Again! Current-limiting resistor for LEDs

[Mr. Man-wai Chang]

The supplied voltage is 3V.

The forward voltage drop over LED is 3V.

What is the resistance of current-limiting resistor?

 

[Daniel Pitts]

The supplied voltage is 3V.

The forward voltage drop over LED is 3V.

What is the resistance of current-limiting resistor?

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.

 

[Tom Biasi]

Having read a few of this guy's posts - I think he should take up woodwork.

Please Ian, he would need sharp tools!

 

[Daniel Pitts]

Having read a few of this guy's posts - I think he should take up woodwork.

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

 

[Mr. Man-wai Chang]

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.

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!

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

Later... A LED may be dirt cheap, but blowing one up is still money.

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[Mr. Man-wai Chang]

Zero ohms.

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

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[Mr. Man-wai Chang]

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

It could be fun to ask EE students to write a short paper about this
simple LED thingy...

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[whit3rd]

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

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.

 

[Jasen Betts]

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

It depends on the 6V and on the LEDs.

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.

it's a cheap education.

 

[Mr. Man-wai Chang]

It could be fun to ask EE students to write a short paper about this
simple LED thingy...

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

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[Cydrome Leader]

The supplied voltage is 3V.

The forward voltage drop over LED is 3V.

What is the resistance of current-limiting resistor?

nice one.

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

 

[Mr. Man-wai Chang]

Many discount stores carry 1 or 2 cell flashlights that contain a small
boost converter - probably based on the PR4401 chip.
http://www.prema.com/pdf/pr4401.pdf

Sounds interesting...

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[Mr. Man-wai Chang]

The reason for that is because the VI curve for an LED is steep and if
you apply 3.5V across an LED which drops 3V with 20mA through it you
could easily overcurrent the LED.

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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[Mr. Man-wai Chang]

You could say that - but the LED won't do much.

How do you prevent a LED from shorting itself?

Is the value of input voltage related?

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[Mr. Man-wai Chang]

LEDs are current operated - a fixed voltage is the least suitable way to
drive them.

What's wrong about that?

All those motherboards out there uses fixed voltage to drive indicator
LEDs, I supposed.

You have been told this by several different people, if you still don't
understand - take up woodwork!

I think you are asking for too much from simple LED applications.

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[Mr. Man-wai Chang]

I think you are asking for too much from simple LED applications.

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

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[P E Schoen]

"Ian Field" wrote in message

You could say that - but the LED won't do much.

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

 

[Shaun]

"Mr. Man-wai Chang" wrote in message

You could say that - but the LED won't do much.

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
a bookstore a buy a book on basic electronics instead of asking all these
dumb questions.

Shaun


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[Mr. Man-wai Chang]

Is the value of input voltage related?
Why don't you look up basic electronics on the internet (a course) or go
to a bookstore a buy a book on basic electronics instead of asking all
these dumb questions.

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

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