Tip - Zener Diodes

Zener diodes are common very basic voltage regulation and voltage referencing. They are designed to break down in a non-destructive manner when subject to reverse voltage. This is why zeners are generally always connected in reverse bias, and depending on the type of zener, will depend on its reverse breakdown voltage.

When the zener reaches this reverse voltage breakdown, it will do its best to maintain that voltage, and will sink a lot of current to try to achieve this. Without some sort of current limiting resistor, the diode would 'melt' fairly quickly.

Have a read of your zeners datasheet, and find its nominal current (or test current) to calculate the current limiting resistor required.

Lets say you had a 12V DC signal from your car somewhere that you want to interrogate with your PIC micro to check if something is on or off. Now car voltages are anything but stable, they have plenty of dependencies, e.g. the current load on the batteries, the charging voltage from the alternator, spikes from different sources, so how can we interface this with our PIC micro safely? By using a 4.7V zener!

The zener I have chosen (1N4732) has a tested current of 53mA. So to calculate the current limiting resistor, the equation would be;

R = V / I

R = 12 / 0.053

R = 226ohm (220ohm is the closest standard resistance to this value)

Click Here to view a Video of simulating the differences of Zener regulation and Voltage dividers!