1. Special Diodes

2. Describe and analyze the function and applications of:  Zener Diodes

3.  A zener diode is a diode with a defined value of reverse breakdown voltage  A zener is used in reverse breakdown mode  The voltage across a zener is more or less independent of the current through it  The function of a zener is to provide a voltage reference or regulation in a circuit

4. Some important zener characteristics:  Nominal Zener Voltage : 5.1V zener, 12V zener, etc.  Maximum current is Maximum Power / Zener voltage.  Nominal Bias Current: calculated as 10% of Maximum current.  Tolerance on zener voltage, e.g. : 12V  5%,  Maximum Power: 1 Watt zener, 5 Watt zener, etc.  Temperature coefficient: by what % does zener voltage change as diode temp. changes 1 O C  Dynamic Resistance (Rd): the change in zener voltage (  V) caused by a change in zener current (  I): Rd =  V/  I

6. Key points:  Vin > Vz  Iz = (Vin – Vz)/Rs > Load Current 8.2V 1N4738A

7. Key points:  Vin > Vz  Iz = (Vin – Vz)/Rs > Load Current

8. Key points:  Vin > Vz  I RS = (Vin – Vz)/Rs > Load Current 8.2V

9. Suppose a 8.2 Volt zener is connected to a 12 Volt supply through a resistor. If we design the zener circuit to have a 15 mA zener current, and the load (RL) is 510 Ohms. Find the required resistor value to limit current (Rs). 8.2V 510 ohms 12V

10. Suppose a 8.2 Volt zener is connected to a 12 Volt supply through a resistor. The zener requires a 15 mA bias, and the load is 510 Ohms. Find the required resistor value. 1. Find load current: I L = 8.2V / 510  = 16.08 mA 2. Find total current: I T = I L + I Z = (16.08mA + 15mA) = 31.08 mA 3. Find drop across Rs: V R = 12V – 8.2V = 3.8 V 4. Find R: Rs = V R / I T = 3.8 V / 31.08 mA = 122.27 Ohms 5. Select standard value resistor that is less than calculated: R = 120 Ohms.