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Lightbulb Limiter High-Power

Why

In many cases, a power-consuming device has to be protected against overcurrent; alternatively, the current flow has to be stabilized on certain reasonable value. Lightbulbs have a heated filament.

In other cases, it is necessary to sink a significant amount of electrical power. A resistive load that transfers the electricity into waste heat is then desired.

And sometimes the powers necessary to be handled aren't just few dozen to hundred watts anymore - kilowatt range of power is needed.

Lightbulbs are especially suitable for current limiting; the filament, when cold, can be over ten times less resistive than when hot. When more current is forced through the filament connected in series with a load, it heats up, its resistance increases, and it takes progressively higher share of voltage, lowering the voltage across the load, until a steady state is reached.

What

A Lightbulb Limiter was already built earlier, based on the E27 socket lightbulbs. This is however suitable only for low power operations; the available lightbulbs aren't exactly powerful.

Luckily, higher-power lamps are available. The cheapest ones at hundred watt to kilowatt range are the R7S linear halogen lamps. It was decided to build the device around these lamps. They come in several standard lengths corresponding to the lamp's power; in a rough ballpark, 78mm for lowest powers, 118mm for 120-500 watts, 189mm for 1000 watts, 254mm for 1500 watts, 331mm for yet higher powers.

Support for 118 and 189 mm lamps was chosen, allowing configuration ranging between 3x100W to 3x1000W. Typically the lamps used will be 2x500+1x1000W, or 230+500+1000W.

Design

Circuit

An IEC 60320 C14 power socket was chosen as input due to easy availability of the corresponding power cables. It is rated only for 10 amps, but the load at 3 kW is about 13 amps, which may be acceptable for shorter period. A pair of 19mm-spaced binding posts was also added.

For the output, a lidded CE 7/5 socket, with the left pin designated as L and right as N, was chosen. A pair of binding posts was added as well, to facilitate attachment of unexpected devices.

The R7s sockets are mounted on a custom-made steel holder, with option to configure each position for either 118 or 189mm length.

The main power switch, the switches for enabling the individual lamps (all connected in parallel), the lamps bypass switch and the selector between acting as a series limiter or a resistive load are mounted on the lid.

The lamp bypass switch is interlocked with the load/limiter selector to not allow lamp bypassing into load function, which would equal a hard short of the input.

Modification of design

The design was field-tested. The original positions of the binding posts were found to be wrong, as they were poorly accessible. There was also missing attachment for an ammeter, which turned out to be sorely needed.

The binding posts were moved to the front panel. Another pair of the posts was added, together with a bypass switch, to facilitate attaching an optional ammeter.

The holes were cut into the panel without fully stripping the box, using aluminium foil stuffed into the box as protection against the beam. The beam however still managed to scorch a few wires, but the damage on the insulation was not full-thickness and was judged just superficial.

Holes for the original binding post positions were covered with custom-made 3d-printed plugs.

Bug and further mod

A bug was discovered during documenting. The added ammeter connection would not function in load mode, the terminals would be shortcircuited. The ammeter posts will have to be rewired.

The schematics are ugly and are scheduled for being redrawn properly.

Connection box

An electrical installation case was chosen for the user interface housing. The Marlanvil 008.PL box was chosen on the basis of availability and suitable size, 150x110x70 mm.

The material is an electrical-grade ABS.

Cutting holes

The holes on the top and side panels were drawn in OpenSCAD, exported to DXF, and cut on the laser cutter. The laser cutter was modded earlier by grinding out a large hole in the bottom to allow insertion of taller objects. This allowed insertion of the entire box bottom, so the sides can be cut. The cutting required several passes, at higher speed (12-15cm/s) and lower current (8mA) to reduce overheating of kerf sides and the corresponding widening and spreading of its edges.

The material produces vapors when cut, which tend to get ignited and produce copious amounts of soot. The flame extinguishes immediately after cessation of the beam. The soot tends to soak into the surface of the plastic and stay there and be difficult to remove.

The laser cutter has tendency to sometimes run the laser and move over the bed diagonally, then grind against the stops. This can damage the workpiece when not avoided and not caught on time.


Lasercutting input side

Lasercutting input side, showing diagonal line damage

Lasercutting output side

Lasercutting output side

Lasercutting output side

Lasercutting lid

Lasercutting lid

Lasercutting lid

The position of the output socket was a little high, interfering with the lid. Two millimeters of the lid were manually ground away using a handheld Dremel mill. A little piece of reinforcing/mount bar on the bottom had to be ground away to provide space for socket mounting nut.


Output side lid cut

Output side lid cut

Output side lid cut

Output socket

Output socket

Output socket

Modifications were needed to be made; they were performed on an assembled box.


Lasercutting front panel mod

Lasercutting front panel mod, clamp view from bottom

Lasercutting front panel mod, clamp view from bottom

Lasercutting front panel mod

The laser caused some superficial damage on the wires.


Laser scorch marks

Lightbulb holder

The R7s sockets needed to be mounted on some support construction. This was quite a problem, as specific spacing was needed in two different distances, and the sockets came as loose ones with a M4 screw built in but protruding only a short distance.

For attaching the sockets a steel 20x20mm square tube was chosen. To accommodate the protrusions on the ceramic body of the socket, cutouts were made on the tubes with a band saw, as pairs of tightly spaced parallel cuts.

Holes were drilled and M4 threads tapped for the holding screws.

A frame for mounting the socket holders was made from 10x10mm square tubes. Holes with M3 threads were used to hold the assembly together, and for attachment to the connection box. The horizontal beams were made longer than strictly needed for the socket holders, in order to facilitate later attaching of a protective cage.

The holding tubes were attached to the frame using pairs of M3 screws.


Lightbulb socket holder, slit being cut

Lightbulb socket holder

Lightbulb socket holder

Lightbulb socket holder

Lightbulb socket holder

Lightbulb socket holder

Lightbulb socket holder

Lightbulb socket holder

Lightbulb socket holder

R7s socket

R7s socket

R7s socket

R7s socket

Sockets on holder

Sockets on holder

Sockets on holder

Sockets on holder

Sockets on holder

Sockets on holder

Sockets on holder

Sockets on holder

Sockets on holder

Sockets on holder

Sockets with bulbs

Sockets with bulbs

Wiring

The wiring was done by wires soldered to the switch contacts. A better way would be using crimped blade connectors ("fastons"). The lightbulb sockets were connected using a length of electrical terminal strip.


Assembling the lid

Assembling the lid

Wiring

Wiring

Wiring

Wiring

Wiring

Case labels printout

After first field test, the binding posts were moved to the front panel, and the ammeter attachment pair and a bypass switch were added.


Ammeter bypass switch

Modded wiring

Modded wiring

Front panel

Front panel

A problem was discovered during documenting. The ammeter was connected after the lamps, so it was shorted and nonfunctional in the load mode. It was moved between the main power switch and the lamp switches.


Ammeter correction wiring

Ammeter correction wiring

Ammeter correction wiring

Ammeter correction wiring

Ammeter correction wiring

Final assembly and tests

The unit was tested in the mode of artificial load, first on a variac, then directly on mains. The lamps when run on full voltage are BRIGHT and HOT.

Adhesive aluminium foil was attached to the lamps-facing side of the connection box to provide at least some protection against radiated heat.

Covers for binding posts were designed and 3d-printed to avoid accidental touch of live metal connectors.


First assembly

First assembly

First assembly

First assembly

First assembly

First assembly

First assembly

First assembly

Aluminium foil for heat reflection

Lit-up assembly

Lit-up assembly

Lit-up assembly

Lit-up assembly

Lit-up assembly

Finished assembly

Finished assembly

Finished assembly

Finished assembly

Finished assembly

Finished assembly

Finished assembly

Finished assembly

Field deployment

The device was tested on a prototype electric car, as both a load (for discharging a 100V battery and for sinking power from an engine controller) and a series current limiter (for charging test, using motor controller as a rectifier).


Field deployment, battery load

Field deployment, battery load

Field deployment, battery load

Field deployment, variac charging

Field deployment, variac charging

Field deployment, variac charging

Field deployment, variac charging

Field deployment, variac charging

Field deployment, variac charging

Field deployment, variac charging

Field deployment, variac charging

Field deployment, direct charging

Field deployment, direct charging

Field deployment, direct charging

Field deployment, direct charging

Caveats

At full power, the rig produces a copious amount of light and heat. The amount of light can be downright blinding. A shield may be needed.

The rig heats up fast. The lamps were run for only few dozens of seconds at time, so major issues were not encountered. The construction can however be expected to warm up considerably. A cooling fan may be needed.

The lamps may have to be mounted further away from the box to avoid heat damage. The cover will need to have an integrated heat reflector.

Files

DXF files for panel cutting (caveat, left and right have wrong holes for binding posts)

TODO


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