ZX hardware - Backup Power Protects ZX Programs

ZX Hardware

1. Program Protector

This circuit was designed for the ZX81, but it can be used with any microcomputer powered by 9 V through an adapter. In principle, it is a simple circuit consisting of six 1.2 V nickel-cadmium batteries (7.2 V in total). As long as the adapter supplies power normally, the batteries receive some charge. If the power is interrupted, they automatically take over the microcomputer's power supply, providing sufficient voltage for it to operate correctly.

First, let us recall one feature of microcomputer power adapters: they provide filtered, unregulated voltage, which is higher than the 9 V used to charge the batteries.

This circuit is not, however, a charger for these batteries: after a complete discharge, they need about 50 hours to recharge.

Now let us look at the circuit diagram.

The charging current flows through resistor R2 and LED D2, which will glow very dimly when the batteries are full. This indicates that the microcomputer is being powered correctly by the adapter. Diode D1 is reverse-biased and has no effect in this situation. If the mains power fails, D2 will go out and D1 will conduct, since it is forward-biased; the voltage from the battery pack then goes directly to the microcomputer. Capacitors C1 and C2 are connected at the output to soften voltage surges when switching from the adapter to backup power or back again. An abrupt change in the supply voltage can also cause stored instructions to be lost.

Since this circuit has few components, it is neither expensive nor difficult to build. As always, take care to observe the polarity of the diodes and batteries.

Note that on the ZX81, the positive pole is at the tip of the jack and the negative pole is on its body. On the ZX Spectrum, it is the other way around.

When connecting any microcomputer, pay attention to the power-supply polarity: reverse polarity can cause serious damage. Starting the circuit is simple: plug the adapter's connector into the female jack on the circuit, then connect its jack to the microcomputer. Once switched on, the microcomputer should work without any problems. To test the circuit, simply unplug the adapter from the mains; the microcomputer should continue to operate as before. When you plug the adapter back in, the microcomputer should continue working without disruption.

Parts list

• R1, R2: 470 ohms
• D1: 1N4001 or similar
• D2: red LED
• C1: 0.22 microfarads
• C2: 2,200 microfarads
• Batteries: 6 × 1.2 V, 500 mAh
• Battery holder
• Male jack
• Female PCB jack

Contribution prepared by:

Nenad Pajović
V. Karađorđa br.22
15004 Jelenča