A Simple Zener Diode Circuit

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I looked at the output of simple circuit composed of a reversed biased 14 volt Zener diode and a 10K resistor, connected in series across a DC power source. If the voltage source was less than the Zener breakdown rating then the voltage at the diode (Vz) is the same as the source voltage and there is no AC waveform at the output. As the source voltage approaches the Zener breakdown, a series of little sawtooth pulses can be see at the output that are of a few mV in height. Increasing the source voltage to about 14.7, the number and the height of the pulses increase to a maximum peak to peak of about 200 mV.

Viewing the output on a Sain Smart DDS140 Digital oscilloscope, I can see that the output is a sawtooth pattern with a rise time of about 2 microseconds and a more rapid fall time of 80 nanoseconds. The slope of the rising edge is dependent on value of the resistor. The frequency does appear to be random, in that peaks beginnings and endings are at irregular intervals and voltages. The scope has a fast Fourier transformation FFT display window that shows a fairly flat frequency spread  between   KHz and 6 MHz with predominant frequencies about 1 MHz. I don't think the frequency spectrum is very useful. I need a way to characterize each pulse.

Some VB6 Tools

Each pulse can be described by three values: the length of the pulse, the high peak voltage where the zener break-down occurs, and the depth of the zener pulse fall. These three values can determine the position of a point in three dimensions and can produce a 3D scattergram of one point for each measured peak. The digital scope outputs a text file with 8000 voltage readings taken across 100 microseconds. I wrote a VB6 program to plot those points, find the high and low peaks and plot a 3D scattergram of the 369 random pulses observed. Yes, it looks to be a fairly random distribution of points in 3D but I cannot think of a practical use for this cool looking 3d data.

As I was looking at random distributions of sawtooth peeks, it occurred to me that given enough data, I could find the probability of a peek occurring at a given voltage. I wrote RandomWaveAnalyzer.exe to analyze raw data text files exported from the SimSmart DD140 digital scope. It looks at the distribution of the sawtooth peeks (and troughs) to find what is the probability that a peek will occur at a given voltage. Surprisingly, the data falls along exponential curves, where the probability equals a coefficient A times the constant e to the power of exponent B times the voltage or,  p = A * e ^ (B * Volts). The program has the ability to perform a least squares regression to find values of A and B for the best curve fit. Typically, the program finds about 20,000 peeks among one million data points.

I wrote RanSim.exe to use the coefficient and exponent values (A & B) that were found with the Analyzer program, to simulate random sawtooth waveforms that are generated by zener diode circuits. About 3,600 peeks are generated during a 10 mS simulation. Probabilities of a peek (or of a trough) occurring at a specific voltage are then calculated on the simulation data and then overlain onto the ideal probability curve graph. I can even export the simulation data and analyze it using the RandomWaveAnalyser.exe tool.

Light and Dark Experiments

Measuring from the digital scope and using the analyze program, I looked at the effect that light had on a 14v zener diode circuit with a 10K resistor at 14.7 volts.

A little bit of light, generally effects the waveform by making the peaks smaller and more frequent. There is no change in the voltage drop across the diode and no significant change in the rate of charge (sawtooth slopes). Light has the effect of narrowing the space between the peak and trough probability curves and markedly increasing the size of the B terms (exponent parameters). Clearly I will have to conduct my experiments in a light proof and in an EMF wave proof box. I would not be surprised if the output was temperature sensitive or sensitive to vibration (sound) or to radiation. I will have to be careful to keep conditions controlled.