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     Northam, Western Australia
     QRZ Page: VK6YSF

 

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REGULATOR 12VDC TO 5.0VDC 2.0A

12V to 5.0V DC/DC 2.0 Amp voltage regulator. December 2024.


 

A 5.0VDC supply was required to power radio receiving equipment from a 12VDC main supply. A more efficient switch mode power supply was rejected as there was concern that this type of equipment may generate RF noise, therefore an older regulator type supply was chosen.

The TIP2955 transistor provides the necessary current when the load requires is more than 1A, while the 7505 ensures the voltage stays constant at 5V. The 7505 continues to regulate the voltage to 5V, while the TIP2955 handles the increased load, ensuring the output remains stable at 5V and can supply up to approximately 2A.

  

Fig 1 Schematic diagram

 

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.

 

Since the circuit uses linear regulation, it is less efficient compared to switching regulators, especially at higher currents. The power dissipation in the form of heat can be significant, which will require a large heat-sink for the TIP2955 and 7505. The transistor and regulator chip are fixed to the heat-sink with heat conducting grease to aid the heat transfer.

 

Testing results

 

Load Amps Input Voltage Out put Voltage Temperature TIP2955 
0 13.58v 5.06v Ambient, 21.0°C
2.07A 12.8v 5.06v 86°C. Device rated at 130°C

Fig 3 Load test

Power

Output Power (5.0v): 10.4W

Input Power (12.8v: 26.5w

Efficiency: 39%

 

TIP2955 Pin-outs

 

The TIP2955 is a PNP power transistor commonly used for high-power applications. Its key specifications are as follows:
 
Electrical Characteristics
Transistor Type: PNP
Collector-Emitter Voltage (Vceo): -60V (maximum)
Collector-Base Voltage (Vcbo): -100V (maximum)
Emitter-Base Voltage (Vebo): -5V (maximum)
Collector Current (Ic): -15A (maximum continuous)
Peak Collector Current (Icm): -25A (non-repetitive)

 
Thermal and Power Characteristics
Total Power Dissipation (Ptot): 90W (with proper heatsinking at Tc = 25°C)
Junction-to-Case Thermal Resistance (Rθjc): Typically 1.39°C/W
Operating Junction Temperature (Tj): -65°C to +150°C

 
Gain Characteristics
DC Current Gain (hFE):
Typically 20–70 (at Ic = -4A, Vce = -4V)
Minimum of 15 at higher currents (Ic = -10A)

 
Switching Characteristics
Transition Frequency (fT): 3 MHz (typical)
Collector Capacitance (Cc): Approximately 300 pF (at Vcb = -10V, f = 1 MHz)

7805 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.

 

Fig 4 TIP2955 transistor and 7805 regulator pin-outs and general specifications.

 

 

 

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Page initiated 01 December, 2024 

Page last revised 09 December, 2024

 

 

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