Incandescent Light Bulb as Current Limiter

Introduction

After prior experience showed that I need to have an easy way to test short circuit or big inrush current, I decided to make something straight out of electricians’ bag of tricks, that is to use an incandescent light bulb as current limiter.

It works like this:

• An incandescent light bulb is wired in series with the DUT.
• If the DUT has short circuited (or draws current near the bulb’s power rating) the bulb will glow bright continuously.
• If the DUT has big inrush current the bulb will initally glow but then stop glowing.
• The DUT is probably fine if the bulb does not glow at all, or only glow dimly.
• Something funky with the DUT if the bulb is pulsing.

Having this current limiter will prevent mains circuit breaker from tripping if the DUT has a short in it, because the bulb will use all the drawn power. This works because of how the incandescent light bulb works: initially incandescent light bulb is low resistance, but then as current passes the temperature rises (that’s why it glows) and as such the resistance also rises thus dropping the voltage (that’s what prevents the circuit breaker from tripping). If I use a 200W 220V AC lamp, the maximum current flow in the circuit will be limited to approximately 0.9A AC RMS. There’s a better explanation at Instructables (Edit 2021-07-11: Gone, Instructables removed that page and I am unable to find a replacement).

Materials

So the materials required for this project were:

• An incandescent light bulb: 200W 220V AC, E27 screw type.
• A light fixture/fitting: fixed ceiling E27 screw type
• A Schuko socket.
• A Schuko plug.
• Mains-rated stranded cable.

Not required, but these made for nicer result:

• A 3mm thick cardboard piece to bolt everything together.
• Electric wire ferrules to prevent the wire strands from fraying when screwed in place.
• Cable tie to tie the cable to the cardboard.
• Spring-cage splice connector to splice the wires between the light fitting and the Schuko socket. I used a fake Wago 2x2 splice connectors, like the ones in Big Clive’s video. John Ward tested different fake Wago connector’s flammability.
• Screws, washers, and nuts to bolt the light fitting and the Schuko socket to the cardboard. M4 size for the lamp fitting, M3 for the the Schuko socket. The screw pitch doesn’t matter, the length should be at least be 12mm.

The tools I used were:

• Screw driver.
• Pliers.
• Utility knife.
• Scissors.
• Wire stripper.
• Ferrule crimping tool.

Construction

I began with layout: where to put things.

After that I used the utility knife to make the screw holes on the cardboard for the light fitting and the Schuko socket, and tried to screw them in place.

Next I cut two lengths of cable. One length is to connect the Schuko socket to the light fitting. The other length is to connect the light fitting to the Schuko plug.

As I sized the wires to be inserted into the Schuko socket’s screw terminal, I found out that the insulated ferrule’s profile to be too long and will made the wires not fit inside the socket housing. So I popped the sleeve off the ferrules using the pliers. Then I crimped the bare ferrules to the wires and the screwed them to the screw terminals.

Turned out that they were still too long. So I shortened them using the pliers until they fit.

I continued with the other length of cable, the one that had the Schuko plug.

I proceed with the other end of this cable: the light fitting. I striped the wire insulation, crimped the ferrules, and spliced the neutrals together and the grounds together.

There was a little setback. As I was screwing the live wires to the light fitting’s screw terminal, the terminal broke apart.

No wonder, because the I bought the cheapest light fitting I could find. Anyway, I bought a more expensive replacement light fitting which appears to had nicer terminals, and the project continued.

At last moment, I decided that the cables are to be passed behind the cardboard. This was because I wanted the fake Wago connector to be contained inside the light fitting. This meant I had to make big holes on the cardboard for the cables. So I did that.

As it was, the mains cable can be tugged off from the cardboard because nothing was securing it in place. So I made two small holes so I can use the cable tie to affix the mains cable to the cardboard, to prevent it from pulling the wires out of the light fitting when tugged.

The build result could use some more work. The screws and one cable length are sticking out from the back of the cardboard, making the profile unbalanced. This made the cardboard would not stay flat on the table. But for now, I’m calling it done.

Testing

Time for testing:

No Load

With no load, the light bulb shouldn’t glow.

Load: Small Desk Fan (45W)

The light bulb shouldn’t glow, or glow very dimly.

Load: Rice Cooker’s Warming Function (48W)

This should not be any different from the fan test: the light bulb should either not glow, or glow very dimly.

Load: Rice Cooker’s Cook Function (350W)

The power draw exceeded the bulb’s power rating and thus should be current limited. The light bulb should glow brightly.

Tests Shortcoming

• I should measure the voltage drop caused by the bulb during each of these test, and also measure actual current draw.
• All the DUT/loads above were inductive. I should test other kinds: capacitive loads and resistive loads. I also should try other DUT such as a SMPS like a laptop power brick.

I’m not going to do these today, though.

Conclusion

• During testing, it functioned as expected.
• Not enough kinds of loads / DUT for the tests.
• The build quality is awful.
• I consider this project done.

That’s all folks.