13,8v/5Amp 78H05 based power supply for portable use:

FT817ND QRP all band transceiver

By Guy, de ON6MU

I wanted a compact but powerful and versatile power supply to take along when I go on vacation, camping etc.
I use this power supply for a Yaesu FT-817ND and similar (QRP) transceivers. This transceiver uses 2 amps at full 5 watt TX and so the power supply has a large enough margin to let the build-in battery cells to charge and let your FT-817 work at full power and still have power to spare. Still, this power supply is compact enough to be used for portable use and the FT-817 fits perfectly on it HI. Ok, the power supply could be made smaller, but I needed 5 amps to use with my power hungry TM-255 @ 5 TX watt power.

The 78H05 can easily manage 5 amps at constant full load, has thermal overload protection, short-circuit protection and safe area protection! If the safe operating area exceeds, the device shuts down rather then failing and damaging your expensive transceiver/equipment!

If you do not need 5 amps you can always use lesser diode bridge and transfo amps which allows you to reduce the size of the power supply. The FT817 and simular QRP transceivers often don't need more then 2 to 3 amps anyway. An ideal power supply for SWL's too, like for FRG-100 and so many others.

Remember to isolate the 78H05 and the 7805 from the chassis! This is very important! Also use thermal cooling paste/grease and a conservative but efficient heat sink.

The zenerdiode is a 5watt (1,3 watt should be sufficient too) 8,2 volt type and the extra diode in series gives you an extra 0,6 volt which gives you exactly 13,8 volts (5volt + 8,2volt + 0,6volt). Most transceivers can work with lower voltages, so if you want you can leave out the diode and so giving you an output voltage of 13,2 volt.
S1 is used to switch and monitor between the two voltage regulaters output. Calibrate the meter by using the 4k7 pot.

For personal use I included a little 1 amp circuit along with it to be used for other equipment (or batteries etc.) when I'm at /A or /P. Switching S2 and S3 can obtain you 4 different voltages depending on the two zenerdiodes zd1 and zd2. This saved me carry space because I do not need to take along other power supplies for my little radio, portables etc. For this reason I made a charger interface (or you can choose to build it in) and a cable with 6 different types of connectors (also a 9 volt clip). You can change zd1 & zd2 to whatever voltages you might need on your /A adventure. The connector is been fed through a 25 Ohm 10 watt resistor which can be switched on and off to be used as a simple current reducer for battery charging. I mounted the resistor agains the metal chassis of the switch which helps to cool it down a little if using 12 (or more) volts via the 78M05 (1AMP).

The second way (instead of switching between fixed voltages) is a schematic that uses the 7805 to continues regulate the output voltage bewteen 5 to 14 volts. You may choose whatever principle you have the best use for in your power supply.

The multi-functional power connector
And the little schematic:
As some devices uses reverse polarity (negative on the center pin) I placed two LED's to show you which polarity is used.

S2 S3 V
a a 5
b a 7,4
a b 9,3v
b b 12

RE-PSF14A5 Schematic

Regulated 12 volt power supply using 78H05

The power supply insides


Alu box: 132x55x128


Technical specs:
Description = Fixed Positive Voltage Regulator
Output Voltage Nominal (V) = 5.0
Load Current Max. (A) = 5.0 - 6.0 peak
Load regulation = 0.2% V-Out
Tolerance (%) = 1.0
Drop-Out Volt Max. = 2.5
P(D) Max.(W) Power Dissipation = 50.0
Supply Voltage Maximum (V) = 30.0
Temperature = -55C to 150C
Package = TO-3
Pins = 3
Military = N


More of my projects: 12-20amps power supply

The power supply in my quick-take-along-emergency-ft817-carry-case (what a name!)