basics
basics
What is a zener diode? What is it used for and what's the difference between a regular diode and a Zener?
Let me dust the cobwebs out of my head and see if I can explain this so it makes sense.
A bipolar semiconductor, such as a silicon diode, experiences what is called reverse breakdown voltage. For instance the peak reverse voltage of a diode such as the 1N4001 series is 50 volts. It CAN be higher but the most the specs will state it will handle is 50 volts. For years I understood the reverse breakdown voltage is the voltage at which the diode is destroyed. This is simply not the case. This is the voltage that the diode ceases to block current flow. A diode or transistor can safely and repeatedly enter reverse voltage breakdown as long as you do not exceed the reverse current. This will be important later.
So basically what you have is a diode that has a forward voltage drop of something like 0.6V DC when conducting forward polarity current. The 0.6V voltage is fairly flat across a wide current range. This voltage is known as the knee voltage. But there is another point where the reverse voltage breaks down as well; usually much higher. As in the case of the 1N4001 this is 50 volts (but can be higher as the diode can perform better than specified). Now this reverse breakdown voltage can be important. This means if you had a 120VAC supply and you wanted to turn it into DC you would not use 1N4001 diodes as they could not block the reverse voltage.
"So what!?" you say. "What does this have to do with Zener diodes?" The reverse breakdown voltage can be made much lower. Such as 5.2V. So the Zener acts as a regular diode when forward biased as well as when it is reverse biased. But it has been designed such that the reverse breakdown happens at a much lower voltage; a specific voltage. Thus what does this do for you? You will know what the voltage will be across this diode when you put a reverse current through it. Since you know what the voltage across this diode will be it gives you a VOLTAGE REGULATOR!
How does this work? Let's say you need a 5VDC source to power some chip (let's say an LCD clock chip) in something like a radio. But the radio runs on 9VDC for the amplifiers and what not. You want a regulated 5V for the chip. You get yourself a Zener diode. For argument let's say the reverse current through the diode needs to be 5mA. The zener diode is a 5.2V model. 9V - 5.2V gives 3.8V. You now find the bias resistor. You know the current (5mA) and you know the voltage across the resistor (3.8V). Your resistor will be 760 Ohms. Usually in case like this you would go to the next higher standard value resistor or 820 Ohms. But 750 Ohms is a bit closer and will work just fine.
Usually, though, Zener diodes will not be used alone. You will typically see them driving the base of a power transistor and the current will flow will be taken through the transistor. But for simple circuits that need regulation you can take the voltage right across the diode itself.
A bipolar semiconductor, such as a silicon diode, experiences what is called reverse breakdown voltage. For instance the peak reverse voltage of a diode such as the 1N4001 series is 50 volts. It CAN be higher but the most the specs will state it will handle is 50 volts. For years I understood the reverse breakdown voltage is the voltage at which the diode is destroyed. This is simply not the case. This is the voltage that the diode ceases to block current flow. A diode or transistor can safely and repeatedly enter reverse voltage breakdown as long as you do not exceed the reverse current. This will be important later.
So basically what you have is a diode that has a forward voltage drop of something like 0.6V DC when conducting forward polarity current. The 0.6V voltage is fairly flat across a wide current range. This voltage is known as the knee voltage. But there is another point where the reverse voltage breaks down as well; usually much higher. As in the case of the 1N4001 this is 50 volts (but can be higher as the diode can perform better than specified). Now this reverse breakdown voltage can be important. This means if you had a 120VAC supply and you wanted to turn it into DC you would not use 1N4001 diodes as they could not block the reverse voltage.
"So what!?" you say. "What does this have to do with Zener diodes?" The reverse breakdown voltage can be made much lower. Such as 5.2V. So the Zener acts as a regular diode when forward biased as well as when it is reverse biased. But it has been designed such that the reverse breakdown happens at a much lower voltage; a specific voltage. Thus what does this do for you? You will know what the voltage will be across this diode when you put a reverse current through it. Since you know what the voltage across this diode will be it gives you a VOLTAGE REGULATOR!
How does this work? Let's say you need a 5VDC source to power some chip (let's say an LCD clock chip) in something like a radio. But the radio runs on 9VDC for the amplifiers and what not. You want a regulated 5V for the chip. You get yourself a Zener diode. For argument let's say the reverse current through the diode needs to be 5mA. The zener diode is a 5.2V model. 9V - 5.2V gives 3.8V. You now find the bias resistor. You know the current (5mA) and you know the voltage across the resistor (3.8V). Your resistor will be 760 Ohms. Usually in case like this you would go to the next higher standard value resistor or 820 Ohms. But 750 Ohms is a bit closer and will work just fine.
Usually, though, Zener diodes will not be used alone. You will typically see them driving the base of a power transistor and the current will flow will be taken through the transistor. But for simple circuits that need regulation you can take the voltage right across the diode itself.
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Jolly,
That was a very thorough explanation but it needs some correction. In your calculation for picking the resistor, you assumed a current of 5mA. I assumed you meant that was the load current and not the zener current. Ideally you will want nearly all the current to go to the load and very little for the zener. The zener keeps the voltage constant by passing the current that the load does not use. The less the zener current the more efficient the regulator. Also, if your calculation yields 760 ohms for the proper current, you wouldn't want to pick the next highest value you would pick a value low enough to supply the highest current needed. Usually the next lowest value would work. Choosing a higher value would cheat the load out of the current it needs.
Hope this helps the new guy.
That was a very thorough explanation but it needs some correction. In your calculation for picking the resistor, you assumed a current of 5mA. I assumed you meant that was the load current and not the zener current. Ideally you will want nearly all the current to go to the load and very little for the zener. The zener keeps the voltage constant by passing the current that the load does not use. The less the zener current the more efficient the regulator. Also, if your calculation yields 760 ohms for the proper current, you wouldn't want to pick the next highest value you would pick a value low enough to supply the highest current needed. Usually the next lowest value would work. Choosing a higher value would cheat the load out of the current it needs.
Hope this helps the new guy.
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