Zener Diode Clipping Circuits
Therefore the zener diode can be taken in use to regulate a constant DC source but what if the input signal is not steady state DC but an alternating AC waveform how would zener diode react to the constantly changing signal.
The diode clamping and clipping circuits are circuits which are used to modify an input AC waveform producing a differently shape output waveform depending on circuit arrangement. Diode clipper circuits are also called as limiters because they limit or clip-off the positive or negative part of the input AC signal. As zener clipper circuits limit or cut-off part of waveform across them, they are mostly used for circuit protection or in the waveform shaping circuits. For instance, if we wanted to clip the output waveform at +7.5V, we would use a zener diode of range 7.5V. If the output waveform exceed the 7.5V limit, the zener diode will "clip-off" the excess voltage from the input producing a waveform with a flat top still keeping the output constant at +7.5V. Note that in forward bias condition a zener diode is still a diode and when the AC waveform output goes negative below -0.7V, the zener diode turns "ON" like normal silicon diode would and clips output at -0.7V as shown below.
Square Wave Signal
The zener diodes connected back to back can be used as an AC regulator producing what is teasingly called a "poor man's square wave generator". By using this systematic arrangement we can clip the waveform between a positive value of +7.5V and a negative value of -7.5V. If we wanted to clip the output waveform in between different minimum and maximum values for instance, +8V and - 6V, use would simply use two differently rated zener diodes.
Note that output will clip the AC waveform actually in between +8.7V and -6.7V due to the addition of the forward biasing diode voltage, which adds another 0.7V voltage drop to it. This kind of clipper configuration is ordinary for protecting an electronic circuit from over voltage. The 2 zeners are usually placed across the power supply input terminals and during the normal operation, one of the zener diodes is "OFF" and the diodes have less affect. However, if input voltage waveform exceeds limit of it, then zeners turn "ON" and clip the input to protect circuit.
In the next topic about diodes, we will look at using the forward biased PN junction of a diode to produce light. We know from the earlier tutorials that when charge carriers move across the junction, electrons combine with holes and energy is lost in the form of heat, but some of this energy is dissipated as photons also but we cannot see them. If we place a translucent lens around junction, visible light will be produced and diode becomes a light source. This effect produces another type of the diode commonly known as the Light Emitting Diode which takes advantage of this light producing characteristic to emit light (photons) in a range of colors and wavelengths.
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