The brutal fact is that repairing electrical equipment that is plugged into the an AC wall socket can kill you. It only takes few 10’s of mA to flow through your heart before it stops or through your muscles before they can’t let go. To reduce this risk, you have to understand the hazards, use tools like the isolation transformer and variac, and rigidly maintain safety habits. In addition, the AC plug can supply lots of power that can take a fault and cause explosions, smoke, and fire. I’m sure I’m not the only one that when repairing power supplies or amplifiers have had MOSFETs and diodes explode, capacitors pop and smoke, or PCB traces burn as circuit failures cause the power of the AC plug to be dissipated into the limited size of devices and PCB traces. And this can occur even when using a variac and isolation transformer. While you must always use these tools when repairing this type of equipment, an AC current limiter is a great addition that will help eliminate these additional explosive and fire dangers. In this blog, the design, material and instructions to build a 1Amp rms current limiter with power switch and limit light is presented. Build this current limiter to help you safely repair, without explosions, your next AC plug connected project. As always, the schematic, and BOM are available for you do design and build yourself.

Disclaimer: This Blog works with 120VAC wiring and connections. It is extremely dangerous! You are doing this at your own risk. If this is new to you, research extensively, ask lots of questions of experts, and be vary CAREFUL.

AC current Limiter with Limit Switch Light

AC Current Limiter Design

AC Current Limiter Schematic

This AC current limiter design adds a defeat switch with power and limit switch indicators for both flexibility and simple state display. When both indicator lamps are lit, The power is on to the AC plug and the current limit protection circuit is working. The bottom switch turns on the power and will light up the power on indicator lamp. The upper switch turns on the current limiter and current limit indicator lamp. The two large lamps on the top of the design are in series with the AC plug load and limit the current to approximately 1 amps. This configuration allows the brightness of these lamps to determine how much limiting is being engaged. When little current is flowing to the load, the lamps are off. During heavy current draw like during a short (see the video below) the lamps will light to their rated 60 Watt level. Now in the case of a circuit fault, The power from the AC grid will be dissipated In these two lamps instead of the MOSFET, diodes, or PCB traces of the electronic device being repaired.

Danger! Shorting the line and neutral to test the AC current limit.

Parts and Construction

The parts should all be available at you local hardware store.

  1. Two 60 watt light bulbs
  2. Two lamp mounts boxes
  3. One AC switch
  4. One 3-way AC switch
  5. One AC wall outlet socket
  6. One 14 AWG three wire AC plug
  7. Three AC switch boxes
  8. Ten wire clamps
  9. Two 15 Watt lamps
  10. Two 15 Watt lamp holders with connection wires.
  11. A package of yellow wire nuts for connecting up to four 14 AWG wires
  12. 14-2 electrical wire (14 AWG with insulated line, neutral and non-insulated ground conductor)

The Construction is based on proven electrical safety standards. You should familiarize yourself with these standards and ask lots of questions of experts if you have any questions. Here is a great video for showing the basic of performing safe wiring. Mr. Carlson’s Lab video channel also designed a similar dim bulb tester a couple of years ago. It does not include the AC limit on lamp but includes the variac and isolation transformer into the design. Mr. Carlson’s videos are great and always include lots of safety information! You should also have an electrical wiring book available as a reference. The earth ground connector is connected to each of the lamp and switch boxes to help protect against anything touchable to be live with AC voltage. All electrical connections between the lamp and switch boxes are with 120 AC voltage rated 14-2 or equivalent wiring. Each wire input to the lamp and switch boxes are tightened with the wire clamps, ensuring no movement after assembly.

The best way is go through this is to print out the schematic and map how each of the wires will go from one box to the next. The directions below describe the basic connections.

Power On Switch Box

The power switch box contains the input AC plug, AC switch and connects the AC power on lamp indicator. The three wire (Line, Neutral, Ground) AC plug is brought into the bottom of the switch box. These wires are attached to AC Switch and box with pig tails and yellow wire nuts. The switched line, neutral and ground are then distributed to the Left AC Current Lamp Box. The power on lamp wires are input from the top of the switch box. There is also a single conductor neutral wire that connects to the AC limit on Lamp Indicator and is thread through the limit switch box. Use the yellow wire nuts and wires to connect the wires together According to the schematic. I like to tug on them after attaching the wire nuts to ensure they fit snugly (see this youtube video for example).

Limit Switch Box

The limit switch box contains the thee way AC switch, and AC limiter on lamp connection. The wiring to the switch box comes from the left lamp box. This 14-2 cable bundle contains the switched line, the AC limited line, and ground wire. There is also two conductors from the AC limiter on lamp that will connect to the switched line in this switch box and the neutral to the power switch box. The neutral wires is routed through the bottom of the limit switch box to the power switch box.

Left Lamp Box

This lamp box has a lot of connections and careful layout is required. The Line, neutral, and ground come from the power switch box, and it distributes wires to the limit switch box, the right lamp box and the AC Output Limited Current Socket. The schematic will help you see the wires that are required by each of these boxes, and you must also allow for a ground connector to the switch box itself.

Right Lamp Box

This lamp box receives wire the Line, AC limited line, and the ground connector from the left Lamp box. In addition to connecting the Line, and AC limited line to the lamp box, the ground wire is connected to the Lamp Box.

Testing the Current Limiter Safely

The goal of this project is to provide better safety for your electronic repairs. The irony is that testing the current limiter to ensure you have set thing ups will also be dangerous, so a good plan is required.

Step 1: Use a multi-meter and and ensure all input and output resistance match the switch options. Check that the power switch turns off the line from the AC Output Limited Current Plug. Also ensure that the line connector impedance drops when the limit switch is turned off. Check that each box is shorted to the AC Plug ground connector.

Step 2: Plug the AC current limiter into the isolation transformer and Variac with the voltage turned down and turn on the power switch and limit switch. Slowly raise the variac to 120v and ensure the limit lamps don’t turn on, but the power on and AC limit on lamp indicators turn on.

Step 3: Turn the variac to zero volts and short the line and neutral from the output of the AC current limiter socket. Now slowly rise the variac voltage and you should see the AC limiter lamps start to turn on along with the power on and AC limit on lamp indicators.

After passing this the tests above you should be set to now try to plug it into the AC wall plug. If anything seems off stop and ask questions of your experts.


Final Thoughts

Are you scared to do this project yourself? OK good, because being scared is important. I’ve been working with high voltage electronics for the last 20+ years and I’m still scared. Take this feeling and convert it into being vigilant and learning to use your safety tools properly and when needed. The isolation transformer and Variac are great tools you should always employ. But now you can use your AC current limiter to add to your tools and become even safer. You should always feel scared when repairing AC connected equipment, just use this feeling to remain vigilant. I would love to hear your comments below.