Diode selection to isolate headlight

I am re-wiring a motorcycle that originally had a dual filament headlight. I've changed it to (2) single filament bulbs. My problem is I want to run both lights on high beam and only one on low beam. To do this, I have to run a diode across the low beam to high beam headlight relay to isolate the low beam from the high during low beam application. No one here locally seems to be able to help me select an appropriate diode. Can you help? The low beam bulb is a 20W (12V) and the high is a 20W (12V) for a combined 40W load. What diode do I need to put in the low beam to high beam (relay)? How do you calculate diode size? Where can I order one? TIA, Roger

Diodes have a current rating and a peak inverse voltage rating(piv). The piv will be 12 volts, and your current will be calculated from the PIE formula: P - Power, watts ---- I - Current, amps --- E - Volts P=I x E I= P / E I= 20w / 12v I~ 1.7 amps I generally double this as a safety measure, and for you I would recommed a 5 amp diode. Connect cathode to low beam, anode to high beam.

OK, I did some searching. It seems a 5amp is a Heavy Duty diode and most have a 1000V rating. Could you point me to an actual part at a place I could buy one? Signed, Helpless in Illinois.

Radio Shack - Part No. 276-1661  6A  50v diode