# Why is Zener Diode connected in parallel to the load?

I’m a newbie to the topic of circuits so sorry if this seems a stupid question.

This is the picture given in my textbook of a Zener circuit.

First of all, isn’t Vz = the fluctuating DC input voltage? As far as I understood, potential difference across parallel circuits is the same so it should be same across both the diode and the load right?

Secondly, how does the diode work? Once it’s crossed its ‘breakdown’ stage, what exactly happens when the voltage fluctuates? How does the diode regulate it? My guess is that it ensures the same current flows through the load no matter how much current in the circuit changes (due to change in Voltage) and since V=IR (to reason mathematically) and current and resistance are same, V is also same. Am I right?

• That is a really bad way to try and make a ‘constant DC’ output by the way. Nov 26, 2019 at 13:35
• @JonCuster, that way specifically, yes. But there are shunt regulators and references that behave like zeners (just with better accuracy) that are entirely acceptable in many situations. Nov 26, 2019 at 17:22
• Kathikeya, The "Fluctuating DC input voltage" is not the same as Vz in your drawing. They're separated by the resistor labelled "R". Nov 26, 2019 at 17:22

Think about how the zener diode works. It is conducting for voltages above 0, and for voltages below -$$V_z$$. When the voltage is above $$0$$ the diode will conduct a current and the voltage over $$R_L$$ will be $$0$$. The load will however experience a voltage when the value is $$V_z < V < 0$$ because then the diode is not conducting and we have a voltage over it, the current goes through $$R_L$$. $$V_z$$is the max voltage $$R_L$$ will experience. If this is known the current going through the closed loop can be calculated and thus you can calculate other parameters as well. If you plot the voltage over $$R_L$$ you should get a signal where we only have the lower part of a sinusoid that is clipped at $$V_Z$$ (given that the input voltage is a sinusoid).
When the Zener-voltage is passed, the voltage over the circuit will be $$V_z$$, thus the current/voltage on $$R$$ will be constant as long as the diode is conducting.