This is one of the most essential safety devices you must have if you work with electronics. Not only for the safety of the device you are making/troubleshooting but also for you and your electric installation. It is a famous device - the light bulb current limiter also know as dim bulb . The construction is very simple, that's only a high wattage bulb connected in series between your device and the wall. Here is the schematic: I recommend to use a high wattage filament bulb, like a 150w or 200w, usually the rule is to use a bulb with a higher power than the device do be tested. Please not that the wattage must be the real and not the "equivalent" as we find in modern fluorescent lamps. It is mainly used as a "detector" for short circuits: if your device has a short it will draw a lot of current from the mains and the bulb will be as bright as it can, limiting the current because itself will convert the energy into light and hot. If the device has no shorts, th