Again! Current-limiting resistor for LEDs

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.

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

Please Ian, he would need sharp tools!

Ian should avoid sharp tools... like pencils... >:-} ...Jim Thompson

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

Zero ohms.

Sounds like a lot of cheap LED flashlights and key-ring lights: Battery, switch, LED.

LED voltage-current curves aren't brick walls. The blue (and white) ones can be quite ohmic.

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

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

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Not if the batteries have enough internal resistance to protect the LED.

Some makes of 3D LED flashlight have no limiting resistor, but it doesn't do the LED any good and the light output falls off long before the batteries are fully used up.

You can get 1 or 2 cell LED flashlights very cheaply, they have a flyback boost converter which is better for the LED and keeps on going until the batteries are finished.

It's certainly trickier than using a current limiter of some sort. The loaded battery voltage has to match up with the desired operating point on the LED curve. Any voltage mismatch changes the current by an amount deltaV/Re, where Re is the sum of the battery internal resistance and the slope resistance of the LED curve.

I had a key-ring light that had two 3-volt Li batteries and one white LED. I guess the "designer" figured that the battery internal resistance would limit the current. It didn't. The LED started flashing at high brightness for a while, then failed. I've seen this flash-fail thing happen elsewhere, too.

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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.

It depends on the 6V and on the LEDs.

it's a cheap education.

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

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--- You seem to have missed the point, which is that with a fixed current through the LED the voltage drop across it will be in the range of the voltages specified by the data sheet.

Take, for instance, a white LED with a nominal drop of 3.5V and a range of 3 to 4V with 20 Ma through it.

That means that with 20mA through those LEDs most will drop 3.5V, but the drop can vary from 3 to 4 volts for any of them.

With that in mind you should understand that what's important is to force the desired current through the LED instead of applying a certain voltage across it and expecting the current through it to be what you want.

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.

-- JF

nice one.

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

There are plenty of cheap keychain lights in the discount stores; red LED + CR2032, or blue/white LED + 2x CR2016 each with no limiting resistor.

The lithium coin cells are rated about 3.6V nominal.

If its 2x alkaline cells - they produce just over 1.5V per cell - 2 cells will give a pale light from a blue LED.

My first LED flashlight was a 3D with no limiting resistor - with fresh cells its blindingly bright, but from the point that the beam becomes weak, the batteries will still serve some duty in a non LED application.

Many discount stores carry 1 or 2 cell flashlights that contain a small boost converter - probably based on the PR4401 chip.

Sounds interesting... :)

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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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