I want to shorten my christmas light string. The string is a standard US mini-light string of 50 lights. Each light uses 2.4-2.5V. The string runs off of a standard 125V AC plug. It is wired in series. I want to replace some of the bulbs with resistors. I found that a 50 Ohm wirewound power resistor will equal about 3.5 bulbs. The wattage rating on the resistor is 10V. Does each resistor Volt rating need to be equal to the 125V from the outlet, or just the bulbs it is replacing (in this case 7.5V)?
I wonder if those 'hot' resistors will set something on fire?? How about a Variac? Transformer? Perhaps use a light dimmer. Glue the knob at the 'before if blows' point.
Recheck the string before proceeding. If removing one lamp extinguishes the entire string then it's a single string. There may be more than one circuit, ie 2x25 series lamps in parallel.
At turn-on, the string impedance is quite low. A wirewound resistor will likely be capable of meeting the >10x power surge, so long as it's got welded terminals and isn't one of those cement-filled ceramic jobs, which have little or no overload capability.
Most low value (ohms) resistors will have a votage ratings well in excess of that which would result in over-wattage. The 10W ceramic cased jobs mentioned earlier, for example, typically have voltage ratings in excess of 300V. Note the continuous power rating in watts (W).
Peak pulsed power rating for this type is seldom greater than 3x rated. They do not generally fail in a catastrophic manner, however, they simply fuse open circuit. Welded metal-glazed parts can have pulsed power ratings 10x rated.
You may find that the lamp life is extended, without objectionable reduction in intensity, if you can reduce their operating voltage stress to 75% of the original intended value, by using a larger resistance value than is required for simple direct substitution.
I've done this with larger strings that operate year round to reduce trips up the ladder.
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