When Radio Shack announced that they were closing stores, unfortunately the one in Cupertino (De Anza Blvd.) was one of them. Early this year, one of the clerks in that store told me that March 31st , 2015 was the last day. I made a few visits buying solar cells, boxes, amplifiers, transformers, and incandescent lamps.
Radio Shack has one of the most extensive varieties of small filament lamps. Now why would anyone buy lamps today instead of LEDs? Lamps have nice nonlinear resistive properties that are useful in automatic gain control systems.
I managed to buy all sorts of different low current incandescent lamps whose current draws were less than 100 mA. In particular, the Radio Shack part number 272-1141 is rated at 12 volts, 25 mA.
Although the days of using lamps for automatic gain control systems are essentially over since there are other ways to achieve this, it would be fun to build an oscillator with these types of devices. One of the first uses of a lamp was to stabilize a Wein oscillator’s amplitude that provided low distortion sine waves. We saw this in the 1930s when Hewlett and Packard sold their innovative audio oscillators to Walt Disney for the movie “Fantasia”.
When I thought about writing this paper, I look back to 1952. This is when Dunford A. Kelly filed for a patent pertaining to a phase shift oscillator including an incandescent lamp.
See Figure A.
I started with a simple high pass filter phase shift oscillator as shown in Figure B.
Capacitors C1 to C3 and resistors R1 to R3 form a three section leading phase shift network with about 60 degrees of phase shift per RC section. The values were chosen to be 10x less in capacitance from a succeeding section to another. This approximates as if each section includes a x1 buffer amplifier between each CR section. For example, C1 and R1 is “buffered” from C2 and R2. The approximate oscillation frequency is then:
Fosc ∼ (1/√3)(1/2 π τ )
Where τ = R1C1 = R2C2 = R3C3 = 0.15 uf 820Ω = 123 μ Sec
Fosc ~ (1//√3)(1/2 π 123 μsec) = 747 Hz
The measured frequency was ∼ 700 Hz using 5% capacitors and resistors.
Because there is no amplitude stabilization in this circuit, the “best” adjustment yielded the following distortion-wise at about 12.75 volts peak to peak with some clipping on the negative cycle of the sine wave:
HD2 = 1.00%
HD3 = 0.60%
Now let’s insert a Radio Shack incandescent bulb to provide gain control and hopefully lower the sine wave distortion. See Figure C.
Using the Radio Shack part # 272-1141 (12 volts at 25 mA), and adjusting VR1 to about 6.75 volts peak to peak, all distortion products were less than 0.0055%. In comparison to the circuit Figure B, the incandescent lamp version in Figure C is about 180 times lower in distortion.
So there you have it, a nice low distortion sine wave oscillator using a Radio Shack incandescent lamp. Hopefully, Radio Shack will still have stores opened, including their Website.