LM7905 / 79xx Regulator Calculator

The 79xx series (e.g. LM7905, LM7912) are fixed negative-voltage linear regulators used in dual-supply electronics — and this calculator checks whether your circuit design is thermally safe and efficient. Enter your input voltage, output voltage, load current, capacitor values, and ambient temperature to get power dissipation, junction temperature, efficiency, and heat sink status. Dropout voltage and voltage drop are also returned so you can verify adequate headroom for your chosen regulator.

V

Negative input voltage (must be at least 2V more negative than output)

A

Maximum output current (79xx series supports up to 1.5A)

µF

Recommended: 0.33µF ceramic capacitor

µF

Recommended: 0.1µF ceramic capacitor

°C

Operating ambient temperature

°C/W

Junction to ambient thermal resistance (TO-220: ~65°C/W without heatsink)

Results

Power Dissipation

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Voltage Drop

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Junction Temperature

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Efficiency

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Heat Sink Status

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Dropout Voltage

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Frequently Asked Questions

What is the LM7905 voltage regulator and how does it work?

The LM7905 is a three-terminal negative voltage regulator that provides a fixed -5V output. It belongs to the 79xx series of negative voltage regulators and can handle input voltages from -7V to -35V while maintaining a stable -5V output with up to 1.5A current capability.

Why are negative voltage regulators important in circuits?

Negative voltage regulators are essential for dual-supply circuits, operational amplifiers, audio applications, and digital circuits that require both positive and negative supply rails. They provide stable negative voltages needed for proper circuit operation and signal processing.

What is the minimum input voltage required for LM7905?

The LM7905 requires a minimum input voltage of -7V to maintain regulation. For reliable operation, the input should be at least 2V more negative than the output voltage, so -7V minimum for -5V output.

How do I calculate power dissipation in a 79xx regulator?

Power dissipation equals the voltage drop across the regulator multiplied by the load current: P = (Vin - Vout) × Iload. This heat must be managed through proper heatsinking to prevent thermal shutdown and ensure reliable operation.

What capacitors are needed for LM7905 circuit design?

A typical LM7905 circuit requires a 0.33µF ceramic capacitor on the input and a 0.1µF ceramic capacitor on the output. The input capacitor improves transient response, while the output capacitor enhances stability and reduces output noise.

When do I need a heat sink for 79xx regulators?

A heat sink is needed when power dissipation causes the junction temperature to exceed safe limits (typically 125°C). Higher voltage drops, load currents, or ambient temperatures increase power dissipation and heat sink requirements.

What are the alternatives to LM7905?

Alternatives include other 79xx series regulators (7906, 7908, 7912, 7915), adjustable negative regulators like LM337, or switching regulators for higher efficiency applications. Choose based on output voltage, current requirements, and efficiency needs.