LED

T

terryS

Guest
1) An LED is diode; right? So that it conducts in one direction only?

2) LEDs are normally operated by DC, from a DC supply or battery such
as in an LED flashlight?

3) What happens if I apply AC? Keeping in mind that the AC will have
peak voltage 1.4 times that of a DC supply the AC supply could be at a
lower RMS voltage?

4) So, for example if the DC in item (2) was at 12 volts with
suitable dropping resistor etc. the AC could be at around 8.5 volts
RMS, right? But in the reverse or nonconducting state of the LED that
would result in 12 volts peak (backwards) across the LED.

5) However if the reverse voltage was limited to to no more than the
forward voltage of the LED would that be OK?

6) No particlar application at moment; just curious!
 
"terryS" <tsanford@nf.sympatico.ca> wrote in message
news:aa5a3d02-67ab-4f15-a050-dd165e720afa@41g2000yqf.googlegroups.com...
1) An LED is diode; right? So that it conducts in one direction only?

2) LEDs are normally operated by DC, from a DC supply or battery such
as in an LED flashlight?

3) What happens if I apply AC? Keeping in mind that the AC will have
peak voltage 1.4 times that of a DC supply the AC supply could be at a
lower RMS voltage?

4) So, for example if the DC in item (2) was at 12 volts with
suitable dropping resistor etc. the AC could be at around 8.5 volts
RMS, right? But in the reverse or nonconducting state of the LED that
would result in 12 volts peak (backwards) across the LED.

5) However if the reverse voltage was limited to to no more than the
forward voltage of the LED would that be OK?

6) No particlar application at moment; just curious!
Click to expand...
The LED is indeed a diode.
The reverse peak voltage it can tolerate is not what a diode designed to
rectify could handle.
Depending on the LED there is no guaranty that limiting the revese to the
forward voltage would be OK.
So you could exceed the reverse peak of the LED and burn it out.
Why don't you look at some specs of LEDs?

Tom
 
terryS wrote:
1) An LED is diode; right? So that it conducts in one direction only?

2) LEDs are normally operated by DC, from a DC supply or battery such
as in an LED flashlight?

3) What happens if I apply AC? Keeping in mind that the AC will have
peak voltage 1.4 times that of a DC supply the AC supply could be at a
lower RMS voltage?

4) So, for example if the DC in item (2) was at 12 volts with
suitable dropping resistor etc. the AC could be at around 8.5 volts
RMS, right? But in the reverse or nonconducting state of the LED that
would result in 12 volts peak (backwards) across the LED.

5) However if the reverse voltage was limited to to no more than the
forward voltage of the LED would that be OK?

6) No particlar application at moment; just curious!
If the AC is more tahn a few volts, you will blow the LED.
Click to expand...
The reverse allowed voltage is quite low, but one of the
tricks for using AC is to use two LEDS parallel, but one of them
in the reversed direction.
Each will conduct for half a cycle, and protect the other LED.
 
"Sjouke Burry" <burrynulnulfour@ppllaanneett.nnll> wrote in message
news:49bc6231$0$1645$703f8584@textnews.kpn.nl...
terryS wrote:
1) An LED is diode; right? So that it conducts in one direction only?

2) LEDs are normally operated by DC, from a DC supply or battery such
as in an LED flashlight?

3) What happens if I apply AC? Keeping in mind that the AC will have
peak voltage 1.4 times that of a DC supply the AC supply could be at a
lower RMS voltage?

4) So, for example if the DC in item (2) was at 12 volts with
suitable dropping resistor etc. the AC could be at around 8.5 volts
RMS, right? But in the reverse or nonconducting state of the LED that
would result in 12 volts peak (backwards) across the LED.

5) However if the reverse voltage was limited to to no more than the
forward voltage of the LED would that be OK?

6) No particlar application at moment; just curious!
If the AC is more tahn a few volts, you will blow the LED.
The reverse allowed voltage is quite low, but one of the
tricks for using AC is to use two LEDS parallel, but one of them
in the reversed direction.
Each will conduct for half a cycle, and protect the other LED.
Click to expand...
Or just a normal diode in the reverse direction has the same effect.

John G.
 
"Sjouke Burry" <burrynulnulfour@ppllaanneett.nnll> wrote in message
news:49bc6231$0$1645$703f8584@textnews.kpn.nl...
terryS wrote:
1) An LED is diode; right? So that it conducts in one direction only?

2) LEDs are normally operated by DC, from a DC supply or battery such
as in an LED flashlight?

3) What happens if I apply AC? Keeping in mind that the AC will have
peak voltage 1.4 times that of a DC supply the AC supply could be at a
lower RMS voltage?

4) So, for example if the DC in item (2) was at 12 volts with
suitable dropping resistor etc. the AC could be at around 8.5 volts
RMS, right? But in the reverse or nonconducting state of the LED that
would result in 12 volts peak (backwards) across the LED.

5) However if the reverse voltage was limited to to no more than the
forward voltage of the LED would that be OK?

6) No particlar application at moment; just curious!

If the AC is more tahn a few volts, you will blow the LED.
The reverse allowed voltage is quite low, but one of the
tricks for using AC is to use two LEDS parallel, but one of them
in the reversed direction.
Each will conduct for half a cycle, and protect the other LED.
Click to expand...
I think most LEDs will tolerate about 5 VDC in reverse, and probably will
not blow out unless the current is not limited to the usual forward
current. There are also LEDs with built-in antiparallel diodes, sometimes
one red and one green, so if DC is applied it will tell you the polarity,
and with AC it will light yellow.

Paul
 
On 2009-03-15, terryS <tsanford@nf.sympatico.ca> wrote:
1) An LED is diode; right? So that it conducts in one direction only?
Click to expand...
it conducts better in one direction, forcing it to conduct in the other
direction will damage it.

2) LEDs are normally operated by DC, from a DC supply or battery such
as in an LED flashlight?
Click to expand...
yeah, continuous, or pulsed, DC is the norm.

3) What happens if I apply AC? Keeping in mind that the AC will have
peak voltage 1.4 times that of a DC supply the AC supply could be at a
lower RMS voltage?
Click to expand...
the led will blink really fast :)

4) So, for example if the DC in item (2) was at 12 volts with
suitable dropping resistor etc. the AC could be at around 8.5 volts
RMS, right? But in the reverse or nonconducting state of the LED that
would result in 12 volts peak (backwards) across the LED.
Click to expand...
not good for many leds.

5) However if the reverse voltage was limited to to no more than the
forward voltage of the LED would that be OK?
Click to expand...
most LED datasheets quote 5V as maximum reverse voltage, and
forwards voltage will be less than that, so yes that would be OK.

they make some 2-colour LEDs like that, with a red and a greed led in
antiparallel,
connected one way it lights red, the other way green, and with ac you
get both red and green and the combination appears yellow,
 
On Mar 15, 2:00 am, terryS <tsanf...@nf.sympatico.ca> wrote:

[...]
3) What happens if I apply AC?
Click to expand...
I tried this 3 years ago and made a movie. Have a look:

http://www.midgard.liu.se/~n96rikbo/SED_-_The_Movie_(XviD).avi
 
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