Light Bulb Current Limiter




Introduction: Light Bulb Current Limiter

About: I am a Software Engineer, (formerly) Electronics Technician and practitioner of all things Making. I am very interested in robotics, but I also love building and fabricating things. I am also very interested…

*Disclaimer: I am not an electrician, I am simply documenting the process I took to make this Current Limiter. Please do not attempt this project unless you are comfortable working with high voltage electricity.

This project is to make a Light Bulb Current Limiter used to test electronic devices powered by line voltage. I am formerly an electronics technician where I used one of these on a daily basis to help troubleshoot electronic circuits.

When researching this build I found a particular good explication of this type of circuit by Jack A Lopez in the discussion of this post.


  • Qty 1: Incandescent light bulb (I used a 40 watt, but this would change depending on your application)
  • Qty 1: 2 gang metal junction box
  • Qty 1: 1 gang metal junction box
  • Qty 1: 4" fixture cover raised 1/2" (circular mud ring)
  • Qty 1: Outlet plate
  • Qty 1: 1/2" x 1” conduit nipple
  • Qty 2: 1/2” locknuts
  • Qty 1: 3/8” non-metallic twin-screw cable clamp
  • Qty 1: Switch/Outlet combo
  • Qty 4: 3/4” wood screws
  • Qty 1: 3 prong power cord
  • Qty 1: 3/4" Plywood or MDF (approximately 7" x 5")
  • At least 18" of electrical tape
  • At least 12” each of black and white 14AWG wire


  • Drill driver
  • Philips head screw driver
  • Flat tip screw driver
  • Hammer
  • Pliers (lineman's, Channellock)
  • Wire cutter (aka: dykes or side cutters)
  • Wire stripper
  • Utility knife
  • ¼” drill bit
  • Philips head drill bit

Step 1: Connect Junction Boxes

Remove the center knock-outs on each box using a hammer, flat tip screw driver and a pliers.

Add the locknuts to the conduit nipple, taking care to leave an adequate clearance once the switch is installed.

Tighten them down with a conduit locknut wrench, flat head screw driver and a hammer or a small channellock pliers.

Step 2: Secure Junction Boxes to Wood

Center the junction boxes and install onto the piece of wood using four 3/4" wood screws.

Step 3: Install Light Socket to Mud Ring

Removed plastic from inner screw mounts from the light socket by drilling out the holes with a 1/4" drill bit.

Clean the edges of the mounting holes with a utility knife.

Screw the light socket to mud ring using provided screws.

Step 4: Wire Up Light Socket

Cut a 12" section of each black and white 14AWG wire.

Strip the end of the wire approximately 3/4" of an inch using a wire stripper.

Bend the exposed ends of the wires into a U shape using a pliers.

Secure the wires to the light socket making sure the wire is going clockwise around the screws. This will insure that as you tighten down the wire it will get pulled tighter into the screw. The white wire should be terminated to the silver screw and the black wire terminated to the gold screw.

Step 5: Install the Power Cord

Measure back approximately 5" from the cut end of the power cord and gently score the jacket of the cable. Do this using your wire cutters by applying light pressure and rotating the cutters around the cable. You can also do this step by using a utility knife and rotating the cable on a flat work surface under the blade. Your goal is to break through the jacked of the cable but NOT score or scratch the jackets of the wires inside. Once you have broken through in one spot, you can stop scoring.

Now that you have broken all the way through the jacket in one spot and scored the rest of the cable jacket, you can bend the cable back and fourth around the scored line. The jacked of the wire should continue to break along that line until completely cut all the way around. You can then remove the jacket by pulling off of then of the cable.

Inspect the wires to make sure you did not score or cut the jackets of the wires in the process.

Your cable might have a foil shield, a small wire run along that shield as well as some paper/plastic (used to help make the cable more flexible). If so, you can cut those pieces off, we only the black, white and green wires. *Note: Some cables use a brown, blue and green/yellow color scheme. I these cases brown = black [Hot], blue = white [Neutral] and green/yellow = green [Ground].

You can now strip back the ends of each of the wires approximately 3/4" of an inch using the wire strippers.

Install the 3/8" non-metallic cable clamp to the conduit box using a pliers or a flat tip screw driver and hammer to tighten it down. Make sure the heads of the screws are facing up.

You can now install the cable into the clamp. Make sure the clamp is over the jacket of the cable. Screw down the clamp alternating from one screw to another to apply even pressure on the cable. Tighten down until firmly secured but not crushing or damaging the cable.

Step 6: Wire Up the Light Switch and Receptacle

On the outlet/switch combo, there may be a tab between two of the screws on the hot side of the receptacle. That tab is there to allow you to only need to connect one hot wire to both the switch and the receptacle. In our case, the circuit is different so we need to cut that tab out to allow us to put our light bulb in series. Using a pair of side cutters, cut away the tab connecting the to two hot terminals. Make sure to cut as much of the tab away that the manufacture has allowed for, this is to ensure there is enough of an air gap so the electricity cant arc across.
*Note: this tab is there as a convenience to the installer for most typical applications but is intended to be cut away if not needed.

From the black and wire wires coming from the light socket, measure approximately 6 inches of wire from the inside of the box and cut off any excess using a wire cutters.

Strip back the ends of each of the black and white wires 3/4" using a wire strippers.

Bend the black and white wires in a U shape as we did in Step 4 using a pair of pliers.

Twist the ends of the stranded wires coming from out power cord so that there aren't any stray strands.

Connect the wires to the switch/outlet as shown in the circuit diagram. Make sure to bend the wires clockwise around the screw terminals as we did in Step 4 so that they become more secure as you tighten them down.

Wrap the outlet in 2 layers of electrical tape, making sure that there are no terminations exposed.

*Tip: Do not stretch the electrical tape too tightly around the switch. If the tap is stretched, it may shrink over time which would just cause the tape to pop off. Instead of adhering the tape as you are unrolling it (causing it to stretch), unroll it a little ways and then gently adhere it to the switch/outlet.

Step 7: Attach Switch, Switch Plate and Mud Ring to Junction Boxes

Depending on the switch plate you have, you may need to remove the little taps or ears that would normally help hold it flush with the finished surface. If so, simple just use a pair of pliers and rock them back and forth along the scored edges.

Attach the switch/outlet to the junction box using the provided screws.

Attach the outlet cover to the switch/outlet using the screw provided.

Now, you can finally attach the mud ring and light socket to the the other junction box.
*Note: I did these two steps backwards in the video so I had to unscrew the mud ring in order to install the light switch.

Step 8: Test the Current Limiter

Its finally time to test the circuit (so that you can test a circuit)!

Install the light bulb into the light socket.

With the switch in the off position, plug the end of the power cord into an outlet on your wall supplying our circuit with power.

You can now flip the switch to on. If you'd done everything correctly to this point nothing will happen (including tripping a breaker in your circuit box). This is because our circuit through our light bulb isn't yet complete.

Now, turn off the switch and unplugging the power cord from the wall.

You can now take a short section of left over wire and strip both ends using a wire stripper. Carefully insert the wire across the hot and neutral terminals of the receptacle.

Plug the power cord back into the wall and making sure to be clear of the wire (even more distance than I show in the picture) you can turn the switch on. If everything is wired correctly, this should operate like a normal light switch and bulb at full brightness.

Now, turn off the switch and unplugging the power cord from the wall.

You can now remove the piece of wire from the receptacle.

Step 9: Testing Our Electronic Device Using the Current Limiter

Plug your currently limiter into the wall.

Make sure the switch on the current limiter is off and then plug your electronic device into the current limiter.

Now, with your device off, turn on the current limiter, supplying power to your device.

You can now turn your device on and watch the light bulb to see how bright it gets. The more current your device draws, the brighter the light will get.

For the example in the video, I was using a guitar amp I built myself. In the video I turned on the amp, the light bulb glowed and then slowly faded out. This is exactly what I expected to see when testing an amplifier like this. The initial glow was caused by and inrush of current as it was charging the power capacitors. As, the capacitors were fully charged the current draw went down which caused the light bulb to dim. Good news for me as it showed I didn't wire anything wrong enough to cause a dead short (which would cause the light bulb to glow at full brightness).

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I just built this, which will be a great help in rebuilding old amplifiers. I had a few comments.

I used a switch and outlet unit with the decora rectangular format, which only fit in the 2x4 box with some finagling. I put an o-ring on the connector on the 4x4 box side, which created a bit more space in the 2x4 box. Alternatively, one could use two 4x4 boxes.

I put in a bare wire from the outlet ground to one of the screws holding the box to the wood piece to be absolutely sure the boxes were grounded.

I found it helpful to understand that the neutral wire coming from the lamp becomes the hot wire for the outlet.

For the final test, I used a fan rather than a bare wire shorting the outlet.


2 years ago

I suppose if the bulb is burned out and still in the socket then the apparatus will no longer provide any protection?

If so, then there is a need to check the bulb each time before you use it... in that case the simple wire plugged into the outlet inputs would be the test to do each time?


Reply 2 years ago

That is correct, the bulb is acting sort of as an in-line fuse


2 years ago

This is great! I will definitely make one for myself.
Thank you for sharing :)


Reply 2 years ago

Thank you! Hope you do. Post pictures if you do.


2 years ago

Very well made!!!! and Neat!!!!
This is an great oldtime trick I learned back in my amplifier repair days. I've also used hot water heater elements. These can also act as inrush limiters with a time delay relay to short across it after its limited the inrush.


Reply 2 years ago

Nice suggestion, I've never heard of the hot water heater trick. I'll have to try it!

Works by Solo
Works by Solo

2 years ago

Sweet project! Welcome to Instructables!