| Let's say that the supply
is 12 volts and the load requires 6 volts, then a 6 volt zener is required
and 6 volts must be dropped across the resistor.
Again, let's say that the maximum load current is 50 mA and the zener current is 10 mA, giving a total current of 60 mA through the resistor. The value of the resistor from Ohms Law is V/I = 6V/0.06A = 100 ohms. The total current taken from the supply is shared by the zener and the load. If the load takes more current then the zener takes less, and if the load takes less then the zener takes more. If the load current drops to 25 mA then the zener current rises to 35 mA. If the load is disconnected then the zener current rises to 60mA. The zener must be able to
handle this current without exceeding its rated
Watts = volts x amps
6 V x 0.06A = 0.36 watts
Since the same current passes
through R and it also has 6 volts across it then it also has to dissipate
0.36 watts. A half watt resistor will do here.
|
Copyright Graham Knott 2002