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### Topic: Why my LED dies when connected directly to a 9v battery? (Read 19232 times)previous topic - next topic

#### Paul__B

#30
##### Jan 20, 2015, 05:51 pm
Doesn't it go both ways?
No.

This is a semiconductor diode, not a resistor!

#### racemaniac

#31
##### Jan 20, 2015, 05:56 pm
No.

This is a semiconductor diode, not a resistor!

So you're saying that if i put my power supply on 3.4V constant voltage, and put a led on it, it will short circuit because the led won't limit the current?
For me looking at it from a voltage and a current perspective are pretty interchangable ways of looking at a circuit, and for certain applications one of both ways is often preferred, but that doesn't mean the other suddenly doesn't work anymore.

#### KenF

#32
##### Jan 20, 2015, 05:58 pm
So you're saying that if i put my power supply on 3.4V constant voltage, and put a led on it, it will short circuit because the led won't limit the current?
For me looking at it from a voltage and a current perspective are pretty interchangable ways of looking at a circuit, and for certain applications one of both ways is often preferred, but that doesn't mean the other suddenly doesn't work anymore.
I've not been paying much attention to this thread but we're on page three already so I've got to start wondering if racemaniac is on a wind up

#### racemaniac

#33
##### Jan 20, 2015, 06:12 pm
I've not been paying much attention to this thread but we're on page three already so I've got to start wondering if racemaniac is on a wind up
my british english doesn't reach far enough to get what that means :p. (i assume it's not nice :p).
well, at least i'm learning something in this discussion (i hope :p)

#### raschemmel

#34
##### Jan 20, 2015, 06:37 pm
This is all just hypothetical until you measure the voltage drop across the led with a 220 ohm resistor and tell us the forward voltage for the led you are using for the experiment. We can't do the math until we know that. So far that remains unknown, therefore the math is just hypothetical.
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#### raschemmel

#35
##### Jan 20, 2015, 07:58 pmLast Edit: Jan 20, 2015, 07:58 pm by raschemmel
We're all speaking english. Ken is basically asking how long is it going to take you to see the light ?
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#### nickgammon

#36
##### Jan 20, 2015, 08:35 pm
my british english doesn't reach far enough to get what that means :p. (i assume it's not nice :p).
A "wind up" is where you are taking the contrary view just to see how people react. A similar expression would be "tease".
Please post technical questions on the forum, not by personal message. Thanks!

#### raschemmel

#37
##### Jan 20, 2015, 09:26 pmLast Edit: Jan 20, 2015, 09:38 pm by raschemmel
I'm in the USA so that went right over my head...

If anything I would have thought it meant something like "winding up for a pitch." (baseball)
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

#### nickgammon

#38
##### Jan 20, 2015, 10:01 pm
The Urban Dictionary has a reasonable definition of it: http://www.urbandictionary.com/define.php?term=wind-up

Funny it should be a south-eastern England term, I understood what it meant.
Please post technical questions on the forum, not by personal message. Thanks!

#### Paul__B

#39
##### Jan 20, 2015, 10:36 pm
So you're saying that if i put my power supply on 3.4V constant voltage, and put a LED on it, it will short circuit because the LED won't limit the current?
Precisely.

It's not strictly a "short circuit", but the current will not be limited to  a reasonable value.

The "equivalent circuit" of the LED and its behaviour is that it acts as if it were actually a series combination of three parts; a battery which corresponds to the threshold voltage (somewhere about, but not exactly 3.4V), a perfect diode (so that you can only put current in to the battery, not take current out) and a very small resistance of a few ohms.

#### KeithRB

#40
##### Jan 20, 2015, 10:40 pm
I think it would be very instructive for the OP to perform two experiments.

Use two 270 ohm resistors in series and monitor the voltage at the junction between them as the voltage across them goes from 0 to 5 V.

Use an LED and a 270 ohm resistor and perform the same experiment.

#### Grumpy_Mike

#41
##### Jan 20, 2015, 10:58 pm
Quote
if i put my lab supply to 3.4V constant voltage mode, and only put the led on it, the led will only allow 20mA trough it.
ONLY if the forward voltage drop of that LED on that day at that temperature is 3.4V. This is highly unlikely and not very stable.

Quote
getting leds to work would be hell i think.
No that is why god gave us resistors, to approximate a constant current supply from a constant voltage one.

#### nickgammon

#42
##### Jan 20, 2015, 11:02 pm
No that is why god gave us resistors, to approximate a constant current supply from a constant voltage one.
I thought that was Mr. Ohm.
Please post technical questions on the forum, not by personal message. Thanks!

#### raschemmel

#43
##### Jan 20, 2015, 11:06 pmLast Edit: Jan 20, 2015, 11:58 pm by raschemmel
Quote
I've not been paying much attention to this thread but we're on page three already so I've got to start wondering if racemaniac is on a wind up
Quote
Funny it should be a south-eastern England term, I understood what it meant.
You understood what you thought he meant. He hasn't clarified if that's what he meant and his comment
above implies he is wondering if Racemaniac is winding up his post, which is entirely different from wondering if Racemaniac is teasing us. I don't think it is clear what he meant. Perhaps Ken will clarify what
he meant.  (it isn't far fetched that racemaniac is just jerking our chain but I actually got the impression he
was trying to learn. I don't think I am the only one.)

@Racemaniac,
3.4V is not a common forward voltage for a led. The generic red 5mm are usually about 2.2 to 2.4V.
Anyone with a variable bench supply can test the theory that a led won't burn out if supplied only with it's
forward voltage. For a led with a Vf of 2.2V, the current limiting resistor should be:
Let Vf = 2.2V @ 20mA
Let Vcc = 5V

(Vcc-Vf)/Iled @ Vf=(5V-2.2V)/0.02=2.8V/0.02 A = 140 ohms.
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

#### nickgammon

#44
##### Jan 20, 2015, 11:26 pm
Every now and then I think someone is winding us up and then jerking our chain, however I don't necessarily think Racemaniac is one of them.

As for what KenF really meant, perhaps we'll let him speak for himself, but possibly the UK version is not totally unlikely.
Please post technical questions on the forum, not by personal message. Thanks!