I first came across this circuit when I was looking for a way of driving LEDs over a voltage range from 5 volts to more than 30 volts. My specific case was to drive an opto-isolatator from a source that could vary widely; no single resistor value would provide satisfactory operation over the possible voltage range.
This simple circuit, consisting of two NPN transistors and two resistors solves the problem nicely. It doesn't get much simpler than this.
Ok, it can get a little simpler. Dual transistors in an SMT package are available, like the MMDT3904_R1_00001 from LCSC, which is available for JLCPCB assembly. Other similar dual NPN transistors are available.
It can't get much simpler than this! Or can it? In a recent thread, rjenkinsgb mentioned the ON Semiconductor NSM4002MR6 Dual NPN Transistors for Driving LEDs. This SC-74-6-packaged part is specifically designed for this application. Where making the necessary connections on the above dual transistor package is a bit untidy, the pinout of the NSM4002MR6 arranges the pins in a convenient order, and makes some of the connections internally. Up to 45 volts (plus the voltage drop across the load) may be applied, with the current delivered as much as 200mA. If the LED is an opto-isolator, this is a simple, wide-range driver. The NSM4002MR6 is available for about 12 cents a piece at qty 100.
Whether this circuit is implemented with discrete transistors, a general purpose dual-NPN package or the NSM4002MR6, it's a handy trick to have in your arsenal.
This simple circuit, consisting of two NPN transistors and two resistors solves the problem nicely. It doesn't get much simpler than this.
Ok, it can get a little simpler. Dual transistors in an SMT package are available, like the MMDT3904_R1_00001 from LCSC, which is available for JLCPCB assembly. Other similar dual NPN transistors are available.
It can't get much simpler than this! Or can it? In a recent thread, rjenkinsgb mentioned the ON Semiconductor NSM4002MR6 Dual NPN Transistors for Driving LEDs. This SC-74-6-packaged part is specifically designed for this application. Where making the necessary connections on the above dual transistor package is a bit untidy, the pinout of the NSM4002MR6 arranges the pins in a convenient order, and makes some of the connections internally. Up to 45 volts (plus the voltage drop across the load) may be applied, with the current delivered as much as 200mA. If the LED is an opto-isolator, this is a simple, wide-range driver. The NSM4002MR6 is available for about 12 cents a piece at qty 100.
Whether this circuit is implemented with discrete transistors, a general purpose dual-NPN package or the NSM4002MR6, it's a handy trick to have in your arsenal.