, I would be wary of the LED's
> peak-reverse-voltage rating (typically only 5 or 10V)

One of my earliest discoveries about LEDs was a little circuit for
running them on 230V. 0.47uF and 1k in series, with a 1N4148
anti-parallel across the LED. I used a yellow LED, and it's still
there after mumble-mumble-mumble years, as an illuminator for
the counter on a reel-to-reel tape deck

2008\08\24@173128 by Jinx

face picon face
> I would put the diode across the LED, not in series

Similar to adding an anti-parallel 1N4148, as in another post

Then the configuration in that pdf could be looked at as a series of
anti-parallel pairs rather than two strings mashed together

2008\08\24@182130 by Sean Breheny

face picon face
Hi Mike,

Well, I can think of some design mistakes which could lead to a
resistor failing with a lower resistance than nominal. For example,
some small resistors have a maximum working voltage lower than what
you would expect based on power dissipation (200 to 250V for 1/4W thru
hole carbon film resistors). If you exceed this, I would imagine that
you could get arcing across part or all of the resistor. Also, if the
resistor overheats and burns material around it, any carbon-containing
material around it might be able to conduct once it burns.

Yes, those are a bit of a stretch, so I suppose that you are correct.

Sean

On Sun, Aug 24, 2008 at 3:30 PM, Mike Harrison <mikespamKILLspamwhitewing.co.uk> wrote:
{Quote hidden}

2008\08\24@182702 by Sean Breheny

face picon face
Hi Wouter,

I thought about that but decided that putting the diode anti-parallel
to the LED would double the power dissipation of the current limiting
resistor.  Also, the series diode would limit the inverse current
through the LED to (at maximum) the reverse leakage current of the
diode. I think it would be very difficult to damage the LED with such
a tiny current. I suppose what you say would be safer and if resistor
dissipation were an issue, TWO silicon diodes could be used, one
anti-parallel to the LED and one in series with the LED/diode
combination. This would guarantee that the reverse voltage on the LED
would be never more than 0.6V but there would still be negligible
current during the "off" part of the AC cycle.

Sean


On Sun, Aug 24, 2008 at 5:11 PM, Wouter van Ooijen <.....wouterKILLspamspam.....voti.nl> wrote:
{Quote hidden}

> -

2008\08\24@210708 by Xiaofan Chen

face picon face
On Mon, Aug 25, 2008 at 3:30 AM, Mike Harrison <EraseMEmikespam_OUTspamTakeThisOuTwhitewing.co.uk> wrote:

> Resistors don't fail short - even the standards for Intrinsically safe
> equipment for use in flammable atmospheres state that resistors
> can be assumed to be infallible as regards failure to
> lower values.

Not all resistors are equal. For IS (intrinsic safety) application, what you
say can only apply to certain type of resistors (eg: metal film and wire-wound).

Xiaofan

2008\08\25@105442 by Harold Hallikainen

face
flavicon
face

> On Mon, Aug 25, 2008 at 3:30 AM, Mike Harrison <mikespamspam_OUTwhitewing.co.uk>
> wrote:
>
>> Resistors don't fail short - even the standards for Intrinsically safe
>> equipment for use in flammable atmospheres state that resistors
>> can be assumed to be infallible as regards failure to
>> lower values.
>
> Not all resistors are equal. For IS (intrinsic safety) application, what
> you
> say can only apply to certain type of resistors (eg: metal film and
> wire-wound).
>
> Xiaofan


I believe carbon composition resistors fail to a short. Film and wire
wound fail to an open. When I started in electronics, carbon composition
resistors were the only ones around. They had a cylindrical body. I
remember seeing my first carbon film resistor with the now familiar "dog
bone" shape and thought they were quite strange.

I haven't been following this thread closely, but have seen stuff about
protecting the LEDs from reverse voltage. As I recall from the original
schematic on the thread, short strings of LEDs were connected in parallel
with polarity reversed. That handles the reverse polarity protection and
also allows AC current to flow so the current limit capacitor does not
just charge up to the peak voltage and have everything stop.

Harold


--
FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
opportunities available!

More... (looser matching)
- Last day of these posts
- In 2008 , 2009 only
- Today
- New search...