Yes, I was aware of the cable losses, and where I was thinking LM317 at=20 the end of cable, I do have 2 of those MPJA regulators per the link that=20 I have recently got. Thanks again for the input. On 10/19/13 11:10 AM, Denny Esterline wrote: > Missing in the discussion is the non-trivial resistance in the cable... > 24Ga wire is variously reported at about 0.025 ohms per foot. 1000' down > and 1000' back and you have 50ohms of added resistance to the circuit. > If I'm reading your description right, 130mA for the camera and 50mA for > LEDs, 50ohms*180mA, you'll be loosing 9 volts to the cable alone. I'd be > surprised if this works at all. Probably getting by at the moment because > you're only using 320' of cable and doubling up the pairs - which cuts yo= ur > cable loss to about 1.5 volts. > > I'd recommend increasing voltage and putting a switching regulator on the > end. > > I tripped on this one the other day > http://www.mpja.com/LM2596-Step-Down-Adjustable-15-37V-DC_DC-Converter/pr= oductinfo/30148%20PS/ > Probably > not the "best" depending on your metrics, but rates high in the "easy", > "cheap for a one-off", "small enough" and "available off the shelf" > categories. > > -Denny > > > > On Sat, Oct 19, 2013 at 7:33 AM, RussellMc wrote: > >> LEDs should (~~=3D must) be driven by constant current. >> Trying to drive them with ~=3D their notional Vf will lead to a substant= ial >> variation in most cases. >> When operating on 12V you should have only 3 LEDs in series, plus curren= t >> 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 balanc= e. >> 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 consta= nt >> current source taking up the difference. You can use as little as an LM3= 17 >> 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 o= ne >> out per group and probably get a better result. >> >> If these are on only occasionally then 'any brand will do", almost. If t= hey >> 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 lice= nce >> to them. LED lifetimes are still woefully low in many cases - despite >> claims to the contrary. >> >> >> Russell >> -- >> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist >> --=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 .