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PICList Thread
'[EE]: LED current'
2003\04\04@142154 by Pat Smith

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Hello,
Im trying to make a variable Constant Current source into an led. I was
going to use a D/A converter into a Voltage to Current  OP-Amp. The
problem Im having is the anode of the LED is hard wired to 5 volts. I
cant see how to make the current constant when the anode is hardwired.

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2003\04\04@142959 by Ned Konz

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On Friday 04 April 2003 11:08 am, Pat Smith wrote:
> Hello,
> Im trying to make a variable Constant Current source into an led. I
> was going to use a D/A converter into a Voltage to Current  OP-Amp.
> The problem Im having is the anode of the LED is hard wired to 5
> volts. I cant see how to make the current constant when the anode
> is hardwired.

So use an NPN or NMOS transistor from the cathode of the LED to the
current sense resistor; the other end of the current sense resistor
goes to ground.

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GPG key ID: BEEA7EFE

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2003\04\04@144934 by SavanaPics

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Need a was to calculate the resistor calue.  Look nearthe bottom of the site
and you will fin a resistor calculator for LEDs  I found it by accident while
designing a colorimeter

<A HREF="http://www.aaroncake.net/electronics/math.htm">Electronics Calculators</A>

Eddie Turner, kc4awz

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2003\04\04@152225 by Brendan Moran

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At 01:08 PM 04/04/2003 -0600, you wrote:
>Hello,
>Im trying to make a variable Constant Current source into an led. I was
>going to use a D/A converter into a Voltage to Current  OP-Amp. The
>problem Im having is the anode of the LED is hard wired to 5 volts. I
>cant see how to make the current constant when the anode is hardwired.

How about a D/A converter into a current mirror?

(view this in notepad or similar, must be monospaced font)


  +5V
   |
   |
  ---
  \ /  LED
  ---
   |                       R          /-----
   |         +-----+-----/\/\/\/-----<  DAC |
   |         |     |                  \-----
    \|       |   |/
     |-------+---|
  E /|           |\ E
   |               |
   +-------+-------+
           |
          ---
         ///

I_R should be very close to I_LED

If you don't care about the accuracy, you can use two normal transistors,
but since this is a very temperature dependant circuit, if you want more
accuracy, you'll need to get the two transistors on a heatsink together, or
buy a current mirror in a single package.

Depending on the current draw of the LED, you might also want to have a
resistor between the LED and the transistor, though at normal LED currents,
it shouldn't be necessary.

--Brendan

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2003\04\04@153403 by Brendan Moran

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Olin has a point, (His post was mistagged) a PWM (even without the low pass
filter, since we're talking about the human eye) would work fine in place
of the DAC.  I feel that an opamp is overkill for this application.

--Brendan

{Quote hidden}

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2003\04\05@004603 by Pat Smith

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Thx to everone who replied , It works!

-----Original Message-----
From: pic microcontroller discussion list
[spam_OUTPICLISTTakeThisOuTspamMITVMA.MIT.EDU] On Behalf Of Brendan Moran
Sent: Friday, April 04, 2003 2:33 PM
To: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
Subject: Re: [EE]: LED current


Olin has a point, (His post was mistagged) a PWM (even without the low
pass filter, since we're talking about the human eye) would work fine in
place of the DAC.  I feel that an opamp is overkill for this
application.

--Brendan

{Quote hidden}

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