Introduction to Zener Diodes
Zener diodes allow conventional current to flow normally from the anode to the cathode, as any other diode. The Zener diode has a special feature, however. When we use this in reverse bias mode, the zener will begin to conduct current at a certain voltage. Current will flow “Backward” through the zener diode once a certain threshold voltage has been made.
In this configuration, we will use the Zener as a voltage regulator. It may not be the most efficient voltage regulator to use for regular loads, but it does work great for providing a reference voltage when little current flow is needed. We can then use this reference voltage in an operational amplifier.
Below Threshold
Let’s look at the operation of the Zener Diode when the supply voltage is below the threshold of the Zener.
I created this diagram with TinkerCad. Here we have a 10K resistor in series with a 5.1v Zener. Look at the power supply voltage. The Power Supply voltage is 4.8v. The threshold of the zener that I chose is 5.1v. Since we are below the threshold, all of the voltage is being dropped by the Zener.
Above Threshold
Now, let’s increase the power supply voltage. This time we will try 6.4 volts on our supply. You will notice the Zener (in reverse bias) began to conduct. Any voltage above 5.1v is drained off by the diode. This holds the potential across the diode at 5.11v.
Due to the resistance of the contacts, and the inherent characteristics of the diode, we are dropping just a little more than 5.1v. This drop becomes more apparent as we continue to increase the voltage.
You will notice if we go all the way up to 30v, our Zener Diode is still doing a pretty good job at holding the voltage close to it’s breakdown. Remember the breakdown voltage is 5.1v.
You will also notice that our current consumed by the Zener is much higher. We are now “wasting” 2.47 milliAmps instead of the 129 microAmps that we had before. If we are using this circuit as a simple reference to an operational Amplifier, 2.47ma is still a very small amount of current.
Typical Configuration
Let’s try this in a more typical configuration. If we are running two 18650 batteries in series, we will have a maximum of about 8.2 volts.
We are drawing 306 micro amps. If the batteries used are rated at 3500mAh (MilliAmp-Hour), the current drain from this particular circuit would still last well over a year, or 477 days. This would usually be negligible compared to the rest of the circuit that is utilizing the Zener.
— Ricky Bryce