Another constant-current circuit

Here's a constant-current LED driver circuit meant for use with wall-wart power supplies

The general idea is much like the boost-mode regulator, but with an adjustable linear regulator instead. The voltage across R1 is 1.25V, and the dropout voltage is around 2V, so if the forward voltage across one strand of LEDs is V, then the wall-wart should be at least V + 3.25. If the wall-wart is not sufficiently regulated, then the value of C1 may need to be increased.

Any number of strands of LEDs can be connected, but one strand must always be connected at X3-4 or regulation won't take place. The Vf of each strand must be equal, but you could mix different LED types in one strand (for instance, 2 strands each with 1 red, 1 green and 1 blue LED).

Since the voltage between ADJ and OUT is regulated to be 1.25V (and Iadj is small, typically 50μA), the current is determined by the value of R1. 1/10W resistors suffice to 80mA or so.

Current R1..R4
15mA 83.33 Ω
20mA 62.50 Ω
25mA 50 Ω
30mA 41.67 Ω

Entry first conceived on 19 March 2005, 17:55 UTC, last modified on 15 January 2012, 3:46 UTC
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