Table of Contents
Zener diodes are specially made to conduct current while reverse biased (RB).
Once the RB voltage reaches their Zener voltage, they behave kind of like a pressure release valve. They conduct current as needed to maintain that voltage across them.
Most diodes are damaged if you exceed their reverse bias breakdown voltage, but Zener diodes are made to conduct safely as long as you stay below their wattage rating.
The voltage across the Zener diode is usually used as a reference voltage. It is a signal voltage that the input of other circuitry looks at and responds to in some way.
The Zener diode needs a power source that has a higher voltage than their Zener voltage, and they need to have current limited through them.
Zener diodes come in many values. Affiliate link below has a number of values of Zener diodes, and many other semiconductors that I have used in my videos. It included the S102T current regulating diode component mentioned in the next section of this page when I bought my kit.
Series current source:
A resistor is commonly used to set the current through a Zener diode, but a current source will get a better Zener voltage output. That is because the Zener voltage does vary a bit based on how much current is flowing through the Zener diode.
Resistors pass more current as voltage across it rises, whereas a current source passed the same amount of current regardless of the voltage across it.
As long as an S102 current regulating diode has about 2 or more,volts across it, then it will pass almost exactly 1mA of current through it. That same amount of current will flow through a series Zener diode as long as the output does not have to provide any current. 1mA is usually a good amount of current passing through a Zener diode in order to get it close to it’s Zener voltage across it.
Remember that the Zener voltage is dropped from the resistor, or current source, that is setting the current through the Zener diode. That is why the power supply should be 2 or more volts higher than the Zener voltage. It needs to provide 2 or more volts in addition to the voltage needed to get the RB zener diode to conduct.
Transistor amplified:
The output of a Zener voltage reference circuit fragment, does a bad job of providing current on it’s own.
It does a much better job of setting a voltage at an NPN bipolar junction transistor (BJT) base. The NPN BJT can then be wired as a current source, or other type of circuit that relies on a fixed voltage input.
An emitter resistor will have the base voltage, minus approx. 0.6V (a diode drop) across it. The collector to emitter will pass current as needed to maintain that voltage across the emitter resistor. The collector load is in series with the emitter resistor and collector to emitter. Therefore it will pass the same amount of current. Of course, the load can’t drop more voltage than is needed to be across the emitter resistor. If it does so, then the current will be limited even more by the load.
The NPN BJT wired as a current source only takes a very small amount of current through the base. So, it should not affect the Zener voltage any noticeable amount.
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