Power-on delay for Atari ST: When the Atari ST is provided with an external hard-disk drive, it has to be reset after about 15 seconds, as otherwise the drive is not enabled. The delay circuit presented here obviates that inconvenience by ensuring that the computer is not switched on until a (presettable) period of time has elapsed. The circuit may also be used with the combination of MS-DOS computer and HP DeskJet printer since the latter can be switched on only after the MS-DOS machine is powered. If that type of printer is used with an Atari ST. the printer must be switched on before the computer is. No doubt, there are other situations where this delay may prove useful.
Operation of the circuit is fairly simple. After the mains has been switched on, capacitor C3 is charged via R5 and P1. When, after a period of time set with P1, the potential across C3 has reached 12 V, zener diode D1 conducts and switches on thyristor Thi. That device ensures that bridge B1 provides an a.c. connection ‘ between the mains input and the mains output. At •• the instant the thyristor is switched on, owing to the anode-cathode current, its gate voltage rises slightly. This results in C3 being charged a little more 1 and thus serves as a gate-current buffer to I ensure that the thyristor remains on during the mains zero voltage crossings. 1 Therefore, once the thyristor has been I switched on, it stays on.
The delay is best built into a small man-made fiber box with integral mains connectors. The circuit is best built on a small piece of Veroboard or other prototyping board, bearing in mind that it will carry the full mains voltage. This means that tracks carrying the mains must be separated by at least 3 mm and preferably 5 mm. This probably entails the removal of tracks between the mains-carrying ones.