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Please comment on my LED driver.

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EngIntoHW

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I'd like to opearate a Green LED (3.3Vtyp @ 20mA), using a 3.3V digital I/O pin.

The problem is that VF can reach up to 4.1V, but I'm afraid that using a current source will need more components which will make the design more expensive and physically larger.

Have I designed it well?

Thanks!

**broken link removed**
 
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well it looks good to me! Of course, unless its a specialist LED, like a power LED, then there aren't any real restrictions on driving them.

I must say, 4.1Vf fora green LED seems high to me... highest I've measured/seen is 2.2V, but then again there are different materials used for different greens.

Also, you're running it at 30mA. R= 30, v = 5 - 4.1. Therefore with R = 30. I = V/R = 0.9/30 = 30mA.

That is a limit for most standard 3/5mm LED's. I'm sure you're not planning on running this permenantly on for years, but that much current will dramatically lower its operating life. No problem for the odd project, but if its an 'on' indicator for something which is powered up regularly, the hours add up!

You have 30 ohms as the series current limiting resistor, which isn't a standard value (33 is),I would raise that to say, 47ohms. Easier to get hold of, and this will lower your current to 19mA. I generally run LED's at 10mA, perhaps higher if they need to be brighter. So you could get away with 100ohms for the series resistor.

LOL you edited the post at the same time as me! gawd, so my comments are now out of date :(
 
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I would likely try an 82 Ohm resistor in series with the LED but what you have should work.

Ron
 
Simply calculate the current if the LED is its minimum voltage (maybe 3.1V) and if the LED is its maximum voltage of 4.1V.
 
Hi,
And then what to do with these 2 Current Values?

Make sure that the max current when the LED has a low voltage of only about 3.1V will not burn it out and see if the current when the LED has a max voltage of 4.1V makes it bright enough.
 
Thanks!

Make sure that the max current when the LED has a low voltage of only about 3.1V will not burn it out
I should check the max allowed current, right?

see if the current when the LED has a max voltage of 4.1V makes it bright enough
I should look at the graph of luminious intensity VS Forward Current, right?
 
I should check the max allowed current, right?
Of course.
If your LED does not have a datasheet then throw it away and buy an LED that has a detailed datasheet.

I should look at the graph of luminious intensity VS Forward Current, right?
The graph might not mean anything since it is just a bunch of numbers. Connect the LED with your driver circuit and look at it. Change its current-limiting resistor value so that its current is the minimum and maximum that you calculated to see if it is bright enough.
 
This is the LED data sheet. Typical forward voltage is 3.3 with a maximum of 4.1 and the current is 20 mA. Thus 5 - 3.3 / .020 = 85 and thus I suggested 82 Ohms in the interest of using a common off the shelf standard resistor.

Ron
 
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The minimum forward voltage is not shown so i guessed at 3.1V. then the current with an 82 ohms current-limiting resistor is 23mA which is fine.
If the LED is actually 4.1V then its current will be 11mA which is also fine.
 

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Here is how I would do it: The Sim shows the LED current and LED Vf as a function of the emitter resistor. This method draws less current from the port pin, and has a more constant current through the LED if the 5V supply changes than the circuit the OP posted.

The collector current is nearly the same as the emitter current. Since the transistor is operated as an emitter follower, it is not saturated, and the base current is tiny compared to the common-emitter configuration.
 

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how can you know that Ic/Ib = 10 if your transistor is saturated?
The datasheet for nearly every transistor says that the base current must be 1/10th the collector current for the transistor to saturate properly. You calculate the currents and select the value of the base resistor that does it.
 
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