I'm building some LED based lights for my work trike, intended as safety daytime running lights, but with the ability to be used as headlights at night.
Using a pair of LM317s in constant current configuration, with the input source switched between them.
Would a diode be needed on the output to prevent reverse current damage to the inactive current source, like this? Yes, I know a diode will add an additional voltage drop of ~1.7V to the circuit, but given the power is being supplied by an 8Ah SLA, I think I have the headroom available, since I am driving a trio of 1W Osram type white LEDs wired in series with a typical voltage drop of 3.2-3.6V
Low beam would be configured for 90mA and high beam for 250mA.
I set up a test bed for 3v0's circuit, and found that 60mA -->320mA gives me the effect I want.
Resistor values are 17ohm for 60mA, and 5ohm for 320mA.
This keeps me comfortably below the max constant current for my 1W LEDs on a freshly charged battery. This is with the ammeter in the circuit, so I would expect a very slight difference without it.