Pro/Con

A series resistor to limit LED current has the following advantages and disadvantages.

Advantages:

1. Given the right size resistor it is simple to implement.
2. Is relatively small and non-breakable.
3. Can be used safely when operating current does not need to be controlled over a broad range.

Disadvantage:

1. Straight line characteristics make for energy inefficient current protection.
2. Should not be used when "design to" voltages are over a broad range. Has the least protection for the high current end of all of the 3 driver methods tested. Because primitive environments need to operate in broad voltage and current ranges this approach becomes the least workable one.

X-mass miniature tree bulbs used as variable resistor to limit LED current have the following advantages and disadvantages.

Advantages:

1. Is the most energy efficient method of driving an LED to produce light. Allows more light in usable current range while protecting a bit better than a resistor at the high end.
2. Is more likely to be available after a PS than resistors.
3. Will last a long time at reduced currents before burn out.

Disadvantages:

1. More prone to developing corrosion at the socket to bulb mechanical connection. Recommend soldering when one can do this. They do need special cleaning if soldered.
2. One needs to match typical current flow to be greater than bend in resistance curve. Takes a bit of trial an error.
3. The bulbs are fragile and can break if not protected.
4. Current surge when cold is more than a resistor and may in the long run slightly shorten LED lifetime.

The use of an LM-317 integrated circuit in a constant current circuit configuration driven by a wide ranging voltage values as a driver circuit for LEDs has the following advantages and disadvantages.

Advantages:

1. Will work up to voltages of 37 volts and still hold the current to a given maximum depending the value of the control resistor.
2. Low cost and easy to implement. Provided some chips are purchased and stored before the PS.
3. This is by far the safest most protected way to drive LEDs. Keep the current below a given value for all input voltages.

Disadvantages:

1. They use a bit of energy taking in about 2.3 volts of overhead. This is the minimum drop across the unit that is needed to control current. Thus they are not the most energy efficient way of driving LEDs.
2. They would not be readily available after a PS. Would need to purchase them before.

Availability:

LM317LZ (small size no heat sink) Jameco# 23552 cost $.23/each
LM317T (bigger with heat sink) Jameco# 23579 cost $.45/each
Go to Jameco h and type in the part number.

Resistors can be ordered from Digikey. As examples:

82 ohm 82H-ND cost $1.89/100
160 ohm 160H-ND cost $1.89/100
110 ohm 110H-ND cost $1.89/100

LEDs can be ordered from Ebay. Search for "white led mcd 100" or whatever color you want. Some of the Highest MCD are the newest or the narrowest beam angle. These are not necessary the longest lasting. To determine lifetime one needs to ask detailed questions to the seller. Even then it is not that easy. Good luck.

Offered by Mike.