# Why does not all the current flows through Zener diode?

Let's assume the following circuit, where Zener diode is used as a voltage regulator. R16 (R18 later) is the load. When the input voltage is sufficiently high for Zener diode to be reverse-biased, according to the current-voltage curve of the Zener diode, the current will flow through it. Also, I know that the resistance of the diode is very small (milliOhms) when it is opened.

Considering all of the above, I guess we can exchange the diode with the resistor of low resistance (R20 in the image below). Now we can calculate the currents through R20 So, if the load has a small resistance, then most of the current will flow through Zener diode.

My questions are:

• Is it the correct interpretation of Zener diode's functioning?
• Do we use it for the loads, where only the voltage is important?
• – G36 Mar 2 at 13:29

## 1 Answer

No, you cannot replace a zener diode with a resistor. You can replace the diode with a voltage source, equal to the zener voltage, in series with a small resistor. But this model only applies if the zener is reverse biased, meaning that current flows into the positive terminal of the voltage source.

In your second paragraph you say "the current will flow through it [the zener]" but it is more accurate to say "some current will flow". Some current also flows through R16, because it has a voltage across it (the zener voltage).