What is zinar diode & working as a voltage regulator

  Welcome. Today I will explain function of zener diode & its applications. This is symbol of zener diode. It has 2 terminals, one is anode, one is cathode. This is special type of diode. In this if we flow the current in forward direction, Forward means anode to cathode, current is flowing like this. Then it behaves like normal diode. But if we apply the voltage in reverse direction, Reverse means, cathod voltage +ve and anode voltage negative, or cathode voltage more than anode voltage. And voltage in reverse direction, becomes equal or greater than break down voltage of zener diode. Then current starts flowing. So in zener diode, current can flow in both the directions. This is characteristic curve of zener diode. This is V. This is I. This operates in 2 ways. This side is +ve biasing. This side is negative biasing. This side it behaves like normal diode. This side it behaves like zener diode. In this side, when voltage exceeds more than 0.7 V, say for silicone diode, current will increase suddenly. But we will discuss here. Zener diode behaviour. In reverse side ,when we apply the voltage, small leakage current flows. When reverse side voltage crosses, break down voltage, this is breakdown voltage, Then current will increase suddenly. And the voltage remains less or more constant. Zener diode has one dynamic resistance. Which is equal to delta V/ delta I. Suppose we take 2 point A & B, This line is not vertical, it has some angle, you see like this. This ∆V is the voltage difference between A & B. And ∆I is the difference of current at point A & B. Ratio ∆V/∆I is called the dynamic resistance. It can be10 ohm, 20 ohm, 100 ohm & so on. Now we come to specifications. This is one example. Say zener diode voltage is 5.6 volt. If you see the data sheet, There will be tolerance say +/- 5%. So 5.6 volt will not be correct value, When you measure, it may be 5.6, + 5% or minus 5% or in between. If you want accurate value, You can take 5 or 10 numbers of zerer diodes, measure the value, and select one which has closure value. Now zener diode has 3 currents. One is test current. One is normal current. 3rd is surge current. Test current means, you will get 5.6 volt at this current. This is the test current. For example if 45mA flows in a zener diode, Voltage measured across this will be 5.6 V. 2nd is 160 mA, Which is continous current it can take. 3rd is surge current. Now you see this current 810 mA, is almost 5 times, compare to 160 normal current. This means for short time, It (zener) can take much more current, say 810mA, this is just example. 810 mA for 10 mSec only, not for longer time. Then power loss. Suppose this zener diode has 5.6 V, Current flowing in this is Iz. Loss will be VzIz. Nor this loss or this current. Current flowing in this, that will decide the loss. I have written 2 figures. 900 mW & 1300 mW. In this case, This zener diode is rated 1300mW at 25 degree. That is this point. This is power derating curve. This is temperature. This is power. Till 25, it is 1300 mW. If you increase the ambient temperature, You have to derate. It can not take 1300 mW. It can take less. For example at 70 degree, It can take only 900 mW. This is just example. So we have to derate. One more thing is there, When they define this power, or this current, They define lead length also. You have to see data sheet carefully. Now we come to applications. This is list of 8 applications. Out of 8, two are shown here. Remaining 6 are in other sheet. Now here I have shown, Zener diode application in waveform clipper. I have connected 2 zener diode in series. But polarity is reverse. In one case cathode is at top. Another case anode is at top. So total voltage will be this voltage 5.6V + this drop 0.7V=6.3V. This is resistor to limit the current. When we give input voltage AC, Output voltage will not exceed more than 6.3V. Output will be red color like this. This drop, this voltage will be 6.3V. Then zener diode application in voltage shift. This is voltage. This is time. Green color is input voltage. This volatge will be always less than this equal to this drop. Suppose at this point, at input voltage is this. So output voltage will be this voltage, minus this zener diode drop. So we get this. Now this is another application of zener diode, in voltage regulation. I have connected one resistor here. This resistor is connected to limit the current in this. And ouput voltage will be equal to zener diode voltage. And current flowing in load will be Vz/RL. But limitation of this circuit is, To much variation in load is not allowed. It has limitation. To improve that this is better way. Here I have connected one transistor. NPN transistor. In zener diode, current flows like this. Now suppose zener diode voltage is 5.6 V, Then voltage at this point at output will be equal to this voltage minus this voltage, Vbe voltage. Suppose Vbe voltage is 0.6 V, Then voltage at this point will be You will get 5 volt here, not 5.6V in this load connected. You can select bigger size of NPN transistor. In this case load variation can be to much. It has lot of range to change the load. When we use this kind of circuit, Then load current flows like this, through transistor. This is load current. And power loss across this transistor, will be load current multiply by drop across transistor Vce. Now this is another application of zener diode, in over voltage protection. This is thyristor. I have connected one zener diode in gate. This resistor is connected to suppress noise. When voltage across this, become equal or more than this zener diode, Zener diode will conduct. And the gate current will flow like this. Because of this gate current, this thyristor will trigger. and this will become short. It will not be fully short because there will be drop of thyristor small drop. Now current flows like this. Now there are 2 ways we can use this. We connect a fuse here. When this becomes short, very high current will flow. And fuse here will blow. 2nd application is, we do not want fuse to blow. We want zero volt itself. Whenever voltage across this, becomes equal or more than this, we want zero volt. In that case fuse is not required. In my one project we wanted like this. We wanted zero volt, whenever this voltage exceeds more than zener diode voltage. Here I am using zener diode, at op amp gain control limiter. I have connected 2 zener diode in series in reverse direction. Assume total voltage is 10V. This is zero, this is zero volt. Normal gain of op amp is R2/R1. If this is not there. When this voltage exceeds more than 10V, Current starts flowing like this. And it will not go more than 10 volt. It provide the dynamic resistance, And limit the voltage at this point. In many control application, This voltage should not go to saturation. If goes to saturation, then response becomes slow. This also protect this IC. As voltage across this will be limited to 10V. Here I have used zener diode as a voltage reference. This is comparator. What happens, When there are so many circuts, Then this supply point will have some spikes. If you connect 2 resistor here, Then spikes will pass over here also. But if we connect a zener diode, Then this point will be stable. So voltage reference will be stable, for the comparator. This resistor I have connected, such that we get the hysteresis. If we do not connect this, This hysteresis function will not be there. This is zener diode application for power supply protection. This is small resistor to limit the current. I have told in the beginning. This zener diode has short time current rating much higher than normal current. That short time rating is used here. Whenever pulse comes here, say spikes comes here. The current flows like this. It can take heavy current. And this voltage will not go more than zener diode voltage. So this R, this C, this zener diode together, provide the power supply protection. But because of this resistance, this voltage will be little lower than this. Or it will have some unregulation. If you can afford that then this circuit is very good for power supply protection. This is meter protection. I have connected one zener diode like this. When voltage across this becomes more than this, then current flows like this. But then full current will flow like this. You have to select proper rating of this zener diode, otherwise it will fail. Or we have to put another fuse or some other thing here, to limit this current. Today we will close here itself.