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### Topic: A noob resistance question (Read 11660 times)previous topic - next topic

#### macweenie

#30
##### Jan 06, 2013, 09:53 pm
Quote
The LEDs voltage tolerances are often more than 0.1V either way, so what voltage do you aim at...?

Well fungus, I would say that I would add the 0.1V to the 4V maximum forward voltage so 5V - 4.1V = 0.9V drop voltage, then to calculate the resistor needed (if I use a resistor vs a chip) it would be 0.9V / .02mA = a 45? resistor (or the next larger value), HOWEVER since I am always wrong about these things my gut is telling me to SUBTRACT 0.1V from the 4.0V BECAUSE it is the MAXIMUM forward voltage and if it is maximum then I can't go beyond it right? So 5V - 3.9V = 1.1V and 1.1V / .02mA = a 195? resistor (or the next larger value). I am aware that you may have been asking a rhetorical question, but I wasn't sure.

#### fungus

#31
##### Jan 06, 2013, 10:17 pm

I am aware that you may have been asking a rhetorical question, but I wasn't sure.

It was rehetorical. The correct answer is not to aim for a voltage.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

#### fungus

#32
##### Jan 06, 2013, 10:18 pm

Quote
This is why controlling LEDs with resistors is a bad idea,

It is not a bad idea it is done all the time in a lot of professional applications and is quite satisfactory technique. It is not the technique to get the absolute exact control over the current but often this is not necessary.

If you're aiming at 10-12mA (or less) on a 20mA LED, sure. The power curve is usually quite flat there.

If you're aiming at 20mA on a 20mA LED then resistors simply don't work.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

#### macweenie

#33
##### Jan 07, 2013, 01:32 am
Since you brought it up, I was wondering, How does using an LED Driver IC differ from powering the LEDs through a PWM enabled pin? Couldn't I use a shift register IC on a PWM pin to get the same result? Not that I have any immediate plans.

#### Grumpy_Mike

#34
##### Jan 07, 2013, 02:46 pm
Quote
How does using an LED Driver IC differ from powering the LEDs through a PWM enabled pin

Rather the same way as driving a car differs from making a cake. No relationship whatsoever.

#### fungus

#35
##### Jan 07, 2013, 03:27 pm

How does using an LED Driver IC differ from powering the LEDs through a PWM enabled pin?

It's completely different.

Not all ICs do PWM.

The IC will clamp the LED current to a fixed value (eg. 20mA), independent of voltage.

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

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