We are trying to use a zener diode shunt circuit to develop a voltage regulator.
- Our input voltage is
5v
. - Our desired output voltage is
3.3v
. - The average load current is
80mA
. - The peak load current is
150mA
.
We assumed the load current to be 150mA
.
We used a Zener diode of 3.3v
and 0.5W
. Based on this, we concluded that the current through the Zener diode is 150mA
.
Thus, the total current is 300mA
.
Based on this, we calculated the current limiting resistor value to be 5.6 ohms
and 0.5W
.
When we actually connected the circuit, we noticed that the output voltage across the Zener diode was coming out to be 4.3v
instead of 3.3v
and it starts to heat up gradually. Why is this happening?
We then replaced the 5.6 ohm
resistor with a 220 ohm
resistor and noted that the output voltage was now 3.3v
but the current was insufficient (as expected). We then replaced the 220 ohm resistor with two 5.6 ohm
resistors in series and found the output voltage to be around 4v
. Similarly, with three 5.6 ohm
resistors in series and found the output voltage to be around 3.7v
.
Questions:
- Are our calculations correct?
- Our assumption is that the voltage across the Zener diode is independent of the current limiting resistor. Why is there an apparent dependence?
- Are there any fundamental errors that we are making?