Electrical theory 1.4 - Light bulb strings

In this section we'll discuss light bulb strings so you can easily repair them.

A light bulb glows incandescently and the filament operates around 600oC. So keep the bulb away from flammable material.

Light bulb strings can be series or parallel connected.

 

Series:

• As shown below, only one wire connects between lights. The second wire runs to the last light for the return path for current.
• Light bulbs are low voltage (2 – 10V) and must be replaced with same voltage and power ratings. Otherwise, it’s a possible fire risk or some lights will be dull or overbright.
• A flasher bulb may be used to turn lights on/off/on… Replace the flasher with the same type otherwise it may burn up or flash too slowly. Flashing reduces the light bulbs’ lives.
• Light bulb has no polarity connections and can be tested with DMM on resistance (range 200Ω); its usually between 2 to 10Ω.
• If a bulb fails, all the bulbs (in the series group) do not light.... except when,
• Some 110V light bulb strings contain a 'shunt in each light bulb. When the bulb's filament goes open, the parallel shunt shorts the light bulb, it stays dark but the remaining lights turn on brighter - refer to the instructions supplied with the light. The shorted light bulb should be replaced ASAP because the remaining light bulbs will fail sooner. Note: sometimes this shunt fails to short and all lights are out.

 

 

 

 

 

 

 

 

 

 

 

Parallel:

• Two wires connect between each light as shown below.
• Bulbs are usually larger and their resistance is around 200Ω each for 120V.
• If a bulb fails, only it does not light. The replacement bulb should have the same voltage and power rating otherwise it will have a different brightness.

 

 

Fuses:
Some light strings may have 1 or 2 fuses in the AC supply. The fuse(s) which are located inside the plug, are intended to "blow" when an overload occurs (eg too many extension light strings plugged in) or there's a major insulation failure such as a person accidentally stapling through the wire. Most strings supply spare fuses which can be replaced in the plug.

SAFETY: only use the same type and rated fuse otherwise you can create a fire or explosion hazard!

 

 

 

 

 

 

 

 

 

 

 

 

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