What can we use zener diodes for? This time, we look at their use as voltage references.
The special property of zener diodes is that they have a specified reverse breakdown voltage. This property especially useful for circuits that require fixed voltage reference. In circuits with varying supply voltage, e.g., battery operated ones, a simple voltage divider is not enough for this. Using a zener diode is one way to solve this issue. In this article we are going to look at how this works and discuss, when to and when not to use a zener for this purpose.
How can we build voltage reference using a zener? Well, we simply select a zener for your desired voltage and use it together with a current limiting resistor.
If the voltage is above the zener voltage, the diode conducts. It clamps the output voltage to the zener voltage while the remaining voltage is dropped across the zener diode itself. Of course, the zener diode can only provide a bypass for a limited amount of current. To prevent it from getting damaged, we need the resistor to limit the maximum current.
That's simple enough, but where would you use such a circuit?
A circuit that outputs a specific voltage? It is easy to think of a voltage source here, but that is not what a voltage reference is. Voltage references should be used together with high impedance inputs like the ones of microcontrollers, op amps or comparators. What does that mean? In practice, it means, that they have an input that draws a very small current. While voltage references can provide a fixed voltage, they can provide almost no current.
Let's have a look at an example circuit that uses a zener diode as a voltage reference in its intended way:
The circuit uses a 9 V zener diode as voltage reference for an LM393 comparator. A comparator compares the voltage as its two inputs: if the voltage at the non-inverting input (+) is higher than the one on its inverting input (-), its output is high otherwise it is low.
In this example, a potentiometer is connected to the non-inverting input and the 9 V reference voltage connects to the inverting input. The comparator's output becomes high, when the output voltage of the potentiometer is above 9 V, otherwise it is low. The circuit has an LED connected to the comparator's output. This LED turns on, if the potentiometers output is below 9V.
Didn't expect that?
The circuit uses the LED in an active-low configuration. Its positive side connects to the supply voltage and the LM393 switches its connection to ground. This might be an odd choice at first, however, the reason for this, is the way the LM393 works. The LM393 has an open-collector output, which means that it only supports the two output states floating (not connected) and low (connected to ground).
What can you do with such a circuit? What is the advantage of using a zener diode?
In a battery to powered circuit, a zener diode is a cheap way to create a fixed voltage reference as threshold voltage. The advantage is, that threshold is independent of the supply voltage. Up next, we will use this to build a low voltage alarm for a 12 V lead acid battery.
When to better use an alternative?
Clear is: if we need a voltage source, we need a voltage regulator and not a voltage reference. But, sadly there is another case in which you need an alternative solution: low voltages.
For our example circuit we used 12 V and not 5 V or 3.3 V used in most microcontroller circuits. Why is that? Only avalanche type zener diodes make good voltage references.
If we look at the I-V-curves for zener diodes below 5 V, we can see that their curves are a lot flatter. Their zener voltage is much more dependent on the current. The declared zener voltage is only valid a specific current. For true zeners this is especially important, otherwise the voltage differs. We cannot guarantee this in our circuit. It is best to just use a commercial voltage reference designed for low voltages here.