How to Reduce the Voltage on a 12 Volt System to 4 Volt

Written by kim lewis
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To reduce a 12-volt system to 4 volts, use a voltage divider or a zener diode circuit. Voltage dividers are made from series resistors. An input voltage is divided into an output dependent on the value of the resistors used. They obey Ohm's Law, where the voltage is proportional to the current with resistance as the proportionality constant. Zener diodes are optimised to operate when they are reverse-biased or placed backwards in circuits. They must be paired with a current limiting resistor to ensure that they stay within the manufacturer's power requirements.

Skill level:
Moderately Challenging

Things you need

• 330 and 660-Ohm resistors
• 1N749 Zener diode
• Zener diode data sheet
• 12-Volt power supply or equivalent
• Digital multimeter
• Jumper wires
• Basic electronics text

Voltage Divider

1. 1

Study Ohm's Law for resistors used as voltage dividers. The formula for a very basic one with two resistors is Vout = Vin * (R2/(R1 + R2)).

2. 2

Construct a voltage divider by creating a series circuit. Attach the positive side of the power source to one side of 660-ohm the resistor, and connect the free end of the component to the one side of the 330-ohm resistor. Wire the remaining terminal to the negative side of the power supply.

3. 3

Place a multimeter on a DC voltage setting. Measure the output voltage on the second resistor, which is the 330-ohm one in this case. It should equal approximately 4 volts.

Zener Regulator

1. 1

Review the specifications and resistance and power formulas for the 1N749 diode. This may be done by studying the data sheet or the device's package. The diode outputs a stable 4.3 volts, and has a 500 mw power rating. The maximum zener current, I, with the 330-ohm resistor, is (Vin -- Vout)/R = 12 V -- 4.3 V/330 ohm = 23 ma. This is within the diode's power rating, since P = IV = 23 mA*4.3 V = 99 mw.

2. 2

Construct a series circuit using the zener diode and 330-ohm resistor. Attach the positive side of the power source to one side of the resistor, and connect the resistor's other end to the reverse-biased side of the zener diode, which is the side indicated by a mark. Wire the remaining diode terminal to the negative side of the power supply.

3. 3

Use the multimeter and measure the voltage across the diode by placing a multimeter lead on each terminal. It should read approximately 4.3 volts.

Tips and warnings

• The zener diode may be paired with an op-amp emitter-follower circuit if higher output current is needed.
• Resistors may be off as much as 20 per cent of their rated value. Use a precision resistor if you need more accuracy.
• The zener must be reverse-biased, or else it will behave like a regular silicon diode. Semiconductors are sensitive devices. Be sure not to exceed the power, current, and temperature ratings specified by the manufacturer. Always exercise caution when building electrical circuits to avoid burning yourself or damaging your equipment.

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