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Topic: Very basic resistors question (Read 1 time) previous topic - next topic

pito

#15
Feb 25, 2013, 10:12 pm Last Edit: Feb 25, 2013, 10:15 pm by pito Reason: 1
A standard EE does it so:
1. Vcc is 3V3
2. my LED lits fine at I=1.0mA
3. My LED is red, red LEDs have Vf=1.5V (for example)
4. R = (Vcc - Vf) / I = (3.3 - 1.5) / 0.001 = 1800 ohm = 1k8
I do not understand who recommends to use 20mA current through an LED today (when used for indication purpose).. We used 20mA currents 40y ago. Today's LEDs lit fine at 300uA.. Unless you going to cut the steel with your LED gun :)

majenko


A standard EE does it so:
1. Vcc is 3V3
2. my LED lits fine at I=1.0mA
3. My LED is red, red LEDs have Vf=1.5V (for example)
4. R = (Vcc - Vf) / I = (3.3 - 1.5) / 0.001 = 1800 ohm = 1k8
I do not understand who recommends to use 20mA current through an LED today (when used for indication purpose).. We used 20mA currents 40y ago. Today's LEDs lit fine at 300uA.. Unless you going to cut the steel with your LED gun :)


That's just what people want to do ;)

I spent some time getting the right combination of LED and resistor for my LED boards to get the LEDs as bright as possible, because that's what people wanted.  Now you can hardly look at the red and blue LEDs without searing your retinas, and people love them.  The LEDs I use have a "rated" forward current of 20mA, an absolute maximum of 30mA.  At If=20mA they're 150mcd.

That's not a patch on the white right-angle LEDs I use.  1300mcd at If=20mA.  Three of them at 20mA (4V Vf each) will light the entire room.  Just with a single one by itself you can't look straight at it.  And if I were to pulse them I could run at up to 100mA with a 1/10 duty cycle.  I haven't done that yet to see how insane they are, but I'll bet they hurt.

So yes, running at lower than the "rated" current is usually pretty bright anyway.  People just "assume" 20mA as most LEDs have their visual data quoted at If=20mA.

Edit: Oh, all those LEDs I spoke of above are all 0805 SMD footprint.

pito

#17
Feb 25, 2013, 10:43 pm Last Edit: Feb 25, 2013, 11:00 pm by pito Reason: 1
@majenko: frankly, the first thing I do with my lovely boards I am getting from ie. ebay or from MCU vendors (kits) is - I replace those 270 and 330ohm resistors they are using because they saw it on a schematics from 1973 with such values, or, somebody told them "an LED needs 20mA"..
I did few measurements in past with a bunch of 3mm dia and 0805 smd LEDs and most of them lit fine (so you can watch the board without sunglasses) at 100-300uA (microamps). I am using 2k2-4k7 resistors with any color..  ]:)


majenko

That's pretty high...  I have traditionally used 1K with LEDs when just messing around, or 470? if I want them bright - mainly because I have thousands of those in ?W kicking around doing nothing.  On boards though I tend to use lower values (usually whatever I have nearby spare on a 4 element array) as kids love the bright lights... It appeals to their simple natures.  And some of my projects demand incredible brightness, as the LEDs are the main feature of it...

pito

Quote
as the LEDs are the main feature of it.

yea, by DUE controlled 2Watts LED lightsaber :)

majenko


Quote
as the LEDs are the main feature of it.

yea, by DUE controlled 2Watts LED lightsaber :)

Close... The PIC32 controlled Tron Disc.

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