13.8V 20A Linear Power Supply circuit and explanation

This linear power supplies project for communication equipment are among the most commonly built electronic projects. Almost every technically inclined radio amateur has built at least one. But unfortunately most designs, even those published in well respected books, are unnecessarily complicated, or have some specific drawbacks. Thedesign presented here is a little bit unusual in its arrangement, but offers some advantages over the usual designs.


Here is the power supply schematic diagram (full resolution version for printing).


This power supply delivers a highly regulated 13.8V, adjustable over a moderate range, at a continuous current of up to 20A. It is current-limited to approximately 25A, and short circuit protected for as long as the heat sink can keep the transistors cool enough. It is probably the simplest design that can accomplish this.

Some notes about this circuit:
  • Use a transformer for the primary voltage you need. The 3A fuse is for 220 or 240V primaries. If you use something in the neighborhood of 110V, use a 6A fuse.
  • The rather high transformer rating of 35A accounts for the losses that occur due to the capacitive input filter. If your transformer is rated for capacitive input, then a 25A value is enough.
  • Of course you can make up C1 by placing several smaller capacitors in parallel. Likewise, the 0.1 Ohm, 5 Watt resistors can be made up by several in parallel, for example by 5 resistors of 0.5 Ohm, 1 Watt each.
  • The LM336Z-5.0 voltage reference IC should not be replaced by a zener diode. Zeners are not nearly as stable. A different voltage reference IC can of course be used, if R2 and R3 are modified for the different voltage.
  • D1 and Q2 through Q6 need heatsinking. Only Q2 needs insulation. D1 dissipates up to 60W, Q2 up to 25W, while the pass transistors dissipate up to 30W each in normal use, but may reach a level of 130W during short circuit! Take this into account when choosing the heat sink!
  • R5 exists only to make sure that the transistors can actually be driven off. The 741 is not a single-supply operationalamplifier, so it cannot drive its output very low. If a true single-supply opamp is used, then R5 becomes unnecessary.