Nuts and Volts - September 2013

READER-TO-READER TECHFORUM

Electrolytic caps are leaky (measured in microamps), have internal resistance that may be significant in some applications, and vary with temperature (±20% typically) and frequency (not useful at high frequency).

After six or more months on the shelf, aluminum electrolytic caps should be re-formed to regain their voltage rating.

Tantalum caps are also an electrolytic but smaller than aluminum, lower leakage, have a better temperature coefficient, and operate at higher temperature. Cost is more than aluminum but does not de-form (lose its voltage rating) with non-use.

There are other capacitor types to consider: mica, polypropylene, metalized polyester, polystyrene, and paper. Each has pluses and minuses to consider.

Russell Kincaid Milford, NH

[#8133 - August 2013]

Antenna Length

Can someone explain how adding inductance or capacitance to an antenna changes the length?

An antenna becomes resonant when the energy that is racing down the wire hits the open end and is reflected back to the sending end; the transit time is equal to the time of one cycle of frequency. The open end has high voltage and low current (there can’t be any current at the open end), and the sending end has high current and low voltage. Adding inductance at the sending end will lower the resonant frequency, thus making the wire appear longer. Adding capacitance to the open end will also reduce the resonant frequency. Since radiation occurs from the wire, adding these L and C elements will reduce the antenna efficiency because it is shorter than it would be if the L or C were not added.

Russell Kincaid Milford, NH

[#8134 - August 2013]

Sine Wave

I would like to know how you can get a positive and a negative part of a sine wave from a circuit that runs on a nine volt battery.

You ask how to get a positive and a negative part of a sine wave from a circuit that runs on a nine volt battery. It is not clear how you are going to use that output, so I'll presume this is to satisfy your curiosity (or perhaps to help on your homework). The circuit suggested is not efficient, and would not make a good power supply.

First, a nine volt battery produces direct current, DC, with a steady amplitude of nine volts (gradually decreasing as it is drained). One simple circuit that produces a fair approximation of a sine wave from DC is a phase shift oscillator, shown below on the left side (adapted from www.learnabout-electronics.org). The sine wave can be observed between the Out and the 0V test points.

Second, the sine wave must be separated into positive-and negative-going signals, which is done by the half-wave bridge rectifier. The two half-sine waves can be observed between the + or - test points and the 0V test point. The waveforms are shown below, (adapted from http://macao. communications.museum). The negative half-wave should look like the second graph flipped upside-down.