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Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
Here, for your amusement, is an unusual electrical demonstration I made a few years ago. What should we call it? It combines synchronous rectification, induction motors, and frequency mixing.
Synchronous rectifiers, increasingly popular in low voltage switching power supplies, were widely used over 100 years ago. DC-motorized trains and streetcars could run on grid power by reversing the connections each half-cycle. Even today it's hard to beat the voltage drop of brushes on a synchronously rotating commutator.
Today's demo uses a neon lamp with two D-shaped electrodes as a polarity indicator. The glow forms around the negative electrode, but in AC service both sides appear lit because of persistence of vision. A commutator with only one conductive segment, and one brush, switches the lamp on and off once each rotation. Duty cycle can be adjusted by axial position of the brush. (I made a less ugly commutator last weekend for the video.) This could rectify the current. Except here, it's spun by an induction motor that slips. The switching frequency is only 59 Hz, so the lamp polarity alternates about once per second. The dees blink back and forth like signal lights at a railroad crossing. Here it is in action. (Please let me know if a smaller file or Youtube posting would be better.)
The effect is achieved without semiconductors.
This was my first project enclosed in a plastic Akro-bin. The containers stack nicely, and their back panels have hooks for rack-mounting. Unfortunately, factory-molded covers don't seem to be available. To bring in AC power without permanently dangling cords, I used "small" IEC receptacles.
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Nearly Synchronous Rectifier (Frequency Mixer?) Demo
