Author Topic: Right resistor for R7  (Read 1974 times)

Offline holland lights

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Right resistor for R7
« on: August 07, 2012, »
I bought a RGB led common anode from radioshack #276-0028 and i want to makee sure i did the calcuations right for the resistor value. <rtt.. 
(green/blue)
    Forward current: 30ma 
    Forward voltage: 3.5 v typ  4.0v max
(Red)
   Forward current: 50 ma
   Forward voltage: 2.0v typ   2.6v max
mA =(1.25/R) x 15].

For 30ma I got
       1.25/650 ohms x 15=0.0288

For 50ma I got
        1.25/400 ohms x 15=0.046875
Because I done the calculations for 20 ma and it gave me
   1.25/1000 ohm x 15=0.01875

Offline rm357

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Right resistor for R7
« Reply #1 on: August 07, 2012, »
Each color needs its own resistor with the common anode connected directly to the positive lead of your power source.

 
Robert
Warner Robins, Georgia, USA

Offline holland lights

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Re: Right resistor for R7
« Reply #2 on: August 07, 2012, »
I know how to connect a led to a resisitor wih the proper value. I am asking about what R7 is suppost to be on the servo board to work with these leds.

Offline n1ist

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Re: Right resistor for R7
« Reply #3 on: August 08, 2012, »
The driver used on the Bobcat servo board is a constant current driver, so you don't use individual series resistors.  R7 is used to set the current drive for each LED.  It's not like the traditional case of a resistor per LED connected to a constant voltage.

The currents spec'ed for that RGB LED are likely maximum currents; I would try driving it at 20mA on each color and see how it looks.  Your calculations look right to me; 1k would give 18.75 mA per channel.  Unfortunately, the way the driver chip is designed, you get the same current on each channel.


/mike

Offline holland lights

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Re: Right resistor for R7
« Reply #4 on: August 08, 2012, »
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The driver used on the Bobcat servo board is a constant current driver, so you don't use individual series resistors.  R7 is used to set the current drive for each LED.  It's not like the traditional case of a resistor per LED connected to a constant voltage.

The currents spec'ed for that RGB LED are likely maximum currents; I would try driving it at 20mA on each color and see how it looks.  Your calculations look right to me; 1k would give 18.75 mA per channel.  Unfortunately, the way the driver chip is designed, you get the same current on each channel.


/mike
yea I thought R7 had to be changed. Then he said individual resistors and i had to wonder why. Yea the leds may run good at 20ma, just not as bright, but the leds is for the eyes of the skull.

Offline holland lights

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Re: Right resistor for R7
« Reply #5 on: August 08, 2012, »
Well after hooking the leds up all 3 colors seem to work fine running on 20ma. When i am running the red led though; the red led causes channel 1 servo to chatter just a little bit. I dont know if it is because i am running on 5 volts still or not. But I guess somebody has to do it and figure out problems, or else it would never get done :).
« Last Edit: August 08, 2012, by holland lights »