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Lightbulb Limiter



In design and repair of mains-powered hardware, there comes the inevitable moment when the device has to be powered up. This is the most critical moment; any mistake or failing part can cause higher than intended energy consumption, leading to release of magic smoke, spark and flame effects, and tripped breakers. As such surprises are costly and generally unpleasant, something that limits the maximum current through the device would be an advantage.

This something, a trick well-known between old-school repairmen, is a humble lightbulb. (An alternative for the hapless serfs in European Union, where lightbulbs are on the index of gradually banned technology, is the HEATBALL®.)

Lightbulbs are load-dependent resistors. The resistance of the filament depends on its temperature, while the temperature depends on the current. A cold filament is about an order of magnitude less resistive than a fully hot one. A lightbulb that's loaded to only a fraction of its nominal current will present very low resistance; the resistor divider between the bulb and the device under test will have most of the mains voltage across the device. When the device's load gets higher, the current through the bulb increases, the temperature of the filament grows, its resistivity increases, and the bulb takes higher proportion of the mains voltage, not just limiting the maximum current through the circuit but also taking greater proportion of the consumed energy; the energy that'd otherwise be all deposited in the faulty parts of the tested device.


A tool was designed to facilitate easy testing of mains-powered devices. The unit contains three lightbulbs (their values can be arbitrarily chosen to suit the applications) and several switches that allow selecting the desired combination of the lightbulbs; single one, or a series or parallel combination of two or all three.

In addition, the tool contains a selectable inrush current limiter (a NTC thermistor, rated to 20 ohm/5 amps), a built-in ammeter, and a pair of terminals for attaching an external ammeter or other device connected in series with the load; these terminals can be bypassed with a switch. The entire lightbulb set can be also bypassed by another switch, for the last test when the operator is confident enough about the tightness of the magic smoke conduits.

Additional switch was added as an afterthought, to turn the unit into a dummy load. (The motivation here was the number of cases when the unit was used with the output terminals shorted, with just the lightbulbs used as a resistive load or power-on indicator.) The switch also acts as an interlock for the load-bypass switch, so both can not be engaged at the same time (which would make the unit act as a short circuit); when it is engaged, the load-bypass current path is interrupted.

For output, a standard European power socket was placed on the back panel, together with a pair of terminals on the front panel. This allows attachments of both finished devices with power cord, and individual components/boards with only bare leads.

The ammeter used was the cheapest panel meter on local market, originally with a 3-amp scale and shunt. The shunt was removed, the resistance of the meter was measured, the voltage across the coil needed for full needle deflection was empirically determined (connected in series with 1 kΩ trimpot and the trimpot adjusted to full deflection using 5V from the USB port, then the resistance of the trimpot as-set was measured, and the voltage across the meter calculated. A range for the meter was chosen (0.8 amps, picked up as a middle-of-the-road suitable value sufficient for most cases). The value of the shunt was calculated and cut from a length of manganin wire. The meter was then connected in series with a 5V power supply, an adjustable resistor, and a digital multimeter, and the current was adjusted to values between 0 and 0.8 amps, with 0.1 A step. A mock-up scale with preprinted angle markers was used and ticks were made on it, the angles then were used to print out a meter scale by a PHP application.

Caveat: Due to a mistake in thinking, the meter was calibrated with DC current and used with AC current. This makes it display the average value instead of the RMS value, which is about 10% lower. As the meter is only an indicator-grade, the error is at this moment considered acceptable; for more accurate measurements, an external ammeter should be used anyway.

It turned out that the lightbulbs are too mechanically exposed. A protective cage was therefore designed. A wire mesh was chosen as a compromise between mechanical integrity, cooling, ease of access, weight, and material availability. Due to availability issues a device for assisting soaking paint rollers with paint (a 4mm-diameter wire frame with a mesh of thinner, about 1mm thick wires) was used as the source of the material. The mesh was welded only at its ends, to the frame; the wires themselves were loose, only intertwined. For the original purpose it did not matter; however for a wire cage this could be fatal, as after cutting off the supporting frame it could fall apart. A recently built spot welder was used for welding the mesh together at its crossings, along the future edges and the bends. The frame was then sawed and the required part of the mesh was cut out. The outermost longitudal wires (two on one side, one on the other) were removed. The sheet was bent twice in a vise, forming the cage profile. The protruding wires from the sides were bent to the inside, forming the sides of the cage. Where needed, the wires were prolonged by welding stubs of wire to their end. Wires were weaved to a mesh, forming the side panels, and welded at each crossing for increased mechanical integrity (due to not optimal design of electrode holders of the welder, some points were rather difficult to access). The protruding ends of the remain of the 4mm frame were used for hinges; two small iron bars were used for the matching hinge parts; stacks of M4 washers were used to adjust the precise width of the cage axis to prevent slipping out of the hinge. A small latch was mounted on the top side of the device, to hold the cage closed while allowing its easy opening for replacing or adjusting the lightbulbs. The cage and the hinges were spray-painted black for aesthetical reasons and to protect the welds against corrosion (the zinc coating vaporized from these areas during welding).



Schematics and truth table

Front view

Back view

Inside view from top

Back side view

Back panel - front side

Front panel - back

Front panel, back side view

Front panel - left back side

Front panel - right back side

Front panel, right back side view

Front panel - left back view

Front panel, right back side

Front panel, left back side

Ammeter dial

Bulb cage

Cage welds, right side

Cage welds, left side

Cage welds, left inner side

Cage welds, right inner side

Cage assembly

Cage hinge, inner side

Cage hinge, outer side

Cage latch

Top view

Front view

Open cage

Closed cage

Limiter, top and front side

Limiter, front side with labels

Limiter, back side

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