Post by thread:[EE]: LED current

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. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

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. -- Ned Konz http://bike-nomad.com GPG key ID: BEEA7EFE -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

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 -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

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 -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

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}>How about a D/A converter into a current mirror? > >(view this in notepad or similar, must be monospaced font) > > > +5V > | > | > --- > \ / LED > --- > | R /--------- > | +-----+-----/\/\/\/-----< DAC/PWM | > | | | \--------- > \| | |/ > |-------+---| > 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. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Thx to everone who replied , It works! -----Original Message----- From: pic microcontroller discussion list [spam_OUTPICLISTTakeThisOuTMITVMA.MIT.EDU] On Behalf Of Brendan Moran Sent: Friday, April 04, 2003 2:33 PM To: .....PICLISTKILLspam@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}>How about a D/A converter into a current mirror? > >(view this in notepad or similar, must be monospaced font) > > > +5V > | > | > --- > \ / LED > --- > | R /--------- > | +-----+-----/\/\/\/-----< DAC/PWM | > | | | \--------- > \| | |/ > |-------+---| > 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. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body