# Again! Current-limiting resistor for LEDs

Discussion in 'Electronic Basics' started by Mr. Man-wai Chang, Jul 22, 2013.

1. ### Mr. Man-wai ChangGuest

The supplied voltage is 3V.

The forward voltage drop over LED is 3V.

What is the resistance of current-limiting resistor?

2. ### Daniel PittsGuest

You'll likely end up with a fairly dim (or completely dark) LED, if the
voltage source is exactly the LED voltage drop. That's because you won't
have any current at all.

Try the experiments I suggested earlier. Use a variable voltage source
and a variable resistance, so that you can play around with both values.

3. ### Tom BiasiGuest

Please Ian, he would need sharp tools!

4. ### Daniel PittsGuest

Perhaps, but I think his questions have helped me to devise explanations
and experiments to validate the assertions we've been making.

5. ### Mr. Man-wai ChangGuest

Then do I need a current-limiting resistor when connecting two 3V LEDs
in series to a 6V supply voltage?

Yes, sort of going back to my question I posted days ago!
Later... A LED may be dirt cheap, but blowing one up is still money.

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6. ### Mr. Man-wai ChangGuest

Should I worry about the LED shorting itself in this scenario?

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7. ### Mr. Man-wai ChangGuest

simple LED thingy...

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8. ### whit3rdGuest

It's not enough information.

Your '6V' nominal supply has a tolerance, and so does the '3V' forward
voltage on the LED. If the LED were a perfect diode, it'd double its forward
current every 15 mV or so, and 6.15V would, while being
nominally still '6V', force about a thousand times the intended current
through the LED.

Your LED choice, and battery choice, includes internal resistances
and temperature coefficients, and measured-value tolerances. LED
direct to battery CAN work, but you have to examine all the vertices
(i.e. all the extreme values of temperature, voltage, etc.) and not
just one point which is the 'nominal' set of values.

9. ### Jasen BettsGuest

It depends on the 6V and on the LEDs.
it's a cheap education.

10. ### Mr. Man-wai ChangGuest

I remember: "essay", not "short paper"!

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nice one.

what's the resistance of your 3 volt power source, and forward current of
the LED?

12. ### Mr. Man-wai ChangGuest

Sounds interesting...

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13. ### Mr. Man-wai ChangGuest

So if the input voltage is guaranteed to be lower than the forward
voltage drop of a LED, no current-limiting resistor would be needed?

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14. ### Mr. Man-wai ChangGuest

How do you prevent a LED from shorting itself?

Is the value of input voltage related?

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15. ### Mr. Man-wai ChangGuest

All those motherboards out there uses fixed voltage to drive indicator
LEDs, I supposed.
I think you are asking for too much from simple LED applications.

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16. ### Mr. Man-wai ChangGuest

I am talking about those simple 3mm/5mm LEDs here, not those hi-power
LED lamps....

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17. ### P E SchoenGuest

"Ian Field" wrote in message
Maybe you could add negative resistance?

When you apply less than the forward voltage drop, the device becomes a
Darkness Emitting Diode, or DED, and it really sucks (light).

Paul

18. ### ShaunGuest

"Mr. Man-wai Chang" wrote in message
How do you prevent a LED from shorting itself?

Is the value of input voltage related?

Why don't you look up basic electronics on the internet (a course) or go to
dumb questions.

Shaun

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19. ### Mr. Man-wai ChangGuest

I expected short answers here. Don't worry, I always google if I know
the right set of keywords.

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