LEDs should (~~=3D must) be driven by constant current. Trying to drive them with ~=3D their notional Vf will lead to a substantial variation in most cases. When operating on 12V you should have only 3 LEDs in series, plus current control.. When operating with 24V you can have up to about 6 LEDs in series plus current control. The more "headroom (aka spare voltage) that you have the easier it is to get ~=3D constant current with simple circuits. It is exceeding bad practice (also exceedingly bad practice) to attempt to drive two parallel strings of LEDs without any attempt to current balance. I have seen some LEDs that are so well matched for Vf that you could 'almost get away with" doing this, but most LEDs have enough variation, even in the same batch, to make this problematic. LEDs should be driven at MORE than their rated combined Vf with a constant current source taking up the difference. You can use as little as an LM317 and one resistor as a constant current source. You'll need about 3.5V "headroom" for an LM317 to operate as a current source. eg for 4 LEDs. V min ~=3D 3.8V x 4 =3D 15.2V. Headroom =3D 3.5V say. Vinmin=3D 3.5 + 15.2 >=3D 18.7V. ie ~=3D 24V would be dandy. Vin =3D LM317 Vin Iout =3D LM317 ADj. Rcc from Vout to Vadj. Rcc =3D 1.25 / I A =3D 1250 / I_ma Dissipation in R =3D~ 6/R or 1.25 mW per mA eg for 25 mA. R =3D 1.25 / .025 =3D 1250 / 25 =3D 50 Ohms. Pd_Rcc =3D 50 x 1.25 ~=3D 63 mW. If you have 4 LEDs in series and this is unchangeable, you could short one out per group and probably get a better result. If these are on only occasionally then 'any brand will do", almost. If they are intended to spend thousands of hours on over their lifetime then you should use LEDs from the major LED manufacturers OR ones made under licence to them. LED lifetimes are still woefully low in many cases - despite claims to the contrary. Russell --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .