For decades, Zener diodes have provided a convenient, inexpensive way to regulate power-supply voltage. Exploiting the phenomenon of reverse breakdown, a Zener is wired backward compared with a standard diode. Zeners have modest current capacity compared with other regulating devices, so check your circuit requirements first. You’ll need one Zener diode for every regulated voltage--positive or negative--your power supply will provide. For every Zener, you’ll also need a current-limiting resistor.
- Skill level:
- Moderately Challenging
Things you need
- Power-supply schematic
- Circuit schematic
- Zener diodes
Determine your circuit's current and voltage needs from its schematic. If you need more than a few hundred milliamps or if the voltage is not covered by available Zener diodes, choose a device other than a Zener for regulation.
Locate your power supply's filter section. This should have one or more electrolytic capacitors. You will place the Zeners after the filter capacitors in the power supply. The supply’s DC voltage before regulation should be a few volts greater than the regulated voltage. The few volts of headroom allow the Zener to regulate effectively.
Calculate series resistance for the Zener using this formula: R = (Vs-Vz) / I Resistance (R) is in ohms, Vs is the supply’s unregulated voltage, Vz is the Zener’s voltage, and I is the maximum regulated current. Calculate the resistor's power by multiplying the square of the current by the resistance, and round up to the next watt. For example, a 10-volt supply and 6.3-volt Zener supplying .25 amps of current gives a resistance of 25 ohms. This resistor needs to be rated for at least 2 watts.
Choose a Zener diode that matches the positive voltage. Connect the resistor to the positive supply and the Zener's cathode to the other side of the resistor. Connect the Zener's anode to ground. The cathode side of the Zener provides the regulated positive-output point.
Choose another Zener that delivers the required negative regulated voltage, as in Step 2. Calculate its current-limiting resistor, as in Step 3. Connect the other side of the resistor to the power supply's negative side, the Zener's anode to the other side of the resistor and the Zener's cathode to the power-supply ground. The anode becomes the tap-off point for negative regulated voltage.
Tips and warnings
- It helps to overrate the supply circuit current by 10 per cent or more to improve your circuit's reliability.
- Choose a current-limiting resistor with plenty of surplus power dissipation. This will improve reliability and let the resistor run cooler.
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