I want to use a zener diode as a voltage reference to the the PIC18F2550, as the analog V+ reference. I am planning to use this zener diode, but I am not sure how to calculate its values. I am planning to have a 4.7V voltage reference, by just using a simple circuit as Vcc->Resistor->Zener->GND (using a reversed zener and gettitng its zener voltage).
looking at the zener's datasheet I cant see the zener curve on it, just the positive, foward voltage graph...why? And I can only see a table with just the values for a 5mA current. I was planning to use a 10K resistor, but by some simulations I am not getting 4.7V, which is probably because I am not giving it 5mA, but how may I calculate this? How may I know how much voltage will it output with a 10K resistor?
Answer
Zener diodes are characterized for reverse-biased operation at a specific current, designated Iz. For the indicated family of Zeners, this current is 5 mA, as given in Table 8 of the Datasheet.
This means, in order to use the Zener diode as a voltage regulator of the specified nominal voltage, it must be passing the stated Iz current, plus or minus a milliampere or so.
Thus, for the BZV55-4V7, the resistance used must be calculated such that it allows approximately 5 mA at (Vcc - 4.7) Volts.
For example: if the supply voltage is 9 Volts, then required resistance R = V / I = 860 Ohms. The nearest standard (E12 series, i.e. 10% tolerance) resistor value is 820 Ohms, which will work fine for the purpose.
The proposed 10 kOhms, on the other hand, will not allow sufficient current to pass for the Zener effect to kick in for usable regulation.
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