12 Volt 20 Amp Power Supply

Description: A 12 Volt high current 20 Amp power supply. The output voltage is variable from 12.2 Volt to 14.4V, allowing it to be set for any device requiring voltage and current within that range. This power supply unit (PSU) utilizes an LM723 as the regulator, with four parallel connected outboard pass transistors and current limiting above 25 amps. The input transformer T1 has a primary rating of 240V for the UK domestic supply and 120V for North America. The secondary must be capable of supplying 20V AC RMS at 25 Amps. F1 is the input fuse and must be a slow-blow type rated at 4 amps. The supply is rectified by BR1, a 35 Amp bridge rectifier type MB356. Capacitors C1 and C2 serve as smoothing capacitors, with the unregulated DC voltage reaching about 26V with no load. The bulk of the regulation work is performed by the LM723 regulator. The preset RV1 adjusts the output voltage, while resistors R1 and R3 set the upper and lower voltage limits. The error signal at pin 10 feeds a Darlington configuration made with transistors Q1 to Q5. The BD131 boosts current from the regulator at pin 10 and provides base current to the parallel connected 2N3055 power transistors (Q2 to Q5), each capable of supplying up to 5 Amps of current into the load. As device parameters vary, particularly the forward current gain (hFE) of a power transistor, resistors R3 to R6 assist in current sharing. With RV1 set to approximately 10%, the output voltage is about 12.3 Volts. The regulation is robust, maintaining a steady output voltage of 12.3 Volts even at 20 Amps. Only when the load increases to 21 Amps does the output begin to drop by 50mV. Current regulation is integrated into the circuit with resistors R7 to R10, a parallel combination of 0.1 ohm, 5-watt power resistors. The effective resistance is 0.025 ohms, connected back to the current sense pins 2 and 3 of the LM723. When the voltage across these pins reaches 0.6 Volts, the IC enters current limiting mode, which begins at 23 Amps, further limiting output current as load current increases. Load regulation at 14.4 Volts is also demonstrated, showing that with progressively higher currents, the output will decrease further, although this design is intended for a maximum of 20 Amps. Capacitors C5 and C4 are utilized for high-frequency decoupling, while diode D7 protects against high voltage back EMF that could damage the IC and semiconductors. LED1 indicates that the supply is functioning correctly, while LED2 indicates that the regulated output is operational. The file 12v20a.zip contains the circuit, symbols for the LM723 and potentiometer, subcircuits for the LM723 and potentiometer model, and SPICE data for the semiconductors. After downloading and unzipping it to the desktop or home folder, it should operate without issues. Individual device currents and voltages can be measured or the circuit can be modified.

This 12 Volt power supply circuit is designed to provide a stable and adjustable output voltage suitable for various electronic devices. The use of the LM723 voltage regulator ensures precise voltage control, while the additional pass transistors provide the necessary current handling capability. The transformer is essential for stepping down the mains voltage to a suitable level for the circuit, and the bridge rectifier converts the AC voltage to DC, which is then smoothed by the capacitors to reduce ripple.

The circuit's design includes several protective features, such as current limiting and high-frequency decoupling, which enhance reliability and performance. The feedback mechanism through the error signal at pin 10 of the LM723 allows for accurate voltage regulation even under varying load conditions. The configuration of the Darlington pair increases the output current capacity, ensuring that the power supply can deliver up to 20 Amps without significant voltage drop.

Incorporating resistors for current sharing among the power transistors is a critical design aspect, as it helps to balance the load and prevent overheating or failure of individual transistors. The current sensing resistors provide an additional layer of protection by limiting the output current when necessary, thereby safeguarding the entire circuit.

Overall, this power supply circuit is well-suited for applications requiring adjustable voltage and high current, making it a versatile solution for powering various electronic devices. The detailed schematic and accompanying files provide all necessary information for replication and testing.A 12 Volt high current 20 Amp power supply. The output voltage is variable from 12. 2 Volt to 14. 4V so can be set for any device requiring voltage and current in that range. This PSU uses an LM723 as the regulator, 4 parallel connected outboard pass transistors and has current limiting above 25 amps. The input transformer T1 has a primary rating of 240V for the Uk domestic supply. For North America the primary needs to be rated at 120V. The secondary must be cable of supplying 20V AC RMS at 25 Amp. F1 is the input fuse and must be a slow blow type rated at 4 amp. The supply is rectified by BR1, a 35 Amp bridge rectifier type MB356. C1 and C2 are the smoothing capacitors and the unregulated DC voltage will now be about 26V with no load. 1 The bulk of the work is performed by an LM723 regulator. The preset RV1 sets the output voltage, R1 and R3 setting upper and lower voltage limits. The error signal at pin 10 feeds a darlington combination made with Q1 to Q5. The BD131 boosts current from the regulator at pin 10 and sources base current to the parallel connected 2N3055 power transistors, Q2 to Q5.

Each transistor supplies up to 5 Amps of current into the load. As device parameters vary, in particular, forward current gain hFE of a power transistor, R3 to R6 aid current sharing. With RV1 at about 10% output voltage is about 12. 3 Volts. The regulation is excellent, the graph below simulates a varying load resistance of 0. 5 © to 1 © measured across the output (Vs) in the circuit diagram. The output voltage is solid at 12. 3 Volt, even at 20 Amp. Only when the load increases to 21 Amp does the output start to drop by 50mV. Current regulation is also built into the circuit. R7 to R10 are a parallel combination of 0. 1 ohm 5 watt power resistors. The effective resistor value is 0. 025 ohm and is connected back to current sense pins 2 and 3 of the LM723. Once the voltage across pins 2 and 3 reaches 0. 6 Volt the IC will go into current limit, limiting output current. This starts to happen at 23 Amp, increase in load current now further the current limiting of the LM723.

Load regulation at 14. 4 V is shown below. With progressively higher currents the output will be reduced further, but this design is for 20 Amps not higher. C5 and C4 are for high frequency decoupling, D7 is to prevent high voltage back EMF from damaging the IC and semiconductors.

LED1 shows the supply is healthy, LED2 shows that the regulated output is working. The file 12v20a. zip contains the circuit, symbols for LM723 and potentiometer, subcircuits for the LM723 and potentiometer model and spice data for the semiconductors. Once downloaded to your desktop or home folder and unzipped, it should work without problems. You can also measure individual device currents and voltages or modify the circuit.