Zener Diodes

In this lesson we are going to talk about Zener

diodes

, but to start let's compare the Zener diode with a conventional diode. On the right, this is the symbol for a Zener diode. Now a conventional diode is designed to allow conventional current to flow forward. direction, so this would be the positive terminal and this would be the negative terminal on a

zener

diode. Current can flow in the direct direction like a conventional diode or it can flow in the opposite direction. Now a xenodiode is designed to operate in reverse bias mode. In this mode, the xenodiode has a certain reverse breakdown voltage and is basically the raden of the

zener

diode, so let's say if we have a 12 volt zener diode, when you put it in reverse mode, if the voltage is 12 or more, will lead in that direction, so let's give some examples, let's apply it to a typical circuit problem, let's say we have a battery and we have a current limiting resistor in series with a Zener diode that is in reverse mode.

Now let's say the rating of the Zener diode is 12. volts and let's say we have a resistance of one kilo ohm, what will happen if the battery is 5 volts? What is the current in this circuit? Is the circuit on or off now because the input voltage of the circuit is less than? the reverse breakdown voltage of the zener diode will not flow current in the circuit, so the circuit is off, the current flowing in this resistor is zero amps. Now let's say if we connect a voltmeter across the zener diode, what voltage will it read now because there is no current flowing in a circuit, the voltage across the resistor is zero volts, therefore the voltage across the zener diode that will be picked up by a voltmeter will be the same as the battery voltage, it will be five volts because these two have to add up to five, so now let's use the same circuit, but this time we are going to increase the battery voltage, so in this case let's make it 14 volts instead of 12 or 5.

Now the circuit is on or off. Because the input voltage exceeds the reverse breakdown voltage of the zener diode, the circuit will be on and therefore a current will flow through this resistor. So how can we calculate the current in this circuit? How can we find the answer in order? To find the answer, it is important to understand that the Zener diode serves as a voltage regulator, so as long as the current is approximately the characteristic current of this particular xenodiode, let's say that if it is small, it is not too large, the voltage at through the xenodiode will remain approx. 12.

It may vary a little bit, but for the most part for all practical purposes we are going to say it is approximately 12. It could be 11.9 12.1 but it will be fairly constant, meaning that the voltage across the resistor has It has to be 2 volts because these two have to add up to the battery voltage, so we have two volts across a one kilo ohm resistor, so if we use v is equal to ir, then the voltage or current that flows through that resistance will be the voltage divided by the resistance, so it will be 2 volts divided by a thousand ohms, which will be 0.002 amps or 2 milliamps, so that is the current that flows in this circuit, but it is important to understand that the zener diode will maintain a relatively constant voltage of 12 volts when it is in reverse bias mode and if the input voltage exceeds that value which is the rating of the zener diode, now let's increase the battery voltage, so let's use the same circuit with the same components to have a 12 volt zener diode, but this time we will use a 50 volt battery, so what is the voltage across the zener diode and what will be the current in the circuit?

In this case, the voltage across the zener diode will still be approximately 12 volts, which means the voltage across the resistor has to be 50 minus 12 or 38 volts, so if we take 38 volts divided by 1 kilo ohm , that will give us a current of 38 milliamps flowing through that resistor and through the entire circuit, that is how the current can be easily determined. flowing in such a circuit as long as you have a resistor in series with a zener diode now there are many other things we can do so here is another circuit let's say if we have a 50 volt battery and let's use a one kilo resistor ohm again, but this time we are going to use two zener

diodes

instead of one and let's say the radian for each zener diode is 12 volts, so what is the current flowing in the circuit now so that the input voltage exceeds the total reverse voltage of the two zener diodes? diodes, which means the circuit will be on and therefore the voltage drop or the voltage across those two xenodials will be 12 plus 12. so we're going to have 24 volts across those two zener diodes, which means that the voltage across the kilo ohm resistor will be 26 because 24 plus 26 equals 50.

So if we take 26 volts and divide it by a 1 kilo ohm resistor, that will give us a current of 26 milliamps flowing in a circuit, but if you want a simple way to calculate the current it will be the input voltage, which is 50 minus the total reverse voltages of the total reverse voltage instead of the two zener diodes, which is 24 and then you divide it by the resistance of a kilo ohm, which is a thousand ohms and then 26 divided by a thousand, that's point zero two six amps and then you can convert that to milliamps by multiplying it by a thousand, so here's a generic formula to calculate the current in the circuits above, it will be the input voltage. minus the total reverse zener voltage divided by the resistance in the circuit and that will give you the current flowing in the circuit.

Now let's say if we have an AC sine wave connected to a resistor and two zener diodes connected like this, what is happening? To make it happen, let's call this point a and let's call this point b and then we'll call it d1 and d2, so when the current flows in this direction, notice that d2 the arrow points in the same direction as the current, so in that case that diode will be in its forward mode of operation which means it will have a voltage drop of about 0.6 and this diode is in reverse mode because its arrow is opposite to the direction of current so its voltage drop will be 12.

Therefore, the voltage across a and b will be 12.6 now let's say if the current travels in the other direction by the way, let's say this voltage is greater than 12.6 let's say it's 20 volts now if the current flows in this direction d2 will have a negative voltage drop 12 because now it is in reverse mode, d1 is in forward mode and therefore the voltage drop will be the same, therefore if you have such a circuit you can convert a sine wave which is you'll see like this in basically a clamp wave that looks more like this with a voltage of 12.6 positive on the top and 12.6 negative on the bottom and that's it for my video on xenodiodes.

I hope you learned a thing or two and thanks for watching.