Transformer Rectifier Units (TRU’s) consist of a unit that combines a transformer and a rectifier. Therefore, it converts Alternating Current (AC) into Direct Current (DC) whilst at the same time transforming a certain voltage into another. One problem appears after the rectification stage of this process as the output is fluctuating. This fluctuation makes the DC unusable for equipment use and it needs to be minimized. This is usually achieved by fitting a filter or smoothing circuit at the output of the rectifier. It is also required to maintain a constant DC supply at the exit of this filter, regardless of the load applied. Therefore, a sort of compensation needs to be fitted to overcome this problem and this solution is called stabilization.

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There’re several sorts of stabilization circuits, but the most common are based around the cold-cathode, gas-filled diode, and the Zener diode (for low voltage circuits). The Zener diode is specifically designed to resist the high reverse current flowing if the breakdown voltage is exceeded, without being damaged. If the Zener diode is reverse biased to a value just beyond the breakdown (stabilizing) voltage, the voltage across the diode remains almost constant even when high current variations occur. Therefore, the Zener diode can be either used to stabilize the voltage over a wide range of circuit currents or to provide a constant reference voltage point in a circuit. Many Zener diodes are produced, corresponding to various values of breakdown (stabilizing) voltage.