Zener diode dropping resistor

[John Limrick]

Hi.. can anyone please explain in simple terms how you calculate the
value of the resistor in series with a zener diode when building a
zener regulated power supply? Any help would be greatly
appreciated...thanks.

 

[scada]

"John Limrick" <johnlimrick@dsl.pipex.com> wrote in message
news:5cab1ff0.0404071209.438ac156@posting.google.com...

Hi.. can anyone please explain in simple terms how you calculate the
value of the resistor in series with a zener diode when building a
zener regulated power supply? Any help would be greatly
appreciated...thanks.

Zeners are rated in Voltage and Power. The voltage is the regulated Volts
across the anode and cathode. The series resistor is calculated to drop the
difference voltage that is between the unregulated supply voltage and the
zener voltage. The current through the circuit is calculated to stay
somewhat below the maximum zener power rating. It's just ohm's law from
here! Follow the example: A 1 Watt, 5 Volt zener is chosen, the unregulated
voltage is 15 volts. We want a maximum of 100ma load @ 5V. If P = I*E then
0.1 * 5 (Z Volt) = 0.5W that's well below the maximum 1W rating of the
zener, all good! We know the circuit current is 100ma and we need to loose
10V across the resistor. We can calculated the resistance. R=E/I or
R=10/0.1 = 100 ohms and P = I*E or P = .1 * 10 so P = 1W. Therefore
you would need a 100 ohm, 1W resistor! I would double the resistor power
rating to at least 2W, as not to be at the limit.