Novice LED questions

[Randy Day]

biscuitbecks@cableone.net wrote:

Hello all,

so I am looking at the back of a package of common LEDs (yes, from
Radio Shack, I have my asbestos flame suit on , and it shows a
schematic for illuminating LEDs in what I perceive to be a parallel
configuration. The schematic demonstrates 2 LEDs, with the anodes and
cathodes in OPPOSITE configuration.

1. Why does it have to be that way, rather than anodes and cathodes
in register?

Let's say you put four 20ma LED's in parallel,
cc-aa, with a resistor to limit the current to
80ma.

The LED with the lowest forward voltage conducts
first, directing all 80ma through it. If/when it
blows, the second diode conducts all the current,
and so on, in a cascade failure.

That's why each LED (or series string) gets its
own limiting resistor.

In an opposite configuration, only one LED can
light at one time no matter what polarity of
voltage is applied.

The schematic you describe sounds like a two-color
LED; one lead goes positive, the red LED lights.
Reverse the polarity, the green LED lights. They
come in various colors.

2. If I were to add subsequent LEDs to the circuit would I have to
obey the opposite configuration?

I'm not sure what you mean here. If you mean you want
to light several of these at once, just hook them in
series with a resistor as you would normally. You can
then reverse the polarity to light the string the
opposite way.

This assumes I'm correct about the devices you're
describing...

 

[Michael Black]

(biscuitbecks@cableone.net) writes:

Hello all,

so I am looking at the back of a package of common LEDs (yes, from
Radio Shack, I have my asbestos flame suit on , and it shows a
schematic for illuminating LEDs in what I perceive to be a parallel
configuration. The schematic demonstrates 2 LEDs, with the anodes and
cathodes in OPPOSITE configuration.

1. Why does it have to be that way, rather than anodes and cathodes
in register?

They aren't merely in parallel. They are arranged so if the voltage is

applied in one direction, one LED will light up, and if the voltage is
applied in the other direction, the other LED will light up. Useful
if you want to determine polarity of a DC voltage. Apply an AC voltage,
and both will appear to light up simultaneously, but in reality they
will be each lighting for half of the time depending on which side
of zero volts is applied to the LEDs at a given time.

Michael

2. If I were to add subsequent LEDs to the circuit would I have to
obey the opposite configuration?

Thank you in advance for any and all responses.

Regards,
Dan

 

Hello all,

so I am looking at the back of a package of common LEDs (yes, from
Radio Shack, I have my asbestos flame suit on , and it shows a
schematic for illuminating LEDs in what I perceive to be a parallel
configuration. The schematic demonstrates 2 LEDs, with the anodes and
cathodes in OPPOSITE configuration.

1. Why does it have to be that way, rather than anodes and cathodes
in register?

2. If I were to add subsequent LEDs to the circuit would I have to
obey the opposite configuration?

Thank you in advance for any and all responses.

Regards,
Dan