Radio Amateurs and Hamnet members might find this project a useful
addition to emergency equipment or general usage in the shack when Eskom decides to go into darkness mode. This controller will automatically switch from a 220v AC powered 12v Power Supply to 12v battery backup power.
I played around with relays but find that there is a power dip visible on the load (12v 50 watt, 4.1 amps globe) side of the circuit as the relay takes a bit long to switch over. Though this will not effect the radio, I do not like this dipping. Now I know this power dipping can be fixed with fast switching diodes and capacitors, I found a far more simple yet effective method that is more trustworthy than relays.
This
circuit will allow any 12 volt radio or related equipment to continue
to operate when commercial power is interrupted to the DC power
supply. In addition, it will maintain a charge on the auxiliary power
source.The
bridge rectifier, F1 and F2, must be large enough to supply the
maximum current drawn by the radio. R1 supplies a trickle voltage
across the reverse biased section of the bridge while the power
supply is in operation. This provides a continuous charge to the
lead acid battery.
Uninterrupted operation will be realized even in the event of a commercial power outage, without the radio "dropping out" at all. This circuit is now used on the 145.550 Mhz RF Link Radio of the GRHub Network and has been found to work great during Eskom power outages. However for now the circuit above must stay operational in the event of a power outage.
Finally: I installed three good quality fuse holders in the positive lines to the battery, power supply and the radio (load). The quality fuse holders I used was salvaged many years ago. It is thus far the best inline fuse holders I have used. Thankfully it is not some of the junk you buy at your automotive parts dealer. I used the ugly style method to build the unit as it will go into a project cabinet if I can find a suitable one. One can further enhance the unit by installing a two color LED as a power source indicator. This unit can also be used for other applications.
Part 2 available HERE
Sources:
1. 73 Amateur Radio Today September 1992
2. Butch Herring KE5V.
3. Johan ZS1I
