Resistors

Way to determine best series resistor or the number of series connected x-mass tree bulbs. Hook up 3 white or 5 yellow LED's in series with a variable resister adjust source voltage to 15-16 V DC. Use the highest voltage that will be seen when in operation (including charging). At this maximum voltage adjust the pot to give 20 ma current flow for each parallel combination of LEDs. The best range for the LED operation at 12.5 volts is 10 ma (white) or 15ma (yellow) current flow for each parallel combination of LEDs. Measure the resulting resistance and you have the best value to use for this circuit. Chouse the next higher standard resistance value that can be found as available.

If using x-mass tree tungsten filament bulbs then start wiring in bulbs in series until the current maximum at maximum voltage is close to being as recommended (10-15 ma for white). When soldering the x-mass tree bulbs together in series the leads need to be sanded or cleaned and tinned with solder. They are thin and will break easily and are hard to tin. Another way is to continue to use the socket. In this case one solders in series the number of sockets needed. If this is done the contacts with the bulb can be expected to give trouble (make poor contact) with age and wet conditions.

Given a limited number of a given type of mini-x-mass tree bulbs, first find the cold resistance (measure a number of them in series and divide by that number to get bulb resistance) and the operating voltage then attempt to match it with my list (see how to drive LEDs write up). Look up the resulting current for the curve bending point as near to but below the design current. Chose a number of parallel series combinations that you think might work and test it. Next adjust the number of bulbs in series until 15-16 volts input gives the intended limiting current flow. If too many bulbs are needed then add more LEDs in parallel if the bulb glows too brightly or near operating voltage then drop the number of LEDs in parallel. In this way one can tune up the result to be a minimum number of x-mass tree bulbs being needed and maximize the life of the x-mass bulbs.

Offered by Mike.