- Published: Saturday, 04 April 2009
- Written by Digital DIY
- Hits: 8211
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 , 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 required.
Lets say you had a 12V DC signal from your car somewhere that you want to interrogate with your 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 safely? By using a 4.7V zener!
The has a tested current of 53mA. So to calculate the current limiting , the equation would be;
R = V / I
R = 12 / 0.053
R = 226ohm (220ohm is the closest standard resistance to this value)