Introduction: Zener Diode Shunt Regulator

Every circuit needs a power source, but different circuits have different power requirements. For most small scale electronics, the power consumption is (is or at least could be with better design) very low. For those times when you don't need a lot of power, just a precise voltage level, the simplest means of regulating the supply voltage is by using a Zener diode.

In this Instructable, I will compare a typical voltage regulation circuit with the Zener diode shunt regulator circuit and show you the proper way of selecting the perfect components for your circuit needs. Correct usage of this circuit can save time and money, but it is not well suited for all designs. So grab those breadboards and let's gown to business!

Step 1: Zener Diode Basics

In case you don't know, a diode is a special type of electrical component that lets current flow freely in one direction, but blocks it from flowing in reverse - in the water analogy, the diode would be a one-way valve, only allowing water to flow in a specific direction. A typical diode will have a voltage drop of around 0.7V when forward biased.

All diodes have a "reverse breakdown voltage" which if applied to the diode in reverse will cause current to flow backwards through the component, typically destroying it in the process. This value is typically in the hundreds to thousands of volts. A Zener diode (similar to an "Avalanche Diode") is a special sub-class of diodes that will allow current to flow in the reverse direction if the applied voltage is above a certain level without damaging the component. Of course, there are limitations to the voltage level and/or current flow, but those are things that the design engineer must take into consideration.

Some common Zener diode breakdown voltages are: 1.8, 3.3, 5.1, 7.5, and 12.6, making them ideal for use in many small circuits.

Step 2: Linear Voltage Regulator Overview

For 95% of small circuits, the voltage regulation is performed by a linear drop out regulator. The common 78xx series is very popular due to the broad range of regulator values represented by the "xx." For example, a 7805 regulator will output 5V. A 7812 regulator will output 12V. These are all DC values. There are two main problems when it comes to using these regulators:

1. They are not very efficient - that excess voltage is wasted as heat.
2. They require a specific range input voltage for operation.

There are some regulators called LDO for "low drop-out," meaning the input doesn't have to be that much higher than the output. These are nice, because a standard 7805 regulator has a drop out voltage of 2V, meaning you need at least 7V at the input for proper operation, but they can cost a lot more. The power inefficiency is a bit more of a problem. For example, if you have a supply of 12V dc regulated down to 3.3V dc with a load drawing 1A, The power in is about 12W while the power out is 3.3W, meaning there is a loss of 8.7W. That is only 27.5% efficient!  It's for this reason that DC-DC switching regulators are used for applications when larger amounts of power are needed and power-line noise (from the switching frequency) is not an issue. These are more complicated to design and also cost a lot more.

In addition to these concerns, most regulators will need a capacitor at both the input and output to stabilize the voltage for proper operation. The good thing about using this type of regulator is that it should work perfectly (without additional consideration) just so long as the supply voltage stays within the operating range and the output current does exceed the maximum.

Step 3: The Zener Diode Shunt Regulator

When you know the level of the supply voltage and you have a small load, using a Zener diode as a regulator can be a great option; however, without proper components, this circuit can be far more inefficient than a linear regulator.

Since the Zener diode is placed in the circuit under a reverse bias, it will allow current to flow through it as long as the supply voltage is above the breakdown voltage of the diode. The series resistor is in place to burn off the excess voltage. Again, this energy is wasted as heat in the resistor. The reason this circuit can be more inefficient is the fact that current will always be flowing through the resistor so long as the supply voltage is above the diode breakdown voltage, even without an attached load.

The value of the resistor determines the current. For example, using our previous numbers of a 12V supply and a 3.3V Zener diode, 8.7V will be dropped across the resistor. The correct value resistor will allow just enough current to pass as is needed to power the load circuitry plus a tiny bit consumed by the Zener diode. If no load is attached, then the entire current will be consumed by the diode.

It is for this reason that knowing the maximum power requirements of the load are very beneficial. Consider a microcontroller circuit that flashes an LED at 20mA. The maximum current consumption of the microcontroller will depend on how fast it is running, among other things, but could easily be less than 100uA. Just for safety's sake, we will say we need 30mA of current supplied to the entire circuit.

To figure out the necessary series resistance, subtract the output voltage from the supply voltage and divide it by the desired current: (12V - 3.3V) / 30mA = 290Ω. The other thing to consider here is power dissipation. The resistor will drop 8.7V at 30mA, dissipating 0.261W of power. A 0.5W resistor should be used. If no load is attached, the Zener diode will consume the entire 30mA dissipating 0.099W of power. A 0.2W or greater diode should be used. Even with our example load attached, the Zener diode will be consuming most of the current when the LED is not being lit. This is why this circuit can be very inefficient.

Step 4: Summary and Thoughts

So, to figure out what parts to use in this circuit:

1. Decide what voltage level you need for the circuit.
2. Calculate what current requirements the circuit will have.
3. Calculate the necessary series resistance to provide the desired current.
4. Calculate the worst case power consumption by the resistor and diode.
5. Pick out parts accordingly.
R = ( VS - VDIODE ) / ITOTAL
PR
( VS - VDIODE ) * ITOTAL   =   ITOTAL2 * R   =   (VS - VDIODE)2 / R
PDIODE = VDIODE * ITOTAL

I have attached a datasheet with some power values for a line of Zener diodes from General Semiconductor. The information is a bit dated, but it provides for a good example. The table shows the Zener part number, breakdown voltage, current consumption, and maximum regulation current. Remember, a safe bet is to use a part rated for at least twice its potential power dissipation.

Practical Considerations
You might wonder, given the fact that this circuit can be so inefficient, when (and why) should we use it? The answer is pretty simple. This circuit can be used when the load is a fairly continuous value (not drastic changes like powering a blinking LED) and is not too large since the components cannot handle very much current. It can be used to power a few chips that need a regulated voltage level lower than the supply. These chips can then control larger loads (like motors or LEDs) connected directly to the source via transistors (BJT or MOSFET).

Another great time to use this regulation circuit is when the supply voltage may be too low for a regular regulator to work. For example, I recently designed a Quiz Game system for a friend. He wanted it to be powered by 4 AA batteries. Here is the problem: 4 rechargeable batteries will produce about 4.8V (4* 1.2V cells) while 4 regular alkaline AA batteries will produce 6V (4 * 1.5V cells). The latter voltage is too high to supply many chips such as a microcontroller, so it needs to be regulated. I really wanted to use 5V to power the µC, so the best solution with the parts I had on hand was to use a Zener diode to regulate the voltage.

If rechargeable batteries were used, the 4.8V supply would pass through the resistor into the circuit, unchanged by the Zener diode. The current draw by the µC would determine the voltage drop across the resistor, but it is a negligible amount. If regular batteries were used, the 6V supply would be regulated down to 5.1V by the Zener diode. There are other obvious solutions, but I didn't feel like ordering any new parts since I had a few Zener diodes and plenty of resistors on hand.

Lastly, this circuit can be used if space or cost is a huge factor in the design. A resistor and Zener diode will cost a lot less than a regulator and a couple of capacitors, and they will take up a lot less space on a circuit board. Hopefully, this information will point you in the right direction when it comes to designing your circuit's power supply!