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28-V, High Current Power Supply - Adjustable?

Apr 20th 2014, 13:35


Joined: Mar 9th 2012, 17:06
Total Topics: 0
Total Posts: 0
I am referring to the project on page 17.36 of the 2014 ARRL Handbook. It is just what I need but for one feature. I need the output voltage to be variable between approximately 5.0V and 24.0V. My question is: Is it possible to make the supply adjustable over that range by changing the values of R8, R9 and R10? I want to build a supply that can output between 5.0V-24.0V at 10A.

I am seeing that it is not easy to find the transformer listed in the project description. Therefore, I would like to substitute it with a Hammond 28V 6.25A (CT @ 14V at 12.5A) . I realize that the supply current rating will drop from 15A to 6A, but that is fine for my needs. Yes, I would like to have a supply that covers the voltage and current requirements of the newer linear amps as well, but it does not seem possible due to the availability of the Avatar transformer.

Thank you,

Steffen (KA2KNX)
Apr 21st 2014, 14:26


Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

Chapter 17 is the RF Power Amplifier Chapter. But, I see a suitable project on page 11.36 of the 1999 ARRL Handbook.

There is an schematic error--rather than go to pin 2, R7 should go to the common of R2, R3, R4, and R5.

Typically, there are two distinctly different 723 circuits--one goes from 2 to 7 volts, and the other goes from 7 to a much higher voltage. Thus, you won't be able to go down to 5 volts with this circuit.

A problem with a wide voltage range linear supply that can supply a lot of current is that you will be dissipating a lot of heat when supplying 5 volts at 6 amps. When putting out 5 volts instead of 24 volts at 6 amps, the pass transistors will need to dissipate an additional 114 watts. De-rating the supply from 10A to 6A isn't enough to compensate for the extra heat--you actually need twice as much heat dissipate to provide 5V at 6A than to provide 24V at 10 amps. Assuming 31 volts, I calculate 156 watts compared to 70 watts.

It may be more practical to built two separate supplies, one to cover 2 to 7 volts, and another to cover 7 to 24 volts.

Zack Lau W1VT
ARRL Senior Lab Engineer

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