Brookings Radio Research Club

Welcome to South Dakota!!

www.w0bxo.org

Brookings, SD
146.940 ( - ) 146.2 tone

A Battery Back-Up Modification of Astron Power Supplies

This modification allows a battery to be floated on the Astron power supplies for battery back up while protecting the regulator chip from damage when commercial AC power mains voltage is removed.

Caution: This mod just adds a current limiting resistor in line with the voltage feed back of the regulator chip, otherwise destruction of the chip will result.

Modification: Installation is straight forward:
A) Cut the trace on the PCB between Pin 4 of IC UA723 and the wiper of R5.
B) Bridge the cut trace with a 10K ohm 1/4 or 1/2 watt resistor
C) Set the output voltage by adjusting R5 to desired "float" voltage with no load or battery connected to the power supply.

Voltage settings:
A) For standard deep cycle marine batteries, set voltage to 13.50 vdc. ( Do not forget to periodically check the water level in battery! )
B) For sealed gel cell batteries, set voltage to 13.80 vdc.
!! Use of standard car batteries is not recommended !!

It is HIGHLY recommended that a fuse rated at equal to or less than the maximum current rating of the power supply be installed between the battery and the power supply. This will protect your system in the event that the power supply's crowbar circuit fires and tries to pull the current of the battery to ground.

Recommended hook up should be:
1.) Hook equipment to the battery through a fuse.
2.) Hook battery to power supply through a second fuse.

In this configuration your equipment is being powered by the battery and the power supply is serving two functions:
One is keeping the battery charged, and second to aid during high current draws. It will also allow the battery to act as a surge protector adding some additional protection to your equipment from line surges.

A couple of additional comments presented by Joe Montierth ....

One, the 10K resistor can be 1/4 watt or even 1/8 watt, the 1/2 watt is way overkill. There is not much current through it or voltage across it to generate any heat.

Two, depending on the size battery it is attached to, you might want to reduce the current limit point on the Astron. The 35 amp Astron will current limit somewhat above the 35 amp point, maybe around 37-39 amps. If you have it attached to a BIG battery, it can run several hours at the current limit, and burn itself out. If you parallel R4 with a 1K pot, you can adjust the current limit to a point less than 38 amps, usually about 60-70% of the power supply rating is good.

Three, always turn the PS "on" before attaching it to the battery. This will keep the internal caps charged up, so they don't have to suddenly charge through the pass transistors on the Astron.

If you have enjoyed this construction article please e-mail me at kuggie //at// kuggie//dot//com and let me know.
I may take the time to do more quality construction projects in the future.

The Astron Corporation Irvine, California 92618
949-458-7277 8a-5p Pacific Time.

Articles also listed on Civil Air Patrols military web site.

RS35 power supply current limiting.

Astron schematic with mod

Astron RS20A-BB battery backup schematic

Pictures supplies by WB0TSR.