It is economical to use battery eliminator but with frequent power failure in most developing country, it becomes annoying to change over from battery eliminator to battery operation every time the electric supply fails.
A simple way to avoid the nuisance is by connecting the battery and power pack in parallel. But the advantage with such a system is that when the eliminator voltage falls, even slightly (say from 6V to 5V), though the performance of the set remains unaffected, the battery starts supplying the receiving set and therefore gets exhausted quickly.
Circuit Description of Simple Battery Changeover Relay
The circuit shown in figure 1 overcomes this problem. An advantage of the circuit is that it requires no extra voltage supply besides the battery and the power pack voltage. It is thus quite easy to include the circuit inside radio receiver or a gramophone player.
The principle of operation is quite simple. T1 is the battery control transistor. A silicon transistor only should be used as it has negligible leakage. Let us assume that the power is on and “Vi’ is present, then VBC (across B and C) will be VB -Vi. Usually, since Vi will be greater, it will be negative, and the transistor will remain at cut-off. Even if Vi is less by l V, than VB, the bias voltage at points C and B is not enough to pull the transistor into conduction. Thus, the battery is protected from discharge even under enough eliminator Output.
- A single DR25 can handle a max. current 250mA. For more current, parallel more DR25 diodes.
- A single diode D2 raises changeover point by 0.6V. For higher changeover voltage, connect more diodes in series.
(At VBB < 6V – 1 no.: at VBB > 9V-2 nos.)
- R1 is subject to variations with VBB. Set for VCE of min. possible.
PARTS LIST OF SIMPLE BATTERY CHANGEOVER RELAY
|Resistor (all ¼-watt, ± 5% Carbon)|
|R1 = 470 Ω|
|T1 = SK100
D1 = DR25
D2 = 1N4001
|SW1 = ON/OFF switch|