Zener voltage drop after a while

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mohammadyou

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one of my friend has a problem with zener. in his circuit, he put a zener(5.6) in series with 20V and 220K resistor.
it works fine but after a while the voltage across the zener drops below 3V when he change the zener the same problem has happened. would you please give a reason about this behavior of the circuit?
 

Is he just measuring the voltage across the zener or is he using it as a regulator for additional circuitry? If there is additional current draw (from some other circuitry), the voltage drop across the 220K is too much and the voltage is falling below the zener voltage. Measure the cuurent draw of the other circuitry and use Ohm's Law to determine a value of the Zener resistor that will keep the voltage across the zener above the zener voltage.
 

he use it as regulator for additional circuitry and the problem is the 220k resistor. he want to limit the maximum current for the other circuit ( around 50ua ) so he has chosen 220k however this reduce the current for the zener and there is a problem. !!
 

Apparently an error of reasoning. The circuit can provide load current limiting and voltage regulation, but not both at the same time.

The current permitted by the resistor must be always higher than the regular load current, e.g. by 100 µA zener current. The zener voltage at 100 µA will be lower than the nominal value, but should be still fairly stable.
 

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