A Zener diode is the simplest known type of voltage limiter (Figure 1) As soon as the voltage exceeds the rated voltage of the Zener diode, a current can flow through the diode to limit the voltage. This is exactly the right answer for many protection circuit applications. However, if it is necessary to limit a signal to a certain voltage in a control circuit, Zener diodes do not provide an adequate solution.
They are only available with fixed values, which are also subject to a tolerance range. What we are looking for is thus an ‘adjustable’ Zener diode. Such a component would be useful in a heating controller with a preheat temperature limiting, for example, or in a battery charger to provide current limiting. The answer to our quest is shown in Figure 2. Assume for example that the output voltage must not exceed 6.5 V. The control voltage on the non-inverting input is thus set to 6.5 V. Now assume that 4.2 V is present at the input. The result is that the maximum positive voltage is present at the opamp output, but the diode prevents this from having any effect on the signal. However, if the voltage rises above 6.5 V, the output of the opamp goes negative and pulls the voltage back down to 6.5 V. The current is limited by R3. Another example is a situation in which exactly the opposite is required. In this case, the voltage must not drop below a certain value. This can be easily achieved by reversing the polarity of the diode. Another option is a voltage that is only allowed to vary within a certain voltage window. It must not rise above a certain value, but it also must not drop below another specific value. In the circuit shown in Figure 3, the left-hand opamp provides the upper limit and the right-hand opamp provides the lower limit. Each opamp is wired as a voltage follower.