# Zener diode circuit-overvoltage protection, problems, calculation

So In the Previous Article, We learned about the Zener diode symbol And Zener diode Construction, So in This article/tutorial, WE Are learning Zener diode with the Zener diode circuit.

## zener diode circuit for overvoltage protection

An overvoltage safety circuit is another kind of Zener diode circuit. The Zener diode is used in a somewhat different manner in this Zener diode circuit, which detects the breakdown current through the diode until a certain voltage is reached.

Though power supplies are usually dependable, a series pass transistor or FET failure can have disastrous consequences. If the series pass system fails due to a short circuit, the entire unchecked voltage is applied to the circuits that are powered by the controlled electricity. This has the potential to ruin all of the chips that are being controlled.

Using a crowbar circuit is one choice. An SCR is enabled when this type of circuit senses an overvoltage condition. This rapidly lowers the output voltage and, in the case shown, blows a fuse, disconnecting the electricity from the input source.

When an overvoltage is observed, the circuit activates the SCR. The Zener diode is selected to have a voltage that is higher than the usual operating voltage – enough to prevent it from igniting under normal conditions, but low enough to allow current to flow rapidly when a fault state is observed.

The output voltage is below the reverse voltage of the Zener diode in usual working conditions, so no current flows through it and the SCR’s gate is not enabled.

The Zener diode will begin to conduct, the SCR will burn, and the fuse will blow if the voltage increases above the permitted voltage, i.e. the Zener diode breakdown voltage.

Also Read: How Electric Motors Work?

## zener diode circuit problems

voltage regulator and we have also seen the few examples based on this Zener regulator circuit. But in those examples during the analysis we have assumed that the Zener diode is ideal meaning that in the breakdown region of operation the Zener resistance is zero.

And we have also assumed that the Zener knee current of the Zener diode is negligible meaning that to turn on the Zener diode in the breakdown region, the required current is almost negligible. But actually this Zener diode requires some finite amount of current or in other words we can say that these Zener knee current is non-zero.

And during the breakdown region of operation, this Zener diode also have some finite Zener resistance. so considering this non-ideal Zener diode let us solve some problems based on this

Zener regulator circuit. So in the first example we have been given this Zener regulator circuit and in this circuit the knee current of the Zener diode is 10 milliampere. And we have been asked to find the minimum value of this load such that the 5 volt of voltage is maintained across this load resistor. And apart from that we have been also asked to find the minimum power rating of this Zener diode.

so for the given regulator circuit first of all let us write down all the currents. so let’s say Is is the current which is supplied by this voltage source and Iz and the IL are the currents that are flowing through the Zener diode and the load resistor respectively.

so from this, we can say that the Is is equal to Iz plus IL. now for the given regulator circuit, as this voltage source and this series resistor are fixed, so this current Is will remain fixed and in this circuit, as we reduce the value of RL then the load current IL will increase.

But at the same time, the current which is flowing through the Zener diode will reduce. But to operate the Zener diode as a voltage regulator, the minimum current which should flow through the Zener diode should be at least equal to 10 milliamperes.

So from this we can say that for the fixed value of Is, the maximum value of the load current should be such that the current which is flowing through the Zener diode is at least equal to Zener Knee current. And from the maximum value of this load current we will find the minimum value of this load resistor.

So first of all let us find out the value of this source current. So the source current Is can be given as, Vs minus VZ divided by R. And here it can be written as 10 minus 5 volt divided by hundred ohm, which is equal to 50 milli ampere. Now here we have been given that the value of Izk is equal to 10 milli ampere. so from this we can say that the maximum value of the load current is equal to Is minus Izk, that means 50 minus 10 milli ampere.

so the maximum value of the load current is equal to 40 milliamperes. And from this, we can say that the minimum value of the load resistor will be equal to VZ divided by IL max. That means 5 volts divided by 40 milliamperes.

So the minimum value of the load resistor will be equal to 125 ohms. So for this circuit to work properly as a voltage regulator the value of the load resistor should be more than 125 ohms. Now, apart from that, we have been also asked to find the minimum power rating of this Zener diode. Now we know that the power rating of the Zener diode or the power of this Zener diode can be given as VZ times Iz, where Vz is the voltage across the Zener diode and Iz is the current which is flowing through this Zener diode.

now for any regulator circuit, the power rating of the Zener diode should be such that it should be able to handle the worst-case scenario. now in the given circuit as we increase the value of RL then the load current IL will reduce and at the same time the Zener current will increase. now in the worst case let’s say this RL is equal to infinite or in the no load condition the entire current Is will flow through this Zener diode.

so in the worst case or in a no-load condition, we can say that this is equal to Iz. and here we have already found the value of Is which is equal to 50 milliamperes so we can say that in the worst-case condition the power which is getting dissipated across the Zener diode is equal to 5-volt times 50 milliampere that is equal to 250 milliwatts.

so the minimum power rating of the Zener diode should be equal to 250 milliwatts. so for the given example, the minimum value of the load resistor should be 125 ohms and the minimum power rating of the Zener diode should be equal to 250 milliwatts.

## Zener diode circuit calculation

Calculate the maximum and minimal load currents (Li), such as 10 mA and 0 mA, respectively.

1. Determine the highest possible supply voltage, such as 12 V, while still maintaining that the minimum supply voltage is still = 1.5 V + Vz (the Zener voltage rating).
2. The needed output voltage, which is the equivalent Zener voltage Vz = 4.7 Volts, is indicated in the simple regulator specification, and the selected lowest Zener current is 100 microamps. This means that the actual Zener current is 100 microamps plus 10 milliamps, or 10.1 milliamps total.
3. The series resistor Rs must make at least 10.1 mA even when the input supply is at the lowest defined level, which is 1.5 V higher than the chosen zener value Vz, which can be measured using Ohms law as Rs = 1.5 / 10.1 x 10-3 = 148.5 Ohms. Rs should be 150 ohms, since it seems to be the nearest norm value.
4. When the supply voltage is increased to 12 V, the voltage decrease through Rs is equal to Iz x Rs, where Iz is the current through the zener. As a result, we get Iz = 12 – 4.7 / 150 = 48.66 mA using Ohm’s rule.
5. The maximum current allowed to travel through the zener diode is as follows. In other words, the maximum current that can flow when the output load is at its maximum or when the required supply voltage input is at its maximum.
6. The zener diode will dissipate a power of Iz x Vz = 48.66 x 4.7 = 228 mW under these conditions. The normal power rating value that comes nearest to this is 400 mW.