The circuit shown is the simplest scheme possible to provide power-down protection for your ZX/TS. You just need a battery holder capable of holding six “one-life” batteries (AA, C or D size), a few silicon diodes (GE531, Radio Shack 276-1141, or other 50 PIV, 2A silicon rectifier) and an on-off switch. An enclosure, some zip cord and hookup wire, a mini phone plug and jack complete the parts list.
As long as the input voltage from the power supply is greater than the battery voltage (about 9.3 volts or up), diode D1 conducts, and no drain is put on the battery. If the supply voltage drops below the battery voltage, D2 conducts, and the battery keeps the machine going. Alkaline AA cells should run about 40 min., C cells over an hour, and D cells up to 3 hours.
Diode D3 drops the voltage to the computer for decreased regulator heating (and lower overall temperature), You can try two diodes in series for even less heating; most machines run fine down to about 8 volts. If your supply does not provide the minimum 9.3 V. under load, chances are you can bring it into spec by connecting a 1000-2000 uF. 16V. electrolytic capacitor across the input jack JL. Observe polarity! Also, always turn the switch off when connecting/ disconnecting the lead to the computer; even a brief short can damage a diode.
In the next installment I’ll show you an enhancement of this circuit by including indicator lights and using rechargeable batteries. After that we’ll get into building an entire replacement power supply for more expanded machines.
