Bi-Color LED Specs?

JBWolf · 2013-02-07 15:27

Hello,
I just purchased part# 350-00005.
The LED didnt come with any info and the store doesnt have any documentation... I dont know how exactly this works or what the max current rating is.
Theres only 2 pins so im assuming just reverse the voltage? Normally this would burn out an LED running it in reverse.

Is there are information on this LED?
http://www.parallax.com/StoreSearchResults/tabid/768/txtSearch/350-00005/List/0/SortField/4/ProductID/569/Default.aspx

Thanks

Chris Savage · 2013-02-07 15:38

Bi-Color LEDs such as this one have two LEDs internally in parallel with one in reverse from the other. Voltage/current flow in one direction lights up one LED while reversing the flow lights the other LED. We don't include technical specs for this LED as the supplier may change and therefore the spces. These are intended for generic use so if you put a 470 ohm resistor in series you'll definitely be in the safe zone on current @ 5V. If you have a multimeter with a diode checker, some report the forward voltage, so you could check in both directions and divide that value by 10mA to get the resistor value. Note that green and red LEDs have different forward voltages so the green will almost always be slightly dimmer than the red.

Mark_T · 2013-02-07 15:43

I would be surprized if it wasn't rated at 20mA - and with 2 leads it is presumably back-to-back. Most LEDs have a reverse voltage rating ot 5V or similar.
so wouldn't mind being reversed at 3V3 for instance. With two LEDs back-to-back the reverse breakdown cannot be reached as the other LED will
conduct first.
Perhaps the documentation for some of the kits it is included with have more information, but if you stick to 5V or below and say a 330 ohm series resistor
it ought to be OK - that's about 10mA so I'm erring on the safe side.

JBWolf · 2013-02-07 16:14

ok, just wanted to make sure, this is my first time using a bi-color with only 2-pins.... thought there would have to be an internal diode to stop flow to one led in a reverse current config.... so I thought maybe I received the wrong product.
Thanks!

JBWolf · 2013-02-07 16:15

Oh, what is a typical method of running this off the prop?
Connect 2 pins to the LED, bring one low and one high, reverse high/low output for other color?

Should I change the pins from input to output to switch from high/low?
or can I just set both pins to output and use !outa[pin1..pin2] to switch between colors?

JBWolf · 2013-02-07 19:33

Works great, red is a bit dim so I might step up current to 20ma.
Here is the code:

{{
Bi-Color LED Test (Red/Green)
Parallax PN# 350-00005
                                                                  
                                                                                       
 P17 >>------- 330&#937; ------                                      
                         |
                         |
                       (LED)               
                         |
                         |
 P16 >>-------------------
 
Starts with LED off...
Button press #1 turns on one color
Button press #2 turns on other color
Button press #3 turns off LED

}}

CON
' Set Processor Speed @ 80mhz
  _clkmode = xtal1 + pll16x
  _xinfreq = 5_000_000

' Global Pin Assignments
  Button = 25
  LEDpin1 = 17
  LEDpin2 = 16

      
   
PUB  Main | Temp
Temp := 0
DIRA[LEDpin1..LEDpin2]~~
DIRA[Button]~
                     
waitcnt(clkfreq / 1000 + cnt)  

     
 repeat                                ' Start Endless Loop
    IF INA[Button] == 1                
     Temp++
       IF Temp > 2                     ' 3-Options Max (0,1,2) 
          Temp := 0
     case Temp
       0 :  OUTA[LEDpin1..LEDpin2]~    ' LED OFF
       
       1 :  OUTA[LEDpin2]~             ' LED Red ON
            OUTA[LEDpin1]~~
            
       2 :  OUTA[LEDpin1]~             ' LED Green ON
            OUTA[LEDpin2]~~
                   
 
   waitcnt(clkfreq / 3 + cnt)

RDL2004 · 2013-02-07 20:42

If that is correct and you're actually using a 330 ohm resistor, that's pretty large for the 3.3 volt Propeller. You're probably only drawing around 5 mA or less from the Prop pins. I'd expect the LED to be kind of dim. You should be able to go down to 220 or even 150 with no problems.

JBWolf · 2013-02-07 22:38

I=V/R
3.3 / 330 = 10ma

I switched to a 178ohm.
3.3 / 178 = 18.5ma

Looks much better

RDL2004 · 2013-02-08 04:52

In this case, you have to subtract the forward voltage drop of the LED (V_LED).

I = V_in - V_LED / R

Red LEDs will normally have a voltage drop of around 1.7 - 2.0 volts or so, green will be a bit higher, maybe 2.0 - 2.2. Any number in those ranges will be close enough.

I = 3.3 - 1.8 /330 = 4.5 mA

So with 178 ohms, you're probably still under 10 mA

ajward · 2013-02-10 21:03

I never checked the actual current flow, but I've used 100 ohm resistors for led's on the prop. No problems!

@

Chris Savage · 2013-02-11 11:00

That sounds right since the Propeller runs at 3.3V instead of 5V.

RDL2004 · 2013-02-11 11:53

Propeller specification is 40 mA per I/O, 3.3 volts / 0.04 amps = 82.5 ohms. This would limit current to the maximum even with a dead short. Given component tolerances and such, 100 ohms is probably a good minimum value.

Chris Savage · 2013-02-11 12:31

Okay, I should have posted this earlier, but I had to look through purchase history to make sure they haven't changed in the last year. The LED we're selling currently and for some time now is the following (datasheet):

http://www.kingbrightusa.com/images/catalog/SPEC/WP57EGW.pdf

According to this datasheet the forward voltage for the red is 2.0V and the forward voltage for the green is 2.2V (typical). Given that and a standard operating current of 20mA, the following resistor values have been calculated:

For Red (5V and 3.3V):
5.0 - 2.0 / .020 = 150 Ohms
3.3 - 2.0 / .020 = 65 Ohms (100 ohms is close standard value)

For Green (5V and 3.3V):
5.0 - 2.2 / .020 = 140 Ohms (150 ohms is close standard value)
3.3 - 2.2 / .020 = 55 Ohms (100 ohms is close standard value)

Keep in mind also that these are for 20mA forward current and that these (according to the datasheet) are both high-efficiency type and do not need 20mA to look good. 10mA probably works fine. I used a 220 ohm in a Propeller project that used this LED giving ~5mA forward current and that works/looks great.

Bi-Color LED Specs?

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