High Voltage To Low Voltage Calculator

Welcome to the High Voltage to Low Voltage Calculator! This handy tool helps you design and analyze circuits where a higher input voltage needs to be safely reduced to a lower, stable output voltage. It is particularly useful when working with Zener diode regulators, simple power supplies, or electronic devices that need consistent voltage levels.

With just a few inputs like supply voltage, Zener voltage, and load resistance, the calculator provides key details such as the series resistor value, load current, and power dissipation. You can start calculating right away, or continue reading to learn about the formulas, examples, and parameter details.

Understanding the Formula

This calculator uses a set of formulas to determine how high voltage can be stepped down into a stable low voltage using a Zener diode and a series resistor.

Series Resistor Calculation
Rs = (Vin - Vz) / (Iz + IL)

• Rs: Resistance of the series resistor (Ω)
• Vin: Input voltage (Volts)
• Vz: Zener voltage (Volts)
• Iz: Current through the Zener diode (Amps)
• IL: Load current (Amps)

Load Current Calculation
IL = Vz / RL

• IL: Load current (Amps)
• Vz: Regulated voltage across the load (Volts)
• RL: Load resistance (Ω)

Zener Diode Power Dissipation
Pz = Vz × Iz

• Pz: Power dissipated by the Zener diode (Watts)
• Vz: Zener voltage (Volts)
• Iz: Current through the Zener diode (Amps)

Series Resistor Power Dissipation
Pr = (Vin - Vz) × (Iz + IL)

• Pr: Power dissipated by the series resistor (Watts)
• Vin: Input voltage (Volts)
• Vz: Zener voltage (Volts)
• Iz: Zener current (Amps)
• IL: Load current (Amps)

In simple terms, the series resistor drops the extra voltage, the Zener diode maintains a stable output, and both components must be rated to handle the power safely.

Parameters Explained

Vin (Input Voltage): The supply voltage provided to the circuit. This is usually higher than the desired regulated voltage.

Vz (Zener Voltage): The fixed output voltage provided by the Zener diode. This defines the low voltage you want to achieve.

RL (Load Resistance): The resistance of the load connected to the output. It determines how much current the load draws.

IL (Load Current): Current consumed by the load. Higher load current requires a lower resistance load.

Iz (Zener Current): The current flowing through the Zener diode, ensuring it regulates properly.

Rs (Series Resistor): The resistor that drops the extra voltage from Vin to Vz while limiting current.

Pz and Pr (Power Dissipation): Heat generated in the diode and resistor. Correct ratings prevent overheating and failure.

How to Use the High Voltage To Low Voltage Calculator — Step-by-Step Example

Let’s take an example to see how this works.

1. Given Values:
   • Vin = 20 V
   • Vz = 12 V
   • RL = 240 Ω
2. Step 1: Load Current
   IL = Vz / RL
   IL = 12 / 240 = 0.05 A (50 mA)
3. Step 2: Assume a Zener Current (Iz):
   Let’s choose Iz = 20 mA for stable operation.
4. Step 3: Series Resistor
   Rs = (Vin - Vz) / (Iz + IL)
   Rs = (20 - 12) / (0.02 + 0.05)
   Rs = 8 / 0.07 = 114.3 Ω
5. Step 4: Power Dissipation
   • Pz = Vz × Iz = 12 × 0.02 = 0.24 W
   • Pr = (Vin - Vz) × (Iz + IL) = 8 × 0.07 = 0.56 W

Result: Use a 115 Ω resistor rated for at least 1 W, and a Zener diode rated above 0.25 W (choose 0.5 W or 1 W for safety).

This ensures the circuit can regulate 20 V down to 12 V reliably.

Additional Information

Here’s a quick reference for standard Zener diodes used in voltage regulation:

Zener Voltage (Vz)	Common Use Case	Typical Power Rating
5.1 V	Logic circuits	0.5 W – 1 W
9.1 V	Small devices	0.5 W – 2 W
12 V	General purpose	0.5 W – 2 W
15 V	Higher loads	1 W – 5 W

FAQs