# Why do we connect a resistor before a Zener diode?

Why do we need a resistor in a Zener diode circuit, like in the diagram below?

I understand it is to limit the current but how so, and why do we need it for a Zener diode?

Does selecting different values of resistors affect the circuit performance? So when we are selecting a Zener diode we look in the specifications at different reverse currents that can flow through them. But if those can be changed through resistors, can we select any Zener diaode at a voltage, without looking at the maximum current?

• Kirchoff's voltage law. Sep 6 '13 at 18:58
• The answers below are good, I think I will just supplement with this Instructible I did on Zener Diode Shunt Regulators: instructables.com/id/Zener-Diode-Shunt-Regulator Sep 6 '13 at 22:13
• I think what people's answers have missed is that the Zener diodes need a minimum current to work properly (usually quite a lot, in the range of 2-10mA at least) and this DOES dictate the resistor used, other than the power rating of both components, and also makes the zeners less useful as a regulator and more useful as a (normally low precision/accuracy) shunt reference voltage. Nov 3 '14 at 8:51

Rather than going to 'no resistor' consider what happens if we just use resistors of different (lower) values and look at the pattern. As we reduce the resistor value the current through the zener will rise. Even if the voltage source is not perfect the power dissipated by the zener will cause it to fail due to overheating.

• Thank You for your answers, Does selecting different values of resistors effect the circuit performance? So when select a zenner we look at different reverse currents in the specs that can flow through them. but if those can be changed through resistors, can we select any zener at a voltage, without looking at the maximum current? Sep 6 '13 at 23:03

The current through the resistor is, by KVL and Ohm's law:

$\dfrac{V_{IN} - V_Z}{R_S}$

This is the maximum current through the zener diode and so, $R_S$ is chosen to limit the current through the diode to some value such that the maximum power dissipated by the diode is below the maximum power rating.

For example, if the Zener is 12V, 1W device, we would want the maximum zener current to be less than $\dfrac{1}{12}A$.

From a comment to another answer:

can we select any zener at a voltage, without looking at the maximum current?

The point of Jim's answer, and mine, is to emphasize the power dissipation associated with the zener diode.

Since our answers are evidently not sufficient in this regard, consider the following excerpt from a zener diode data sheet:

Note the first entry is the absolute maximum power dissipation. In this case, $P_D = 500mW$.

So, if you pick a zener diode from this family with a zener voltage of $V_Z$, you must pick a resistor to limit the maximum current to be (perhaps considerably) less than:

$I_{Z_{max}} < \dfrac{P_D}{V_Z}$

For example, if you pick the 1N5231B with a nominal $V_Z = 5.1V$, you must choose a resistor such that

$I_{Z_{max}} < \dfrac{0.5W}{5.1V} = 98 mA$

Neither a power supply nor a Zener diode is ideal. A power supply has a very low output impedance and a Zener diode has a non-zero impedance. Without a series resistor this means:

1. The Zener diode current will be very high
2. The voltage across the Zener diode will be higher than specified due to its internal resistance
3. Due to the large current that may flow the power dissipation, ($P_{dissipated}=V_{zener}×I_{zener}$), will be huge. Your average Zener diode will have a 400 mW or 500 mW power rating.
• Thank You for your answers, Does selecting different values of resistors effect the circuit performance? Sep 6 '13 at 23:01
• Yes it does effect performance. You have to balance the varying load current with the series resistor. The zener sinks the current that the load doesn't sink. So you really want to know minimum and maximum output currents and figure out what zener and series resistor should go with that. With regular low power zeners, you want about 1-5mA minimum zener current for good regulation. The maximum zener current can be calculated from spec'd power and zener voltage. Sep 7 '13 at 7:14

What is the purpose for using a zener? Short answer: To provide a voltage Vz that is not very sensible to load changes (low dynamical output resistance) - and which can be derived from another voltage source Vo which either is larger and/or does vary too much over time.

Remembering KVL, we need a part which can produce the difference Vo-Vz. A resistor in series with the Zener diode serves this purpose