Choosing an appropriate LED for ATMGE32U4-MU uC

Thread Starter

pedro147

Joined Jan 1, 2013
52
I am trying to replicate an existing circuit for a quad copter flight control board. It has two SMD LED's fitted but no current limiting resistors are used. I realise that this is very bad design practise but I want to recreate the board exactly as it was sold up until recently. My question is on choosing an LED that has the best chance of survival under these arduous conditions :) I thought that this one with the following specs might have a decent chance of survival and am just hoping for an educated opinion but not one that involves fitting a resistor.

Forward current 20 Ma

Forward voltage 3.3v

Source voltage 3.3v

This giving a value of a 1 ohm resistor required. Also how crucial is it that the pad size is roughly the same size as the component footprint or is it more a matter of there being full contact .




Thanks

Alienwii Schematic.jpg
 

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spinnaker

Joined Oct 29, 2009
7,837
Look at the specs for your mcu. If the max current of your binary out cannot exceed 20ma then it is probably the "safe" way to go. As you mentioned not the ideal way to go. Do you already have the PCB?
 

Thread Starter

pedro147

Joined Jan 1, 2013
52
spinnaker thanks for your reply. As you can probably tell I am well out of area of expertise here. I am in the process of trying to develop a board file in Eagle for this project. The schematic and associated libraries are the only information that I have. I really would not know how to find the max current of the Atmega32u4 binary out. If you might have time to have a look for me I would appreciate it, but if not I understand. I cannot directly link the datasheet just the part specs which include that link.
 
Another way to go would be to put a place holder for a current limiting resistor on any new PCB that you create. If you decide to populate it with a 0 Ohm resistor to match the currentboard you have that possibility. If you find it better to have a current limiting resistor you do not have to change the board again later.
 

Thread Starter

pedro147

Joined Jan 1, 2013
52
StayatHomeElectronics thanks good idea. Yes, after the hopefully successful implementation of this board, I wish to incorporate this and some other improvements like larger FET's and a reverse current diode. The current discontinued production board works without any current limiting resistors so I am just hoping to emulate this in my first version of the board.
 
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dl324

Joined Mar 30, 2015
9,326
how crucial is it that the pad size is roughly the same size as the component footprint or is it more a matter of there being full contact
Unless you have a good reason for using non-standard footprints, I'd go with published specs.

The datasheet was ambiguous regarding current sink capabilities. I'd check the IV curve for the LED you plan to use and see what it's current will be at 3.3V. If you're only going to make one board, you could cherry pick an LED for specific IV characteristics.
 

Thread Starter

pedro147

Joined Jan 1, 2013
52
Thanks dl324. So I looked at the IV curve and at 3.3v the forward current is 17.5 mA. How do I "cherry pick an LED for specific IV characteristics" by using this info please. Sorry I am pretty green :) I am finding the use of different terms for the same thing confusing here. So R = voltage source - forward voltage divided by forward current which going from the IV curve is 17.5 mA @3.3v. So obviously regardless of at what forward current I calculate for, the top line will always be zero. So does this mean that because the voltage source and forward voltage are the same value, no current limiting resistor is required in this circuit?

LED Specs.jpg LED IV curve.jpg
 
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dl324

Joined Mar 30, 2015
9,326
Thanks dl324. So I looked at the IV curve and at 3.3v the forward current is 17.5 mA. How do I "cherry pick an LED for specific IV characteristics" by using this info please.
LEDs in the same intensity group will have a range of forward voltages.
ledSpecs.jpg
LED-iv.jpg
For this LED, if you used one that had a Vf of 2V @ 20mA, at 3V it would draw 100 mA. If you used a diode that had a Vf of 3V @ 20mA, at 2V it would draw less than 20mA.
 

Thread Starter

pedro147

Joined Jan 1, 2013
52
Sorry, you beat me to it. I am finding the use of different terms for the same thing confusing here. So R = voltage source - forward voltage divided by forward current which going from the IV curve is 17.5 mA @3.3v. So obviously regardless of at what forward current I calculate for, the top line will always be zero. So does this mean that because the voltage source and forward voltage are the same value, no current limiting resistor is required in this circuit? I appreciate you going to the trouble of trying to "learn me" :) but at this stage I suppose I just want to know if this LED has a decent chance of survival without a resistor.
 
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dl324

Joined Mar 30, 2015
9,326
The LED datasheet should give a range of values for Vf at a specific current, usually 10mA or 20mA. The IV curve is for a typical LED.

To cherry pick means to pick a device from a group of devices for specific characteristics. What you're looking for is one that will conduct no more than a certain current at 3.3V

BTW, since this is a battery powered circuit, I'd be inclined to omit the power on LED.
 

takao21203

Joined Apr 28, 2012
3,696
If theres no resistor most likely it's pulsed. Indication leds need a few Ma.

FYI a single led on atmel controllers used for arduino will draw 60 ma.

So it could be pulsed at 1/20. Just for a single indication led it's not really cutting it to remove the resistor.
 

Thread Starter

pedro147

Joined Jan 1, 2013
52
takao21203 I cannot understand the multi-quote system here so "So it could be pulsed at 1/20. Just for a single indication led it's not really cutting it to remove the resistor"You will have to spell it out very simply for me I'm afraid :) So by "pulsed at1/20" does that mean like the duty cycle. Likewise by "it's not really cutting it to remove the resistor" are you meaning that this not best practise, for want of a better description. My whole issue here is that I am after advice on how to pick an LED for an existing circuit that does not have a current limiting resistor. I know that it is not the correct way to do it but that is the way it has been done by someone else.
 
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