Can a diode light up when it's connected backwards?

[Femme_physics]

http://img560.imageshack.us/img560/453/14led.jpg

Trying to find the answer to the question if a regular basic LED diode can light up when it's connected backwards? I even made a lil' sketch

 

[Averagesupernova]

The only way that LED will light up is if the battery voltage exceeds the breakdown voltage of the LED and it destroys itself in a bright flash. :)
-
The short answer to your question would be NO!

 

[cepheid]

Just to complement what was already said: the whole point of a diode is that it is a device that conducts current in only one direction and not the other.

 

[AlephZero]

Actually the question of whether you could physically make a light emitting zener diode is interesting (and I don't know the answer).

But conventioal diodes usually self-destruct very fast if you try to operate them in the Zener region, because the Zener voltage is high (typically hundreds or thousands of volts) so the heat generated in the diode (volts x amps) when the Zener current starts to flow is also high.

So the answer is "no" for any "mass produced" LED.

 

[mdjensen22]

Reverse biasing LEDs isn't all bad...
a reverse biased LED becomes a photo-diode/light-dependent capacitor.
http://www.edn.com/contents/images/81601di.pdf
http://cs.nyu.edu/~jhan/ledtouch/index.html
http://forums.parallax.com/showthread.php?86035-LED-Touch-Sensor

 

[Carl Pugh]

Well I had one that did, but I think that the manufacture connected the LED backwards when it was manufactured.

 

[Bob S]

Some red LEDs will light up green when connected backwards. There are actually two diodes in the package, wired back to back. See http://www.radioshack.com/product/index.jsp?productId=2062549

Bob S

 

[cepheid]

Some red LEDs will light up green when connected backwards. There are actually two diodes in the package, wired back to back. See http://www.radioshack.com/product/index.jsp?productId=2062549

Bob S

But this can be regarded as a set of two LEDs wired in "inverse parallel" (i.e. cathode-to-anode and anode-to-cathode) correct?

Either way, whichever one is active is forward-biased

 

[Femme_physics]

So if I have this decoder ->

http://img33.imageshack.us/img33/5601/decoderd.jpg

Current can only flow backwards? I thought the point of a decoder is that signals flow based on the signals defined in A0, A1, and A2. But in this case no signal can be received because the LEDS are blocking the way

 

[I like Serena]

So if I have this decoder ->

http://img33.imageshack.us/img33/5601/decoderd.jpg

Current can only flow backwards? I thought the point of a decoder is that signals flow based on the signals defined in A0, A1, and A2. But in this case no signal can be received because the LEDS are blocking the way

Not quite.
At the Q connections the voltage is either high or low.
If the voltage is low, the corresponding LED lights up, with current flowing from Vcc to the low voltage Q connection.

 

[Femme_physics]

Ah, so the current flows FROM Vcc, into the LEDS, to the decoder, and eventually leaves as possibly A0, A1, A2 or possibly neither depends on the combination chosen?

 

[I like Serena]

Ah, so the current flows FROM Vcc, into the LEDS, to the decoder, and eventually leaves as possibly A0, A1, A2 or possibly neither depends on the combination chosen?

No. A0, A1, and A2 are inputs.
The decoder will have separate connections to a voltage supply that is not shown in your drawing (just like an op-amp).

 

[sandy.bridge]

If you change your dc supply to an ac one, it will ;)

 

[Femme_physics]

No. A0, A1, and A2 are inputs.
The decoder will have separate connections to a voltage supply that is not shown in your drawing (just like an op-amp).

Such as...

http://img864.imageshack.us/img864/2757/likethis.jpg


?

 

[I like Serena]

Yes, but the Decoder with have extra 2 connections instead of just one.
One to Vcc, and one to -Vcc or earth.

Btw, I have a little trouble with you naming them Vin and Vout.

 

[Femme_physics]

Yes, but the Decoder with have extra 2 connections instead of just one.
One to Vcc, and one to -Vcc or earth.

Btw, I have a little trouble with you naming them Vin and Vout.
T 07:42 PM

Heh, do you?


Well, current has to come in of somewhere and come out to somewhere, so I figured Vin and Vout are the best choices.

 

[I like Serena]

Yeah well, my problems are:
1. Vin and Vout are not currents but voltages.
2. Vin is usually the input signal (in your case you have 3 input signals: A0, A1, and A2) and not the power supply.
3. Vout is usually the output signal (in your case you have 8 output signals: Q0, Q1, ..., Q7) and not the power supply.

 

[vk6kro]

It is like this:

http://dl.dropbox.com/u/4222062/common%20anode%202.PNG

 

[Femme_physics]

I completely understand now, thank you :)

 

[Femme_physics]

If I have a decoder (same one from page 1) and I am told that the decoder doesn't have a need in a choice signal - CS. What does it mean?

 

[vk6kro]

CS usually means "chip select". So, you have pull this pin high (or low) to make the chip operate.

This is used when several chips are connected but you only want to use one of them.

If you have only one chip and you want to use it alone, you may have to tie this pin high (or low.. read the data sheet) to make the chip work.

 

[Femme_physics]

According to the data the decoder acts on low. So telling me that the decoder has no need for choice signal - CS, seems like an irrelevant bit of information to solve for which code let's out which exit. I mean, it doesn't change the truth table right?

 

[vk6kro]

It just means that the chip has to be wired with that pin grounded all the time, if you want it to work all the time.

 

[Femme_physics]

Oh, ok, so when told to neglect it it's like in mechanics exercises where you're told to neglect friction. :)

 

[psparky]

From what I remember from LED's from senior design...

When you send them a "1"...they turn off.

When you send them a "0"...they turn on.

 

[vk6kro]

Oh, ok, so when told to neglect it it's like in mechanics exercises where you're told to neglect friction. :)

No, not really.

These chips come as an integrated circuit and you have to connect power to them to make sure they work.

They also have to have certain other pins connected in the right way for them to function.

Chip select pins and reset pins are just two of these. If you don't need their function, then you need to connect these in a default way so that the chip functions properly without them.

Once they are connected, then the chip can produce some result. If they are connected wrongly or not connected at all, then they can disrupt the operation of the chip.

 

[AlephZero]

According to the data the decoder acts on low. So telling me that the decoder has no need for choice signal - CS, seems like an irrelevant bit of information to solve for which code let's out which exit. I mean, it doesn't change the truth table right?

It does change the truth table. If CS is "off", you get no output at all, for ANY values of the the other inputs.

For your problem, you probably aren't interested in using that behaviour, so you can ignore it in your truth tables. But if you build or simulate the circuit, you have to connect the CS pin permanently to the right input voltage, otherwise nothing will work.

 

[hxtasy]

Actually the question of whether you could physically make a light emitting zener diode is interesting (and I don't know the answer).

But conventioal diodes usually self-destruct very fast if you try to operate them in the Zener region, because the Zener voltage is high (typically hundreds or thousands of volts) so the heat generated in the diode (volts x amps) when the Zener current starts to flow is also high.

So the answer is "no" for any "mass produced" LED.

i think all electronics emit some amounts of photons, i know all diodes do. LED diodes are just tuned so they emit light energy in the visible light spectrum.

also interesting is that light shining on them will have an affect on it, and this has to be kept in mind in circuit design where ambient light can penetrate onto chip packages causing problems.

 

[NascentOxygen]

Before the availability of cheap LEDs, hobbyists found it possible to make our own solid-state light source by cutting the top off the tin can of particular transistors and reverse-biasing one of the junctions beyond its breakdown voltage. Provided the exposed device was sealed with a clear plastic substitute cap, its useful life could be extended to weeks. :tongue:

The light was a dim violet I think.