On Wed, 8 Nov 2006 18:57:55 -0700, sylvan butler
wrote:
[i]
On Wed, 08 Nov 2006 17:26:09 -0500, Stubby wrote:
sylvan butler wrote:
On Tue, 07 Nov 2006 17:23:08 -0500, Stubby wrote:
Why do you need the diode?
So he could tell how the set of lights was wired.
Is that a question or an unclear statement????
It ends with a period, therefore it is a statement. As for it being
unclear, well, I guess it is since you say it is. To me it seems like a
reasonable answer to your query...
Q: "Why do you need..."
A: "So can tell..."
I'm saying that 70 LEDs in series, all pointing the same way, will not
benefit from having one additional 1N4003 in series with them.
I understood you the first time. And as I said, by using the diode, he
was able to tell that the LEDs WERE NOT "all pointing the same way," to
wit:
Mark Lloyd wrote:
Today I tried connecting some 70-LED light strings to DC (using a
single 1N4003 diode). Half the string would light. Changing the
polarity made the other half light.
Thus we see that using the diode to remove half of the A.C. wave from
the string prevented half of the LEDs from lighting, which effect was
reversed by reversing the polarity to the string. Therefore we know
that the string of LEDs is actually wired as two anti-parallel strings.
Of course, if you were making a string of LEDs intended to run from
A.C., you might also wish to use a series diode in addition to the LEDs
themselves. Why? Because a normal rectifier diode has a much greater
tolerance for inverse or reverse voltage than does a normal LED. If the
peak-inverse-voltage (PIV) specification of a diode is exceeded, the
diode will usually be destroyed.
I once thought so. It's not true. BTW, if it was, it would make it
risky to test LEDs of unknown polarity.
Reverse biasing will not hurt a diode (LED) as long as the proper
series resistor is present. It limits current to a safe value.
Note that exceeding the PIV causes "breakdown" which just means the
diode starts conducting. It is not damaged unless the current becomes
excessive. Zener diodes are operated this way (in breakdown) all the
time. Right now, I have a device (part of a holiday light controller)
that has 2 LEDs on RS232 outputs. these switch polarities (+/-), so
the LEDs are reverse-biased half the time. They haven't been harmed
yet (during about 60 hours of operation).
Some diodes are made to tolerate this
effect, but the LEDs I know of are not intended for such operation.
sdb
--
47 days until the winter solstice celebration
Mark Lloyd
http://notstupid.laughingsquid.com
"God was invented by man for a reason, that
reason is no longer applicable."