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Count number of LEDs and divide by 3 then multiply by 20 ma (3-5mm) for white and 30 ma for Amber to get the maximum current that the unit should use. Design the circuit to draw about half this or 10-15 ma as a max per each LED. Thus for 9 LEDs draw should be no more than 30ma (white) to 45ma (amber) and, for 12 LEDs 40 ma (W) 60 ma (A), for 19 LEDs = 70 ma (W) 105ma (A). If you're chosen LED draws more than this amount at 14 Volts than a resistor in series will defiantly be needed. I have found that typically a 51 ohm ¼ watt will work in most cases. Another very good way to determine resistance needed is to measure the current at about 14 volts and adjust a series variable resistor such that the current ends up be about half of what was measured. Then measure the adjustable resistor and replace it with the nearest fixed resistance standard value.

Note: A word to the wise, I have not had good luck with running LEDs close to rated power. They quickly fade and lose brightness. As a rule of thumb always use more LEDs and design them to run half current where you can. As a result they will become more energy efficient at producing light and save power and end up lasting two to three times longer than at the rated current. The only exception to this rule is short term use emergency task flashlights. These can be run at .15 ma for a max rating of about 20 ma and .22 ma for a maxim rating of 30 ma. The thinking is the mechanics of the flashlight and or the batteries will ware out well before the LED. Use this where light weight or minimum components is desired.

The following table shows my experience with tail light bulbs. Any 12 volt bulb will work it doesn't have to be a one pin type. If two level of brightness bulbs are used they can be wired in parallel if needed to gain more light. Or one can use the lower light output terminal on multi-terminal bulbs if this gives plenty of light (it will defiantly save power).

LEDs	Draw 	light output	Description
No.	ma   	ma 		Tested at 13.6 volts
12	132 	12.5	 	1156 white purchased 4 years ago. 
12	44 	4.8	 	1156 white purchased 4 years ago after 51 ohm
12	8.5 	.83	 	1156 white purchased 4 years ago after 510 ohm

12	34	4.5		1157 white purchased 4 years ago low side.
12	144	14.6		1157 white purchased 4 years ago high side.
12	42	4.4		1157 white purchased 4 years ago high side (51 ohm).
12	7.7	0.7		1157 white purchased 4 years ago high side (510 ohm).

12	94	14.5		1156 White JDM no resistor
12	49	9.2		1156 White JDM after 51 resistor
12	8.3	2.0		1156 White JDM after 510 resistor

19	143	7.0		1156 Amber no resistor
19	74	3.8		1156 Amber after 51 ohm resistor
19	14.4	0.5		1156 Amber after 510 ohm resistor

9	78	10.7		1156 White plastic cone no resistor 
9	42	8.3		1156 White plastic cone after 51 ohm resistor
9	7.9	0.9		1156 White plastic cone after 510 ohm resistor

For a low setting use a 510 ohm resistor. It will use 10 times less current and power. The apparent light brightness between high and low is about one third or one half as bright. A little light at night can be used as nearly as good as a lot of light. Use only what is needed to save power.

Offered by Mike.