I'm planning to get some Luxeon star/O's... fo those i need some current limiting (i want to go a bit safer than the normal resistor) i was wondering if this circuit will work well (with that value of the resistor) (drawn in a hurry.. hehe) i have seen similar circuits with the LM137 (IIRC) (even one here on bit-tech) but i already have the regulators handy, so... will it be correct to calculate the res with 5v/0.35A=~14 ohms...?
What you're suggesting is called a constant current source. You have the right idea but your resistor is in the wrong spot. The GND pin on the regulator connects to the anode (+ pin) of the diode and the resistor goes in series between the VO pin of the regulator and the anode of the diode. So your output current will be... Iout = 5V/R1 or R1 = 5V/Iout like you said. There will be some leakage current, say 5mA or so, from the GND pin on the regulator the will add to the Iout current. So the 14.2Ohm resistor will give a theoretical output current of 350mA but the LED will get 350mA plus the 5mA, so it will get about 355mA. Nothing major but something to keep in mind if it might be important. Also your supply voltage will have to be at least 5V plus the forward voltage drop of the LED. Hope this helps. And by the way, is the 350mA correct or did you mean 35mA
oh! i se the mistake i made now... thanks BTW: 350mA is correct http://www.elfa.se/elfa/produkter/no/7277.htm thanks for the help
You might want to note that the 78L05 regulator is the 100mA version. You want the 78M05, 7805 or 78S05, otherwise you won't be able to run your Luxeon at 350mA (although I'm not sure if the current handling applies to the regulator being used in this configuration)
Good eye!! If you indeed have the 100mA version you can buffer the output with an NPN transistor, then you can pull as much power as the transistor can handle.
i thought the L series was the 1,5A versions... and M 2A... and the ones without a letter was the 100mA versions... ohwell... i have lots of these regulators (all 2 ratings (i think) and many different voltages (even some of the 79 series (negative))
For reference: L series - 100mA (easily identifiable since they aren't heatsink mountable) M series - 500mA S series - 2A
Couple of questions: A. You meant "The GND pin on the regulator connects to the cathode (- pin) of the diode and the resistor goes in series between the VO pin of the regulator and the anode of the diode." right?... B. Just a little reminder, resistance in an ohmic material is not dependent upon current, so you can not say "R1 = 5V/Iout". While a technicality, it is important to note. -special [k]
No, biff was correct. The resistor is connected between the GND pin and the output pin. The GND pin is then connected to the anode of the LED, and the cathode of the LED connected to the true 0V.
