Sunday, February 24, 2019

13.8V 5A Power Supply

13.8V power supplies are commonly used in armature radio experiments. Most of the portable armature radio transceivers are designed to work with a 13.8V power source. We mainly build this power supply unit to power some of our armature radio circuits and modules.

This design is based on the popular LM338 5A voltage regulator. We choose this regulator because of to it’s higher current rating, short-circuit protection feature and higher availability.

The prototype version of 13.8V - 5A power supply unit.

Apart from that, we include MC3423 based crowbar sensing circuit to preventing an over-voltage condition of a power supply unit.

We design this circuit by using commonly available 18V×2 (5A) + 12V (1A) transformer. This transformer is available in the market because it’s commonly used with some AF power amplifier systems/kits. We use its 12V terminal to drive 120mm - 12V cooling fan.

We strongly suggest building this power supply using PCB. During the assembly, we recommend to tin-plate all high current lines of the PCB and also use an IC socket for U1 (MC3423).

The printed circuit board of the power supply unit

In our prototype version, we mount LM338K on 50mm × 155mm × 3mm heatsink. To cool this heatsink we use above mentioned 120mm 12V (100mA) cooling fan.

After everything is assembled, use the following steps to set up the power supply unit’s output voltage and tripping voltage:
  1. Open J3 link.
  2. Power on the circuit and measure the voltage of TP2. It should higher than 18V and less than 30V.
  3. If TP2 voltage readings are correct, connect an accurate voltmeter to output terminal of the power supply unit.
  4. Adjust RV4 and set the output voltage to 14.5V.
  5. Now connect another voltmeter to TP1.
  6. Adjust RV1 until it shows 14.0V – 14.5V output. After this adjustment, PSU starts to trip if it’s output voltage raise beyond 14.5V.
  7. Adjust RV2 and RV3 and set the output voltage of the power supply unit to 13.8V. Use RV3 to precisely adjust the output voltage.
  8. Check the voltage of TP1 again. In 13.8V, it should show voltage less than 2V, if not, calibrate the RV1 again.
  9. After calibrating the TP1 voltage, close the J3 link and disconnect voltmeters from the PSU.

The finished prototype of 13.8V - 5A power supply unit.

Schematic and PCB design files of this power supply unit are available to download at google drive.

1 comment:

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