On using a light bulb as a current limiter (and crude current meter)

If you search on Google for “light bulb current limiter” you should come across several articles, newsgroup and forum posts about the details of building one. (i.e. )

Essentially, you connect a incandescent light bulb in series with an outlet, so you can safely test a device that might be prone to blow a breaker, or draw so much current as to release magic smoke.

One common feature of many of these articles is the idea that the light bulb will “blow” if the device draws too much current. This is totally and completely false.

When a 60-watt light bulb is connected directly to mains, it will draw approximately 60-watts of power, or 500 milliamps at 120-volts. That is the maximum amount of power that it will permit to flow, and it will burn at full brightness.

When you connect a device in series with the light bulb, it is effectively acting as a resistor to the current through the light bulb. With a 60-watt bulb, a device that used 250-milliamps wired in series would cause the bulb to burn at about 50% brightness (although light bulb brightness as a function of current is non-linear so this is not entired true, this is just an example…)
Now let’s imagine a worst case scenario – what could be wrong with an electrical appliance that would cause it to draw the maximum amount of current? Why shorting the power cord of course.

If you short the power cord of a device attached in series with a light bulb, the circuit is 100% IDENTICAL to plugging the light bulb directly into the wall. Nothing blows up, nothing catches fire – the bulb limits the circuit to 60-watts.

So if you are testing an electrical device using a lightbulb current limiter, please be aware: the bulb will not stop the power if too much is used. It is up to you to know how much power your device should be using, selecting a light bulb of the correct wattage to prevent it from going above (or much above) this, and turning off the power if the bulb is burning at full brightness when it should not be.