A diode limiter is an electronic circuit designed to clip or limit an alternating current (AC) supply voltage or an audio signal to a preset value. This is typically necessary where the supply voltage is larger than needed or powerful audio signals require compression to prevent overloads. Diode limiter circuits achieve this clipping function by placing one or two Zener diodes in parallel with the supply or signal. A single diode will only clip one half of the wave form while a pair of diodes will clip both negative and positive parts of the cycle. Accurate limiting is possible due to the known voltage limiting characteristics of different Zener diodes.
Any AC power supply or audio signal is made up of a sinusoidal or sine wave form. A sine wave is a smooth repetitive series of positive and negative peaks with a zero value in the center of the wave form. The height between these peaks and the zero point define the amplitude or size of the signal or supply. A diode limiter can control the amplitude of a signal if it is too large by either clipping one half of the wave or both. Producing a precise reduction in amplitude with a diode limiter is possible due to the unique, accurate voltage limiting abilities of Zener diodes.
This accurate cut-off characteristic allows a Zener diode to limit the voltage it passes at a predetermined level. If a single diode is inserted across the power supply or signal output, it will limit one half of the wave form to its rated voltage. If a second diode is inserted in the circuit in the opposite direction or bias orientation to the first, then both the negative and positive halves of the wave form will be limited. These diodes are typically accompanied by one or more resistors in the voltage limiting circuit which aid in producing a well controlled output voltage.
The diode limiter has many uses in both electronic and audio applications where it is used to produce low working voltages from a single higher supply voltage or to compress excessively powerful audio signals. Diode limiters are simple, efficient, and accurate voltage controllers which take up minimal physical space on circuit boards. These circuits are also a particularly cost effective way of controlling voltage outputs. There are also a myriad of available Zener diode ratings which gives circuit designers flexibility in the exact voltage produced by the diode limiters use in their circuits.