
Fig
1 Schematic diagram
To ensure the two 7809 regulators share the load evenly, small series resistors (typically in the range of 0.1 to 0.5 ohms) should be added between the output of each regulator and the load. These resistors help balance the current distribution. Without these resistors, the regulators may not share the load equally, and one could end up supplying more current, potentially leading to overheating.
Both 7809 regulators will dissipate heat proportional to the voltage drop across them and the current they supply. For example, if the input voltage is significantly higher than 9V (e.g., 12V or 15V), the regulators will convert the excess voltage into heat. Heat sinks or other cooling methods will be required to prevent overheating of the regulators.
To improve stability and reduce ripple, capacitors should be placed at both the input and output of each 7809. Typically:
- Input Capacitor: A 0.33 µF ceramic capacitor should be placed close to the input pins of
the 7809.
-
Output Capacitor: A 0.1 µF ceramic capacitor should be placed close to the output pins of
the 7809.
Both 7809 regulators should be placed on suitable heat sinks to prevent overheating, as they will each dissipate power based on their load.

Fig
2 Component layout with PCB strips glued to a negative/ground PCB
square sheet.

Photo
1 Component layout with in heavy aluminium enclosure and heaver
aluminium back mounting plate to act as a heat sink.

Fig 3
7809 regulator pin-outs
The 7809 is a commonly used linear voltage regulator in the 78xx series. Its key specifications are as follows:
Input and Output Characteristics
Output Voltage: 9V (regulated)
Input Voltage Range: Typically 11.5V to 35V (depends on the manufacturer, but at least 2V higher than the output voltage for proper regulation).
Output Current: Up to 1A (with proper heat sinking).
Line Regulation: Typically 0.01%/V.
Load Regulation: Typically 50mV for 0 to 1A load changes.
Thermal Characteristics
Power Dissipation: Limited by the package and heatsinking, typically 15W with proper heat dissipation.
Thermal Shutdown: Yes, integrated protection against overheating.
Operating Junction Temperature: Usually -40°C to +125°C.
Protection Features
Short-Circuit Protection: Yes, internal limiting of current.
Overload Protection: Thermal and current limiting.
Reverse Polarity Protection: Some manufacturers may include limited protection against reverse polarity at the input.
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Page initiated 05
December, 2024
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last revised 09 December, 2024