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Giant Light Bulb

[Giant Light Bulb]
Giant Light Bulb

I was at a radio swap meet one day, when I spotted a box full of unusual old light bulbs. I knew that these were likely to be in demand, as I was aware that people collect them. However, this proved not to be the case, and at the end of the day I picked up the whole box for $2.

Among the contents of the box was a 500W globe, about 275mm tall and with an E-40 (GES) base. High power bulbs like this were fairly common before the introduction of fluorescent lighting, but they are now no longer manufactured, and most people my age have never seen one.

I decided to make a stand for the bulb, so it could be displayed and operated. As the filament would have a limited life, I decided to power it from less than its 240V nominal supply. A soft start circuit was also designed to minimise the risk of burning out the bulb.

[Light bulb stand]
Light bulb stand

Finding a suitable socket for the bulb was a bit of a problem, but I eventually located one from RS Components, part # 171-051 ($30 - ouch!). This was installed in a wooden mounting block. The underside of the block was hollowed out for the power switch, and passages were also drilled for the wiring.

[Power supply circuit]
Power supply circuit

For the power supply, I used a 240-120V transformer to reduce the voltage to the lamp. At this voltage, its life should be virtually unlimited. The illumination is also reduced to a more manageable level, as the full 500W would be far too bright! To provide a soft start function, there are also two 100 ohm resistors in series with the bulb. When the light is first switched on, the filament is cold and has a low resistance, so most of the supply voltage is dropped across the resistors. As the filament warms up, the voltage across it rises, and eventually this becomes sufficient to engage the relay connected across the bulb through the 10k resistor and rectifier. This shorts out the dropping resistors and feeds the full 120V to the bulb.

If you want to use this circuit, you will need to experiment with the values of the current limiting resistors, and also the dropper resistor for the relay, in order to obtain satisfactory operation. If either is too high in value, the soft start may never disengage, and the series resistors will quickly overheat in this situation. I used a thermal fuse installed adjacent to the resistors to guard against this possibility, which could also occur in the event of low mains voltage.

As the control circuit was to be installed separately from the lamp, it was not practical to have the switch on the lamp control the transformer primary directly. However, I wanted the primary disconnected when the lamp was off, so it used no standby power. Therefore, I used a relay powered by a 9V battery to switch the primary circuit. Once the transformer is on, the relay is supplied from the 6.3V winding via a voltage doubler, to save the battery capacity. One problem with this arrangement is that if the lamp is switched on without a power source connected, the battery will go flat. Therefore, I put an electrolytic capacitor in series with the battery, so it would only supply a short pulse of current to energise the relay. If power is connected to the circuit, the relay will then latch on.

[Light bulb with power applied]
Light bulb with power applied

The light bulb is not very well suited for general illumination, as it is a point source, and is located directly in people's field of vision. Additionally, it uses around 150W at 120V, while the light output is only about that of a 40W incandescent globe. However, it is an interesting object to look at, and it gives an insight into an older lighting technology, now that incandescent lighting is being phased out. Although the LED lighting products on the market now are quite impressive, the don't really have the same appeal.

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