Deluxe A.C. Light Bulb Current Limiter

Convert a $30 Aliexpress Lamp tester into an advanced A.C. current limiter with 4 switched bulbs, fuse, and voltage/current/power meter

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Several sellers on Aliexpress sell an ~$30 advanced light bulb tester with 4 E25/E27 *switched* light sockets, dual wiring-block, plus power switch, fuse, and a digital meter that measures voltage, current, power, and time-on-test. See for example (be careful to select the version with 4 E27 sockets and the power meter).

Converting this to a deluxe current limiter requires only some minor re-wiring (some for safety) plus optionally the addition of a pushbutton "test" switch and a grounded power cord and socket.

Key features are as follows:

  •  The 4 switched bulb sockets allow you to include a geometric series of common bulb wattages (I use 25W, 40W, 60W, 100W) wired in parallel to give a range of current limits (in my case on 110v from about 225mA with just the 25W bulb switched on to about 1.5A with all four bulbs switched on) - choose your bulb wattages to meet your range of current needs
  • The existing lighted (indicator) power switch is SPDT so that it safely disconnects both hot and neutral. Also there is a 5A fuse in series.
  • The dual wiring block (originally wired parallel to the bulbs but converted to be in series ) allows you to connect the load to the wiring block with the magic of current-limiting provided by the non-linear resistance of the bulb filaments
  • You can (optionally) add a standard AC socket (in parallel to the wiring block) to plug in a standard AC-corded device -- I chose also to replace the 2-wire original power cord with a grounded power cord that then allowed me to pass through the ground to yield a grounded socket.
  • The (optionally) added pushbutton "test" button is also wired in parallel to the load (i.e., wiring block and AC socket) allowing you to momentarily test/confirm which bulbs are switched on and to confirm that the circuit is working by "short-circuiting" the load
  • The power meter shows the input voltage, current, power, and power-on time
  • Optionally, one can add an additional voltage AC voltage meter to show the voltage across the load rather than just the input voltage (or you can rewire the existing meter to go across the load -- however, then the meter will cut out when the bulbs light and the load voltage drops below about 40V)

To use:

  1. Determine which bulbs to switch on base don desired maximum current
  2. Turn on the power-switch when ready
  3. Optionally, use the test-button to verify max current flow
  4. Connect load to either wiring block or optional AC socket
  5. Monitor current and power using the meter

  • 1 × Voltage Power Tester LED Quick Test Light Box with (4) E27 Lamp Wattage Tester Light Voltage With Display
  • 1 × Pushbutton switch - NO (normally open), 6A, 5/8" (OPTIONAL)
  • 1 × 16/3 (16 gauge, grounded) extension cord - Approx 8 foot
  • 1 × Miscellaneous short lengths 16AWG stranded hookup wire
  • 2 × Zip ties

View all 7 components

  • 1

    Remove the back by unscrewing the 7 Philips screws (be sure the device is unplugged for safety.

    Before implementing the above changes, I chose to rewire key elements for safety

    1. Make sure the hot wire goes to the base of the bulb. In my case the hot wire went from the thin prong to the switch to the fuse and then to the exterior of the bulb (via the power meter) rather than to the base of the bulb -- this should be reversed -- see later for how it is all wired
    2. Replace the super thin, bare metal wire that daisy chains the bulbs together with insulated 18 gauge stranded wire -- it was super scary to me seeing the loose thin wire (see picture above). Hard to believe that wire wouldn't be near glowing if you used this anywhere near the 5A fuse value.
  • 2
    Re-Wire Loads in *Series* with the 4 parallel-wired Bulbs

    Solder/desolder relevant wires so that the wiring path runs as follows

    1. Hot wire from input Power Cord -> DPDT Switch (top/outer contact)
    2. DPDT Switch (top/inner contact) -> Fuse
    3. Fuse -> 2nd Innermost screw on meter PCB
    4. Innermost screw on meter PCB -> 4-bases of the bulbs (all wired in parallel with 16 gauge wire as above)
    5. 4 sides of the bulbs (all wired in parallel with 16 gauge wire as above)  -> one of the 2 connected Red terminals on the wiring block
    6. One of the 2 connected Black terminals on the wiring block -> Outermost screw on meter PCB
    7. 2nd Outermost screw on meter PCB -> DPDT Switch (bottom/inner contact)
    8. DPDT (bottom/outer contact) -> Neutral wire from input Power Cord
  • 3
    (Optionally) Add "Test" button

    Add a momentary NO (normally open) pushbutton switch in parallel with the load. I used a 6A  pushbutton switch with a 5/8" round base 

    1. Drill a 5/8" hole in the front side of the case next and centered on the terminal block. A step-drill works well.
    2. Secure the button to the case with lock washer and nut as provided
    3. Use short jumper wires to connect one of the pushbutton contacts to one of the Red wiring block terminals and to connect the other contact to one of the Black wiring block terminal.

    Note that pushing the button shorts the load and should light any bulb that is switched on.

View all 7 instructions

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