Lm317
Do a search at national semiconductors site for the LM317. In the data sheet they so how to use it as a constant current source. I think it's rated at 40v thou I don't think it would matter because in their fore mentioned schematic it is not connected to ground(regulating current not voltage). By the way, I use this set up to charge nicads...Works good.
Do a search at national semiconductors site for the LM317. In the data sheet they so how to use it as a constant current source. I think it's rated at 40v thou I don't think it would matter because in their fore mentioned schematic it is not connected to ground(regulating current not voltage). By the way, I use this set up to charge nicads...Works good.
, which is not acceptable.
Linear Tech has an application note for "Current Sources for Laser Diodes" -- you will find schematics, the entire shooting match for CCS using both linear and switched regulators.
If it is a design where "metrics" are important you will probably want to consider using the Linear LT1533 slew controlled switching chip with post regulation.
by the way, Linear now sells their stuff, fewer than 500 pieces, direct to the DIY'r -- and prices are about 40% less than Digikey -- shipping is via FedX.
If it is a design where "metrics" are important you will probably want to consider using the Linear LT1533 slew controlled switching chip with post regulation.
by the way, Linear now sells their stuff, fewer than 500 pieces, direct to the DIY'r -- and prices are about 40% less than Digikey -- shipping is via FedX.
And by the way...
And another thing, a linear regulator would be just fine because "40V supplies have plenty current to spare". I mean.. designing a 21watt constant current (SWITCHING!) supply for a LED is not worth it, and why constant current anyway? If a constant voltage is put across a LED, a constant current would flow thru it. nuff said.
And another thing, a linear regulator would be just fine because "40V supplies have plenty current to spare". I mean.. designing a 21watt constant current (SWITCHING!) supply for a LED is not worth it, and why constant current anyway? If a constant voltage is put across a LED, a constant current would flow thru it. nuff said.
Re: And by the way...
it isn't correct to assume that a constant voltage impressed upon a LED will result in constant light intensity. firstly, not all VF's are equal, secondly, the current in a led is also a function of the LED temperature (didn't I point this out a few days ago?).
you can check out the Agilent or Allegro Microsystems websites for reasons to the above.
a 21 watt constant current/constant voltage supply isn't like splitting atoms -- there are even PCB layouts on some of the manufacturer's websites -- and you can have both constant current and voltage control at the same time.
nine76chris said:
it isn't correct to assume that a constant voltage impressed upon a LED will result in constant light intensity. firstly, not all VF's are equal, secondly, the current in a led is also a function of the LED temperature (didn't I point this out a few days ago?).
you can check out the Agilent or Allegro Microsystems websites for reasons to the above.
a 21 watt constant current/constant voltage supply isn't like splitting atoms -- there are even PCB layouts on some of the manufacturer's websites -- and you can have both constant current and voltage control at the same time.
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