How to Identify a Damaged LED Visually After Incorrect Connection with 74HC00N IC?

I am an engineering student. Recently i was playing round with logic gates. I mistakenly connected a Led and power to the pin 6 or pin7, can't remember. Thankfully i didn't burn the74HC00N IC, but my LED was damaged.Only a small corner of it now lights up.It should be noted that i grounded pin7 of the ic and connected pin 14 to the power supply. Also the LED was connected with a resistor. My LED had some rust on it. Could it have contributed to the damage. Also how do i understand an LED has been damaged by only seeing it and without testing it using any meters or in a circuit.

From the datasheet, It has a rated output source or sink current of ±25 mA, but this may have been exceeded by the low impedance of the LED if it was directly across the 5V power supply, or ground and a high pin (or Vcc and a low pin).
Coloured LEDs quickly burn out if they go much over about 2.5V, white LEDs burn out (almost certainly) by 4V.

It is fairly certain that you have burned out the LED, a faint glow remaining is typical.

Ordinary LEDs have only about 5V peak reverse voltage, so it could be borderline if it was reverse polarity.

Rust on the leads will not affect the LED, it will only change how easy it is to solder.

(LEDs will for a while take considerably more than their rated current. I experimented with a white 30mA LED for 24 hours at 100mA, with no change in brightness or electrical characteristics. But I probably shortened its life a lot.)

The only test (apart from seeing obvious black bits) is to put the correct current through in the correct direction and see if it gives the correct brightness.

Probably pin 7 because I don't think a 74HC00N can sink enough current to damage an LED, at least, not quickly.

What value was the resistor (that you _connected with the LED_) and was it in series with the LED (or in parallel)?

If it's a standard LED for indication, then it's probably designed to run continuously at 20ma (though some are spec'ed as high as 30ma). So, it's important to limit the current to 20ma or below. This is tricky in the configuration you are using. So, best bet is to target a current of 15ma, to give it a little head room. (but, again, that's assuming it's a 20ma LED -- check the data sheet if unsure)

Different color LEDs have different forward voltages. And there is a range of forward voltages possible between even the same color LED and even between the same part number! This is what makes it tricky. You see, an LED is a "current device" and is best run with a current source, not a voltage source. The output of a 74HC' family logic gate is a voltage source. A crude way of converting a voltage source to a current source is to use a resistor in series -- the higher the value of the resistance the better this conversion is, with infinite resistance being the best (something that can be closely achieved with a constant current source or current regulator -- in which case it's a _dynamic resistance_). But, if you were paying attention, the problem with using a resistor for this, is the higher the resister value, the higher the voltage needs to be to achieve the required current (again, dynamic resistance is the solution to this).

BUT, because the current level is so low (assuming that you are using a common indicator type LED -- or even a low current super bright LED) you can get away with a resistor in series (and a poor conversion). Find out what the MINIMUM forward voltage is for the LED in question, and then use the following formula to determine the value of the series limiting resistor:

R = (Vcc - Vf)/I where Vcc is the voltage supplied to the logic gate, Vf is the minimum forward voltage of the LED, and I is the LED current when the LED is on.

You will need to get the LED current from the data sheet (for that LED). You want the max continuous current. If it's 20ma then use that if you are brave, or 75% of that if you're wise. If the spec sheet quotes 30ma, be suspicious -- decades back, I had several LEDs fail over time, that I ran at 30ma ('cuz the spec sheet told me I could).

As for testing an LED in a non "visual" way, when an LED is subjected to excessive current, the forward voltage typically rises. If the forward voltage tests higher than the maximum specified voltage (for the current that that voltage is specified at) then the LED is fried. But, a burned LED can still have a forward voltage within the specified range, so this is not a 100% test. Also, a bad LED can read as an open circuit -- that's pretty much 100%

An LED has a forward transfer curve similar to a diode ('cuz that's what it is As such, the current through an LED changes radically with small changes in forward voltage. The max Vcc on a 74HC' logic device is 6V. So, that is probably worse case, assuming Dipika knows to pay attention to such things.

Also, the minimum voltage for a 74HC' is 2V, which is possible if Dipika is using a red LED with a Vf of 1.7V @ 20ma, we can, really, make no assumptions regarding Dipika's choice of Vcc!

6V across _any_ LED of the "20ma" class will cause EXTREMELY excessive current through the LED -- assuming the source can deliver that much current. Even at 5 volts, the current will be considerable. In both cases the current will surly fry the LED. And, there is even a case where 2V might be excessive.

This is why the forward voltage of an LED cannot be relied on to predict the current that will be demanded by an LED in an arbitrary system. The ideal system will control the current irrespective of the LEDs forward voltage. At the 20-30ma range of currents, this is not as critical, but for LED lighting systems, that demand current in the hundreds to thousands of ma, this is crucial.

bq). Coloured LEDs quickly burn out if they go much over about 2.5V, white LEDs burn out (almost certainly) by 4V.

It's not the forward voltage level that burns out an LED, it's the current. I can apply 2.5V across a blue LED and it won't even light, let alone burn out. A blue LED typically has a forward voltage greater than 3V when properly biased. I have Red LEDs that run at 1.7v when biased to 20ma. And, an IR LED typically runs at voltages close to 1V @ 20ma.

My error, I forgot about the blues. And I was assuming this student was most likely using ordinary little 20-30 mA LEDs for his experiments, so I ignored the high power LEDs.
I generally try to keep things simple because most of our enquirers are usually seeking simple answers.

Possibly, since "decades back", LED reliability may have improved, so that the present specification ratings may be more realistic. But of course, that's just maybe.

Set Digital multi meter in diode test mode and connect LED terminals to meter terminals, light may glow or meter indicates some reading. If nothing happens reverse meter terminals across diode, it may show some reading or may glow if nothing happens both side diode is open / damaged. If it shows some reading in both direction even then diode is damaged, it may not glow. It is not possible to check diode by visualization. If you have found some thing dark inside diode immediately on connection then it might have damaged.

Most digital multimeters won't test LEDs on their diode test ranges, because LEDs have a higher forward voltage than ordinary diodes.

Put a current limiting resistor in series with the led typically for superbright assuming logic 1 or 5 volts out m taking into account the forward drop across the diode typical value 2v1 the limiting resistor should be rated at R470 "470 Ohms " , Thats why it burnt out Im surprised you havent damaged the TTL IC Lucky ots not a processor otherwise normally you can kiss the port goodbye

Use an analogue meter then but you will get a reading in one direction with a DVM anyway half of this is basic maths , basic know how and how to use a DVM

There are plenty of tutorials on the net re this and how to connect an led believe it or not

My ElCheepo DVM shows _infinite_ voltage across an LED
But, my Protek 506 actually lights a white LED and shows the forward voltage! [but it just barely lights it -- 'cuz the max voltage is 3.061V AND is limits the current to a few milliamps, so it's not a very reliable test.

So, it depends on the meter.

It's simple to easy solve your problem's you have to try to check your ic Voltage or check the LED is working corectly in the digital multimeter. you check out throgh multimeter.
"electronic gold tester
(electronic gold tester)":http://www.oceankingindia.com/silver-series-DJD.html

SPAM!