[GreenKeys] [External] Tip on loops

[Bruce Gentry]

The varying resistance of incandescent bulbs has beeen used in many  
ways over the years. One of the more interesting was a volume expander 
for console radios using a bridge of fixed resistors and low voltage 
bulbs between the audio amplifier and loudspeaker. For many years I used 
large light bulbs for troubleshooting relay logic elevator controllers. 
If a 1 or 2 amp fuse kept blowing, I would put a 200 watt light bulb in 
place of the fuse and begin checking things out. It would usually just 
barely glow until an operating sequence powered up a circuit with a 
short. It was far better than blowing a dozen fuses and welding relay 
contacts.  The inrush current of bulbs can be useful as well.  On one 
moderization job, there were several service calls a day, all related to 
bad door interlock contacts. The original interlocks were re-used, but 
they were now carrying 120 volts AC at about 10 MA., instead of  240 
Volts DC at about 2 amps. I added a 100 watt light bulb to load the 
contacts and burn them clean, the service calls dropped to one a month. 
Some sniveling 23 year old in mangement discovered what I had done, and 
ordered the bulb removed. Within a few days, there were several service 
calls a day. I refused to answer them, causing serious conflict. The 
calls suddenly went away. Later, I did go on a call there, and the bulb 
was back in the circuit.  Electromechanical stuff and relay logic 
control circuitry are very interesting, and are becoming a lost 
(supressed) art.


       Bruce Gentry, KA2IVY


On 10/4/19 4:06 PM, Jones, Douglas W wrote:
> On Oct 4, 2019, at 2:28 PM, Jim Haynes wrote:
>
>> ... The reason for this is the temperature
>> coefficient of resistance in the metal filament.  As current increases
>> the filament gets hotter, increasing its resistance and tending to
>> reduce the current.
> I vaguely recall some interesting circuits that I recall seeing in the 1960s that used incandescent bulbs as active elements, exploiting both the non-constant resistance of the bulb and the thermal time-constant of the filament.
>
> 		Doug Jones
> 		jones at cs.uiowa.edu
>
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