The "dim bulb tester"Pieter-Tjerk de Boer, PA3FWM firstname.lastname@example.org
(This is an adapted version of part of an article I wrote for the Dutch amateur radio magazine Electron, February 2022.)
A dim bulb tester is often used by people who repair old radios: connect an incandescent lamp in series with the equipment under test or repair. If the equipment has a catastrophic fault, e.g. a shorted mains transformer, then the lamp will light brightly, without blowing a fuse. On the other hand, if the equipment is fine and draws much less current than the lamp would nominally do, then the lamp will hardly light up, its filament will not get hot, so its resistance stays low, allowing the equipment under test to get almost the full mains voltage. Effectively, the light bulb acts as a current limiter.
A typical schematic of such a dim bulb tester is in the figure. It doesn't amount to much: a light bulb (of the incandescent type) and a power socket, wired in series. The two switches are not necessary, but handy: one to take the bulb out of the circuit if it turns out the equipment plugged into the socket is working fine, and the other to switch off the whole thing. Of course, many extras can be added, such as indicator lamps, volt and ampere meters, and switching between multiple bulbs with different power ratings.
Judging by pictures on the internet, the usual way of construction is mounting the switches, the power socket and a bulb socket on a wooden plank. Mine looks different though, see the picture. It's mostly built from two items from a local home improvement store, costing less than 10 euros together. One is a ready-made box containing a socket and two switches; this one is intended for outdoor usage, is also closed (waterproof) at the backside, and made from a soft type of plastic. The other item is a bakelite E27 lightbulb fitting with a mounting bracket, which can be attached to the box with a simple screw. Adding a few wires finishes the tester. It's a pity though that the switches are single-pole, so one still needs to unplug the equipment under test before touching it. But that's a safe habit anyway.
To better judge how the tester works, I measured the current/voltage characteristics of two incandescent lamps. Have a look at the (red) line of the 60 W bulb. At 230 V it draws 0.255 A: indeed, 230 × 0.255 = 58.65 W, almost exactly 60 W. Now suppose the current is reduced to 0.1 A. If the lamp were a normal resistor, it would still drop 90 V (dotted line). But the actual bulb (solid line) drops only 30 V, leaving 200 V for the equipment under test, almost 90 % of the full mains voltage. For most equipment this would suffice to function normally. But if the equipment tries to draw more current, the voltage across the bulb increases quickly, and more than 0.255 A is completely impossible as then the full mains voltage is across the bulb.
This numerical example already shows that one should use a bulb of the appropriate wattage. With the example 60 W bulb the equipment under test can still draw some 0.1 A and thus use some 20 W. This leads to a rule of thumb: the bulb should have about three times the wattage of the equipment under test. If the equipment under test is not drawing current continuously but only during the peaks of the AC voltage, the same principle holds, but the numbers are different: the bulb's filament temperature (and thus its resistance) is then still determined by the effective value of the current, but the voltage drop is determined by the peak current.
Of course, this principle only works with real incandescent bulbs. Fluorescent and LED lights behave totally differently at low voltage. So it's advisable to preserve a few incandescent bulbs when one replaces them by modern energy-efficient lighting. As an alternative, halogen bulbs can be used: those are alse incandescent, and not (yet?) outlawed.
Connecting a lightbulb in series may seem an "amateur" approach, but it has also been used by professionals. In the 1960s, Tektronix manufactured the TU-75B "Variable Power Source Test Unit", to safely check their oscilloscopes before they left the factory . It contained three switchable bulbs, of no less than 250, 500 and again 500 W.