DC Adapter with Battery Backup

With just a low cost DC adapter and the circuit described here it is possible to build a low cost stabilised, uninterruptible 9 V supply. On the grounds of safety and economy, a simple unstabilised 12 V D.C. adapter is used as the power source, a universal adapter with its output set to 12 V will do equally well. The output voltage of an adapter under low load conditions (up to approximately 1/3 of the rated output current) is over 15 V, even at the rated output current, there will be sufficient voltage to supply a 9 V voltage regulator.