A Zener diode, illustrated in Figure 1, functions as a basic form of voltage limiter. When the voltage surpasses the diode’s rated value, it allows current to pass through, effectively curbing the voltage. This feature proves valuable in numerous protection circuit scenarios. However, in control circuit applications where precise voltage regulation is essential, Zener diodes do not offer a suitable solution.
Seeking an Adjustable Zener Diode Solution
When searching for an adjustable Zener diode, one quickly encounters the limitation of fixed values and their inherent tolerance range. An adjustable variant becomes indispensable in scenarios demanding precise voltage regulation, such as in a heating controller with temperature limitations or in a battery charger necessitating current control. The solution to this requirement is elegantly illustrated in Figure 2. For instance, if the desired output voltage should not surpass 6.5 V, the control voltage at the non-inverting input is set precisely at 6.5 V. If the input voltage, say 4.2 V, is applied, the opamp output registers its maximum positive voltage. The diode, however, prevents this from influencing the signal. Should the voltage exceed 6.5 V, the opamp output turns negative, pulling the voltage back to the desired 6.5 V. The current flow is restricted by resistor R3.
Varied Voltage Control Scenarios
In specific situations, the requirement might be entirely different. For instance, the voltage must not dip below a specific level. In such cases, the diode’s polarity can be reversed to ensure this condition. Another scenario involves maintaining a voltage within a particular range—neither surpassing an upper threshold nor falling below a designated lower limit. Figure 3 depicts a circuit addressing this need. The left-hand opamp establishes the upper threshold, while the right-hand opamp sets the lower limit. Both opamps operate as voltage followers, ensuring precise voltage control within the defined window.