This video describes, in detail, how to construct a Current Limiting Device to protect you, your test equipment, and any electronic gear that you are testing or repairing. When used properly, it prevents short circuits from causing additional damage, particularly to expensive power transformers in amplifiers. It is also quite useful when first powering up previously untested and vintage equipment. It is not suitable for use with medium to large electric motors.
The bulb functions as a high-wattage resistor in which the resistance increases with the increasing heat of the filament. A "cold" bulb may offer only 2 or 3 ohms of resistance (the higher the wattage, the lower the resistance), but this will rapidly rise as the filament heats, particularly in the case of a short circuit.....protecting the circuit and dissipating the resulting filament heat in the form of light (warning you of the existence of the short).
IMPORTANT NOTE: The wattage of the bulb must be tailored to the current draw of the device being protected. The high-wattage bulbs (200W+) recommended in the video are primarily for Tube Amplifiers, which operate at relatively high current levels. With lower-current devices, such as Solid State amps, a lower wattage-higher resistance bulb (even as low as 40W) may be required for circuit protection.
For viewers in areas with 220-240VAC power: Although your voltage is double (220VAC vs 110VAC), your current flow is about half. Since the Current Limiter (CL) depends on current to rapidly heat the filament and increase its resistance, you need a bulb with higher filament resistance. Since the filament resistance of the bulb decreases with higher wattage, you should try a lower wattage, higher resistance bulb.....i.e. a 100W......initially, and then fine tune the wattage depending upon the power demands of the amplifier being protected.
Also, in my wiring diagram within the video, the "hot" AC lead is connected to the threaded socket of the receptacle, which could possibly be touched while changing bulbs. Some viewers have suggested that shock exposure could be reduced if the "hot" lead went to the contact at the bottom (center) of the socket. Please consider this when wiring your own Current Limiter.
The Photoflood Bulb used in this video is available on Amazon. Here is a link: http://www.amazon.com/Eiko-Supreme-Photoflood-Light-Bulb/dp/B000TZQUF0
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