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Transformers vs Voltage Regulators
Transformers (also called “AC/DC adapters”) are very popular and Realist, View-Master, etc., supplied them with their viewers. These transformers have a voltage and current rating. For example, most transformers from the 50s are rated 2.5V @ 500mA. These really output 300mA and will only work well with a No. 14 bulb which operates at 300mA. The No. 245 bulb operates at 500mA and will cause a voltage drop, so it will not be as bright as it could be, when used with the 300 mA transformer.
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| Most transformers from the 1950s are rated 500mA but do not support 500mA at 2.5V. This figure demonstrates how the voltage of a regular transformer (from a View-Master) drops as more current is drawn. The halogen bulb (drawing 0.8 A of current) would operate at only 1V if powered by this transformer. On the other hand, a voltage regulated supply maintains a constant voltage independent of the current drawn. |
The situation is even worse if you try to power the 800 mA halogen bulb with one of these transformers. Extremely dim light with the transformer supplying only about 1 V to the bulb (see Figure above). The answer to this problem is to select a transformer with a current rating that matches that of the bulb. Use a 2.5V @ 800 mA transformer with the halogen bulb (but don't try it with the No. 14 bulb because you will blow it right away!)
A better solution is called voltage regulation. Through the use of electronic components it is possible to have a voltage output independent of the current (see Figure above) and you can also control this output very well. I make a line of voltage-regulated power supplies (PS1) with variable output. The advantage of these units is that they can be used to power any 2.5 V bulb.
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This custom-made device uses an AC transformer and through voltage regulated electronics outputs 1.2-3.0V at 0.8mA. The voltage is variable and controlled via a rheostat. The output voltage is independent of the current drawn by the light bulb. As a result, this unit can be used with any light bulb. |
Updated: February 2003