# Zener diode as voltage regulator and problem with I Kirchoff's Law

I have a question in connection with a Zener voltage regulator. I have a Zener diode with Zener voltage 5V and maximum current 0,1 A. I chose resistor R1 to protect diode from too high current. Then I connect resistor R2=10 ohm in parallel to the diode. The problem is that according to I Kirchoff's Law currents in the node don't equal to each other.

Do you know where I made a mistake?

(The current arrows have been reversed from the previous version of the diagram.)

I'd like to clarify my question. I chose resistor R1 the way I should do this and when the battery is new and has 9V voltage, everything is fine and currents equal to each other. But could anyone explain me how it would look like if the battery discharged a little bit and had only 8V? I've assumed that because battery voltage decreased, current Iz through Zener diode also decreased (value is imaginary) but it should still keep 5V as a voltage regulator. I've got in my calculations that in such situation currents in node won't equal to each other and can't find where is a mistake in the way I think.

• Remove the Zener and re-calculate. Is the voltage where the Zener would be, less than the Zener voltage? If so, you would need the Zener to create voltage and source current from nothing. And if it could do that, you wouldn't need a battery...
– user16324
Commented Jun 9, 2021 at 11:24
• I recalculated it and without the Zener diode the voltage on R2 would be 1,8V Commented Jun 9, 2021 at 11:38
• And what does that tell you?
– user16324
Commented Jun 9, 2021 at 11:39
• You assumed R1 based on Iz rather than Iz+i2 thus calc. wrong value Commented Jun 9, 2021 at 11:41
• In order for the Zener to perform voltage regulation current has to flow from its cathode to the anode. Commented Jun 9, 2021 at 11:43

R1 has the wrong value.

You know how much current must flow through R1 if your circuit is working. You calculated that using KCL.

You know how much voltage must appear across R1 if your circuit is working. You calculated that using KVL.

Use these two values and Ohm's Law to find the correct value for R1.

• Okay so let's assume that I found the correct value for R1, but what if battery voltage decreases from 9V to 8V? The voltage on Zener diode and R2 won't change and will be still 5V so Iz and I2 will be the same, but voltage on R1 decrease to 3V so the current I1 will decrease and then, currents in node won't equal to each other. Commented Jun 9, 2021 at 14:30
• KCL will always be satisfied, but your zener might fail to have $V_Z$ across it. Yes, that's a problem with zener shunt regulators. Choose R1 based on the minimum supply voltage, then calculate $I_Z$ for the maximum supply voltage. Make sure that your zener diode can handle that much current. Commented Jun 9, 2021 at 14:36

Based on the way you've drawn your current arrows, $$\I_1\$$ should be -0.1A and $$\I_2\$$ should be -0.5A.

This means that $$\I_Z\$$ is 0.4A -- i.e. current is flowing from its anode to cathode.

But the condition for the Zener to regulate voltage is that current must flow from its cathode to anode. Therefore the assumption that the voltage across the Zener is 5V is violated.

The basic problem is that your load is drawing too much current.

If it drew less current, say $$\I_2 = -0.03A\$$, then

$$I_Z = I_1 - I_2 = -0.1 - -0.03 = -0.07 A$$ Then $$\I_Z\$$ would be 70 mA in the opposite direction of the $$\I_Z\$$ arrow, i.e. in the cathode-to-anode direction.

Concerning the comment: "Current that flow trough Zener diode is 0,1 A "...

A Zener acting as a regulator will fix the voltage across it, but the current through the Zener can vary as seen in by its V-I curve:

(Image source: Electronics Tutorials - Zener Diode I-V Characteristics)

You picked R1 with the idea of having 0.1A flow through the Zener with no load. But as you add a load some of that current is diverted to the load.

• I corrected arrows in the picture so that they will be in accordance with current flow. It is impossible that I2 be 0,03A if the voltage on R2 is 5V and its resistance is 10 Ohms. Commented Jun 9, 2021 at 13:41
• The voltage drop on R1 is 4V so the current that flows trough it is 0,1A. Current that flow trough Zener diode is 0,1 A and curent trough R2 is 0,5 A, because it has 10 Ohms and there is 5V on it. That causes that Iz+I2 don't equal I1. Commented Jun 9, 2021 at 13:46
• "Current that flow trough Zener diode is 0,1 A" -- why do you think that? This is where your error is. Commented Jun 9, 2021 at 15:21
• Updated answer. Commented Jun 9, 2021 at 15:39
• Okay, now I get it. I have one more question. Is it true that in such circuit voltage on Zener diode will be 5V as soon as battery voltage is higher than 5V? Commented Jun 9, 2021 at 17:24