# Light bulb as limit, to what is current limited to?

I was wondering, if I use a light bulb as a current limiter when working with some electronic device, in case of a short circuit - what determines the current that is allowed to pass to the electronic device, how much of it?

Let me quickly elaborate the question.

If the outlet provides 220 V and I use 150 W light bulb, that is about 0.68 A of current.

Lets say our load in this case, an electronic device, needs to draw 2 A to turn on and work normally.

If a short circuit happens, will the current that is allowed to pass to the load be 0.68 A or will it be 2 - 0.68 A, or am I just missing the point completely?

I am just trying to understand how using a light bulb as current limiter works.

• The continuous short circuit current will be 0.68 A. The instantaneous inrush current will be limited to about 12 times that due to cold filament with high positive temperature coefficient. Commented Nov 7, 2022 at 13:13
• This concept only works with an incandescent light. Do not try this with a LED or fluorescent light. Commented Nov 8, 2022 at 0:25

A filament lamp makes an excellent current limiter, for some applications, precisely because of its non-linear resistance, which is typically 10x its cold resistance when it's hot.

If the lamp is rated at, for instance, 150 W at 220 V, it will pass 150/220 = 682 mA when hot, having a resistance of 220/0.682 = 323 ohms.

When cold, the resistance will be in the 30-40 ohms region. If it is tasked to protect a 200 mA load (for instance), then at this current the lamp will stay fairly cool, and the resistance will be nearer to the cold levels than the hot levels, maybe 50 ohms. It will therefore drop around 10 V, much less that you would expect if you used a 323 ohm resistor, which would also limit the short circuit current to 682 mA.

Unfortunately, the lamp takes several ms or tens of ms to heat up, so the initial current into a short circuit is high. It could be up to 10x the lamp rating, if the short circuit happens when the lamp is cold.

Of course a lamp can only be used with a load that's somewhat lower than its rated current.

No, that's not how it works.

Assuming the lamp is a pure resistance (which it is not), a 150W lamp passes 0.68A only if your other device in series is a short circuit. The other device can never have 2A because the lamp limits to 0.68A.

In practice it is more complex, as the lamp resistance is much lower when cold, and when flipping on a light switch, the lamp can take 10x to 20x times the nominal current.

A light bulb is not really a intended for protecting against short-circuits: for those you want to turn off the load altogether, like with a more typical fuse. What a lamp is being used for is, for example, protecting tweeters against overheating. So it is supposed to get hot instead of the protected device, and the more so, the hotter the device could get.

And it doesn't do a hard cutoff (which isn't exactly tweeter-friendly either), more some sort of squeeze-off. The lamp is dimensioned in a manner that it will have lower resistance than the load when cold, and definitely higher resistance when hot, and with a voltage tolerance that it can stand the maximum voltages to be expected in normal use (if it blows for long exposure beyond the absolute maximum ratings for the protected device, that can be a reasonable part of the circuit design, too).

So there is some handwaving around the specs to use. As a rule of thumb, a light bulb at continuous operating current may have about 15 times the resistance than when cold.

This was common in the past when simplest possible electronics was often desired. A bulb has low resistance when cold, so does little impact when the device is functioning properly and consuming moderate only power. If the short circuit happens, the bulb will light up, limiting current to more than before but not too much because when shining/hot it has much more resistance. So all you get is a shining bulb and only need to care that your power source can handle that much. This protection requires your normal current draw to be much much less than the normal current for the bulb.

The bulb, however, will obviously generate a short high current spike before it lights up.