12vac-12vdc

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L. Daniel Rosa
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Re: 12vac-12vdc

Post by L. Daniel Rosa » Fri Mar 18, 2005 1:43 pm

I only scanned through the posts above, so this may have been answered... Luxeon LEDs by Lumileds are available in 1W and 5W power ratings.<p>Kinda spendy, those. It will probably be worth your while to regulate the power to your LEDs, for longevity's sake. Take a look at http://www.romanblack.com/ for a few variations on a parts-minimal switching supply.

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sofaspud
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Re: 12vac-12vdc

Post by sofaspud » Fri Mar 18, 2005 2:15 pm

OK... I've learned something. There are some high-power LED specs on this page http://www.activeelectronics.com.au/pro ... uxeon.html that include some rated at 5W and some specified for for both 700mA and 1000mA operation.

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MrAl
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Re: 12vac-12vdc

Post by MrAl » Fri Mar 18, 2005 3:13 pm

Hello ILO and others,<p>You're best bet is to use full wave rectification
using four diodes in a bridge configuration.
As someone else said, you dont need filtering
if you lower the average current just a tad.
For example, with three 3.4v LED's in series
also in series with a 6 ohm resistor you'll get
about 300ma through each LED. The reason for
the slightly lower current (300ma instead of
350ma) is too allow for the peak current that
will develop when the sine goes through 90 degrees, and the peak current rating of 500ma of
a typical Luxeon 350ma white LED. With 300ma
average current the peak will be about 470ma,
which is within the peak current spec. With
350ma average current, the peak will be above
the spec, hence the set current should be slightly
lower than the rating if you're not using any
filtering.<p>Whatever you do end up doing, dont use Philba's
idea of using two LED's wired in antiparallel
(as per his drawings) because that will blow
both LED's out due to high reverse voltage.
What MIGHT work is to use ONE resistor in series
with BOTH led's, where the led's are wired in
antiparallel to each other, but i dont like
doing it that way either because it's best not
to apply a reverse voltage to an LED.
The circuit shown (by Philba) with the three
LED's (you would use two or three) in antiparallel
with three more LED's also SEEMS like a good idea
at first, but whether or not the LED's blow out
depends on how well each LED shares the reverse
voltage, which is not something you can depend on,
so it's better not to run LED's in antiparallel
if there is any other choice available.<p>You'll wire either two or three (depending on the
voltage rating of the LED) in series with a single
dropping resistor for the simplest solution.
If you'd like to use an LM317 (or similar) device
for current limiting that's fine too.<p>Philba:
Im sorry to have to say that those circuits will
be very bad for the LED's, but im sure you dont
want to see anyone blow out LED's that might
cost 5 to 10 dollars (or more) each.
Reverse voltage on LED's is a killer, and you can
read the manufacturers specs. A few of them say
"5 volts max" but even that is not recommened.
I accidentally connected one LED up to 9 volts
backwards and consider myself lucky it didnt blow
out, but have no way to access the damage that
may have occurred that i cant see.
Again i appoligize for bringing the bad news,
but there are other ways of doing this that
dont risk anything.<p>
Take care,
Al
LEDs vs Bulbs, LEDs are winning.

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philba
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Re: 12vac-12vdc

Post by philba » Fri Mar 18, 2005 4:30 pm

So far, I haven't heard any reasoning beyond any reverse voltage is bad. I think you are saying the Vmax reverse as spec'd is not to be believed. If what you say is true, why don't they spec zero volts reverse?? its a good idea to be conservative with respects to spec'd maxes.<p>Also, you say that the reverse voltage will be high. Nope. it will be the Vf of the other diode - dropping resistor doing its thing. like I said, do make sure that is below Vmax reverse.<p>What makes the 2 lead bicolor LEDs different? How come they don't fail quickly?<p>I'd like to understand your theory of why this doesn't work.

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sofaspud
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Re: 12vac-12vdc

Post by sofaspud » Fri Mar 18, 2005 5:14 pm

I'm gonna step in here again...
philba, MrAl didn't say the Vmax reverse isn't to be believed. He said it wasn't recommended. Also, I just went to check the Luxeon III datasheet. That they don't even give a Vmax reverse rating tells me something. Your parallel circuit would probably work. But if one LED fails, the other LED will also. For that rea$on alone I'd stick with the tried and true method of a DC source driving a resistor/LED series combo.
I begging your pardon. It's just that it seems that 9 times out of 10, a nuts & volts question that needs a nuts & volts answer becomes a treatise on conceptual physics. I am interested in how 2 colors are obtained from a 2-lead LED.

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Chris Smith
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Re: 12vac-12vdc

Post by Chris Smith » Fri Mar 18, 2005 6:53 pm

In a bi colored LED the current/voltage takes the easy route, and thus the reverse current or voltage doesn’t damage the reverse biased LED.<p>However, piercing any diode can occur if the reverse voltage is sufficiently high, and even if it is short in duration. The same goes for plain diodes although they are far more resilient.<p>While still other diodes, can operate completely in the reverse reigon such as Zeners, and common mode break down cycle usage like a 123/222 used in reverse, to cause “Bursting” for timing cycles and high currents for brief periods of time. [NS]

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sofaspud
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Re: 12vac-12vdc

Post by sofaspud » Fri Mar 18, 2005 7:15 pm

So the 2 leads are each both anode and cathode.
I can buy that.
It also seems that that would require 2 DC sources, one + to - and one - to +. AC won't cut it.

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Re: 12vac-12vdc

Post by terri » Fri Mar 18, 2005 8:25 pm

Very interesting re the illuminators. (Some time passed between reading philba's suggetion about e-bay and this post.) But I think I'm gonna 'sperimink with some Wratten filters ( 87, 87C, and 88A ) over various things like a pressing iron, a floodlamp dimmed down to a dull glow, or maybe even a hair dryer heating element I keep around as a high power variable resistor. All Variac(tm)-controlled, of course.<p>And it just dawned on me to try the IR illumination from a yard light motion detector and see how effective that is.<p>Hey! I did it! No typpos!<p>[ March 18, 2005: Message edited by: terri ]</p>
terri wd0edw

L. Daniel Rosa
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Re: 12vac-12vdc

Post by L. Daniel Rosa » Sat Mar 19, 2005 9:37 am

sofaspud, to use a two lead bicolor LED for yellow (acuually orangish) you need to pulse each direction fast enough that the human eye does not detect the flicker- this qualifies as AC. However what is no problem for die chemistries of LED, the film types are less tolerant of. That is the trade-off for higher power and efficiency I suppose. Of all the supposedly 'static sensitive' components I've mishandled (including a microcontroller I installed backwards), the only things I've ever blown (accidentally, anyways) are white LEDs.

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MrAl
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Re: 12vac-12vdc

Post by MrAl » Sat Mar 19, 2005 9:51 am

Hi again Philba and others,<p>
Let's look at circuit "A"...<p>When LED2 conducts R1 drops the excess voltage, leaving
about 3.4 volts across LED2 and all is well...or is it?
At the peak of the sine with a 12vac input you get 17
volts, which also appears across R2 and LED1. Problem is,
R2 isnt dropping any voltage (only R1 is) so the full
17 volts appears across LED1. When the cycle reverses,
R2 drops voltage and R1 doesnt, so LED2 sees the full
17 volts of reverse voltage.
Tie LED1 and LED2 directly together and use one resistor
and it's a different story. When the resistor drops
voltage the conducting LED sees 3.4 volts while the
non-conducting LED sees a reverse 3.4 volts. This
situation is a little better because the reverse voltage
is limited to below 5 volts, but that's not the end of
the story as there is still a problem with the current
level (below). Note also that some manufacturers state
that their LED's shouldnt be run with any reverse voltage
even though there is a spec for it on their data sheet.<p>Now let's look at circuit "B"...<p>When LED3 and LED4 conduct R3 drops voltage, while R4 doesnt,
meaning LED5 and LED6 see the full 17 volts peak. Now if they
have the exact same reverse characteristics each one will drop
1/2 of the 17 volts which comes out to 8.5 volts each, which is
too high already. If the reverse characteristics are not the
same (and this characteristic isnt specified on any data sheets
so it could be very different for each LED) one LED could get
a much higher reverse voltage across it which is going to blow
it out.<p>Ok, we've looked at the reverse voltage across each LED during
normal operation, now let's look at forward current...<p>Looking back at circuit "A", each LED conducts for only 1/2
the cycle, meaning the ratio of peak current to average current
is more than 3 to 1. This means that the peak current will be
more than 3 times higher than the average current. Since the
average current determines the brightness (and we want that as
high as we can get that to make full use of the LED spec) and
the peak is 3.14 times the average, an LED with a peak current
rating of 500ma (typical 1w white Luxeon) will have to be
operated at 159ma average current. Since this is less than half
of the average current rating (350ma) we'll only be using the
LED to half it's allowable current rating which will result
in only (about) half the light output.
If, on the other hand, we were to drive the LED with a bridge
rectifier with no filtering, we would see a peak current of
1.57 times the average, meaning we could drive the LED's closer
to spec (318ma average) which will result in almost 100 percent
light output.
In circuit "B" the same problem exists, and to help the reverse
voltage situation each pair of LED's should be tied together and
use only one resistor.<p>Yes, 2 color LED's with two leads have two LED's connected in
antiparallel inside, but the operating current problem still
applies. Also, they arent normally used for illumination where you
want to drive the LED's to full spec to get maximum light output
so the operating current problem doesnt hurt as much.<p>
So in the end there are two problems with driving LED's in antiparallel
from an ac source:
1. Reverse voltage could be too high for the device.
2. Peak current to average current ratio will be too high meaning the
device cant be operated at it's full potential.<p>Take care,
Al
LEDs vs Bulbs, LEDs are winning.

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Re: 12vac-12vdc

Post by peter-f » Sat Mar 19, 2005 4:52 pm

you may find this a stupid question from an amateur... but,
resistor + LED... any difference what order they're in?<p>
[other than 'working'?]

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sofaspud
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Re: 12vac-12vdc

Post by sofaspud » Sat Mar 19, 2005 7:54 pm

Before I read MrAl's analysis, I just want to remind folks that if AC is applied to obtain that yellow (orangish) color, it isn't a bicolor LED. You only get one color because the two have mixed.

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sofaspud
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Re: 12vac-12vdc

Post by sofaspud » Sat Mar 19, 2005 8:32 pm

Makes sense to me, MrAl. Interesting info!<p>peter-f, it's probably preferable to put the resistor on the positive side with the cathode of the LED at ground, but it isn't likely much of an issue.

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