Dim Bulb Tester

Building a Dim Bulb Tester

A note before getting started: know what you’re getting into before messing with mains voltages. Do not follow this article if you do not have experience with mains voltages, and even if you do, don’t blame me if you get zapped. 

As time goes on, electronic test equipment has become very advanced, cheap, and available for enthusiasts and professionals alike. This equipment, like oscilloscopes, logic analyzers, function generators, and multimeters, oh my, are all very powerful tools to use. Though, in the day in age of microprocessors, FPGAs, etc. even lower cost equipment is remarkably complex. This complexity is often lost on the user, as they’re using this equipment to design or repair something of equal or greater complexity. However, complexity is not a prerequisite for a powerful piece of test equipment, and that is where the dim bulb tester shines, or doesn’t, depending on what you’re working on.

The dim bulb tester is simple both to understand and to build. The basis of the circuit is to place an incandescent light bulb in series with a problematic piece of equipment; the incandescent bulb being a similar wattage to that of which the piece of equipment draws. Since this bulb is in series with the piece of equipment being tested, if the bulb begins to glow while the piece of equipment, say an audio amplifier, is not under load, this would be indicative of a short, as the equipment is drawing enough current to illuminate the bulb. A popular use of the dim bulb tester is when powering up old analog radios or stereos for the first time, helping ensure the magic smoke is kept in and indicating potential issues. I specifically am building this dim bulb tester to help me repair a Marantz 1060 audio amplifier I purchased broken from an antique radio show.

The parts and tools required for this project are very minimal and inexpensive. I spent approximately $10 on all the parts I needed, and only used very basic hand tools to assemble. I did not need to purchase the appliance cord because I am a packrat that insists on saving cords from broken things. I also did not need to purchase the Romex wiring as a friend is remodeling his kitchen and gave me a length of some of the old wiring (thanks Josh). 

The parts list is as follows: 

  • 2-Gang Electrical Switch Box
  • 15 Amp Toggle Light Switch
  • 15 Amp Duplex Outlet
  • Outlet to Socket Lamp-Holder
  • 2-Gang 1-Toggle/1-Duplex Wall Plate
  • Short Length Romex or 14 Gauge Solid Core Wire (< 1 ft. / ~30 cm)
  • 3-Wire Solid Core Appliance/Tool Cord 

The tools required are:

  • Philips and Flat Head Screwdrivers
  • Wire Cutters/Strippers
  • Pliers
  • Hacksaw (Optional) 

To begin, I removed the nail mounting points from the switch box using a small hacksaw. If you aren’t cheap like me and bought a more expensive box, you wouldn’t have to take this step, but here we are.

Next, it is necessary that the top and bottom outlets of the duplex outlet are electrically isolated from each other. This is done by cutting the metal bus bar connecting the two screw terminals, which will disconnect the terminals from one another. This is done on both sides of the outlet and is illustrated in the photos. I used wire cutters to cut and remove the piece of metal connecting the terminals, but
using needle nose pliers to grip and move the piece back and forth may also
work to weaken and break the metal.

Now comes prepping the Romex to be used for wiring. First, approximately 6 inches (~15 cm) of Romex is cut, and the outer insulation removed. This leaves three wires, white (neutral), black (live), and bare copper (ground). Approximately ¾ inch (~2 cm) of the live and neutral wire insulation is removed on each end of the wire. Now with wires, outlet, and switch box prepped, next begins wiring.

I have created a basic “schematic” (more of a grade school level drawing) of how the dim bulb tester is wired. Note: I did not use a solid core appliance cord for my dim bulb tester, I would recommend against this as it is not as secure of a connection as solid core is. I first inserted the appliance cord into the switch box and tied a knot in the cord so it couldn’t be pulled out. I then wired the live wire from the appliance cord to the bottom terminal of the switch. Then using a piece of white wire from the Romex, wired the other terminal of the switch to the bottom outlet’s live terminal.

The bottom outlet’s neutral terminal is then connected with the black piece of wire from the Romex to the top outlet’s live terminal. The neutral wire from the appliance cord is then connected to the top outlet’s neutral terminal. Now, the bottom outlet will no longer function as a normal outlet, as it will be a pass-through for live current; any component, in this case an incandescent bulb, will be in series with whatever is plugged into the top outlet.

To finish, the ground wire from the appliance cord is connected to the ground terminal of the switch, with the copper wire from the Romex also being connected to the same terminal, and then connected to the ground terminal of the duplex outlet.

The wiring is then bent to conform better to the interior of the switch box and both the switch and duplex outlet screwed in. With the face plate then screwed on, and the outlet to socket converter plugged into the bottom outlet, and incandescent bulb screwed in, the dim bulb tester is complete. Plugging my dodgy Marantz 1060 into the top outlet, pressing the Marantz’s power button, and then flipping on the dim bulb tester led to a dimly glowing bulb. Now to find the issue and fix it…

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