13.8V 20A power supply

Description: This power supply unit (PSU) is specifically designed for high-current ham radio transceivers. It safely provides approximately 20 Amps at 13.8V. For lower current applications, a separate current-limiting output is available, capable of 15 mA up to a total of 20A. The power transformer should be able to deliver at least 25A at 17.5 to 20V. Lower voltage results in reduced power dissipation. The rectified current will be smoothed by capacitor C1, which should have a capacity of no less than 40,000 µF (a general rule of thumb is around 2000 µF per Amp), although a capacity of up to 50,000 µF is recommended. This capacitance can be achieved by connecting several smaller capacitors in parallel. The core of this design is a standard 12V regulator (7812). The output voltage can be adjusted to the desired value (13.8V in this case) using two external resistors (R5 and R6) with a specific formula. The low currents (15 mA) will maintain the regular function of the 7812. When the current exceeds 15 mA, the voltage drop across R4 will activate Q3, which manages the high output current. Q3 is a PNP transistor (Ic > 25) with a current amplification factor of at least 20, and the tested component is the 2N5683. The current-limiting resistor RL, for a maximum output of 20 Amps, should be 0.03 Ohms and rated for at least 15W. Resistance wire or several resistors in parallel can be used to achieve the required resistance and power ratings. Other current values can be calculated based on this rule. The RL and Q2 (a 3A PNP transistor such as the BD330) function as an automatic short-circuit fuse. When the maximum current reaches 20 Amps, the voltage drop across resistor RL will activate Q2, limiting the base-emitter current of Q3. Q1, in parallel with Q2, illuminates LED 1 whenever the current-limiting circuit is active. When the fuse is triggered, Q2 bypasses R3 to prevent excessive current through IC1, ensuring it operates without additional cooling. LED 2 lights up each time the PSU is powered on. This circuit is simple as there is no explicit current-sensing resistor; instead, it utilizes the Rds-on resistance of the N-channel FET, which manages load cutoff from the source. The FET's behavior is depicted in diagram 2. As the current Id increases, the voltage Uds across the Rds begins to rise slowly but accelerates post-threshold. This indicates that prior to the threshold, the FET acts as a resistor, while afterward it functions as a constant current source. The D2, R3, and the base-emitter connection of Q4 monitor the Uds voltage of FET1. When the voltage reaches a sufficient level, Q4 shorts the gate of FET1 to ground, cutting off current flow. A specific gate voltage is required for FET1 to activate, provided by a voltage divider comprising R8, Z1, P1, and R9. The maximum gate voltage is determined by Z1, while the minimum is approximately 3.6V. The Z1 voltage (Uz1) dictates the maximum current through FET1. For example, to achieve 5 Amps, Uz1 should be set to 5.6V, and for 20 Amps, around 9.6V. Capacitor C4 affects the response time of the current limiter; a value of 100 µF yields a reaction time of about 100 ms, while 1 nF results in a 1 µs reaction time. Within the specified limits, P1 adjusts the current output from 15 mA to 20A. Both outputs can be utilized simultaneously, but the total output current is constrained by the value of RL. This PSU can also be configured for higher outputs, provided the transformer meets the current requirements and adequate cooling is supplied for Q3.

The schematic of this power supply unit illustrates various components and their interconnections. The transformer feeds AC voltage to the rectifier circuit, which converts the AC to DC. The smoothing capacitor C1 filters the rectified output, ensuring a stable DC voltage. The 7812 voltage regulator maintains the output at 13.8V, with the adjustment facilitated by resistors R5 and R6. The current limiting mechanism is integrated into the design through the use of the PNP transistors Q2 and Q3, providing both protection and functionality. The LED indicators (LED 1 and LED 2) offer visual feedback on the operational status of the current limiting circuit and the power supply's power state, respectively. The inclusion of a FET for current sensing enhances the efficiency of the circuit, allowing for rapid response to current fluctuations while maintaining overall system integrity. The design accommodates modifications, such as the addition of an ammeter, to enhance usability and monitoring capabilities.This PSU has been especially designed for current-hungry ham radio transceivers. It delivers safely around 20Amps at 13. 8V. For lower currents, a separate current limiting output, capable of 15ma up to a total of 20A has been added. Let us see what we have got here. The power transformer should be capable to deliver at least 25A at 17. 5 to 20V. Th e lower the voltage, the lower power dissipation. The rectified current will be ironed  by the C1, whose capacity should not be less than 40. 000uF, (a golden rule of around 2000uF/A), but we recommend up to 50. 000uF. This capacity can be built up by several smaller capacitors in parallel. The base of this design is a simple 12V regulator (7812). The output voltage can be brought to desired value (here 13. 8V) by two external resistors (R5 and R6) using this formula: The low currents (here 15mA) will keep the 7812 in its regular function. As soon as the current rises over 15ma, the voltage drop on R4 will open  the Q3, actually handling the high output current.

This is a PNP transistor (Ic>25) and current amplification factor of at least 20. The one that has been tested and proven here is the 2N5683. The current limiting resistance RL, for the maximum output of 20 Amps should be 0. 03 Ohms, rated at least 15W. You can use the resistance wire or switch several resistors in parallel, totaling the resistance/power values. Values for other currents can be calculated by the rule: The RL and Q2 (3A PNP such as BD330) form a short circuit automatic fuse .

As soon as the maximum current reaches 20Amps, the voltage drop over the resistor RL will open Q2, and thus limit the B-E Current of Q3. Parallel to Q2 is Q1, which lights the LED 1 whenever the current limiting circuit is active. When the fuse  is active, the Q2 bridges the R3, so the full current would flow through the IC1, and damage it.

Therefore the R4 is inserted, as to limit the IC1 current to 15mA. This makes it possible to run the IC1 without any cooling aid. The LED 2 will light up every time the PSU is switched on. This circuit is very simple too. You will notice that there is no current sensing resistor. But it is really there, in a form of the Rds-on resistance of the N-channel FET, which actually handles the load cutoff from the source. The function of the FET is shown in the diagram 2. When the current Id is rising, the tension Uds over the resistance Rds rises very slowly in the beginning, but very fast after the knick.

This means, that before the knick the FET behaves as a resistor but after it, works as constant current source. The D2, R3 and B-E connection of the Q4 will sense the Uds voltage of the FET1. When the voltage rises enough, the Q4 will shortcut the FET1 gate to mass, and cut the current flow through the FET 1 off.

However, to enable the FET1 to open, there is certain gate voltage necessary, which in this case is brought up by the voltage divider consisting of R8, Z1, P1 and R9. So the maximum Gate voltage will be the one of the Z1, and the minimal will be around 3V6. The Z1 voltage (Uz1) will thus determine the max current flowing through the FET 1. The diagram 2 will show that for 5 Amps the Uz1 should be 5V6, and for 20Amps around 9V6. The Capacitor C4 will determine the velocity  or the reaction time of the limiter. 100 uF will make the reaction time to be around 100ms, and 1n will make it 1us. Within the designed limits, the P1 will limit the current output in the range of 15mA to 20A. You can use both output simultaneously, but the total output current will be limited by the value of the RL.

This PSU can be built also for higher outputs, as long as the transformer will handle the current requirements, and you provide sufficient cooling for the Q3. I have received several requests for some modifications, and the one I find useful is the addition of an amp meter.

Therefore the slightly modified diagram is included in this revision. All