MSE CENTAUR 2, Инструкция по эксплуатации

Страница: 30 / 36

CIRCUIT DESCRIPTION
30
8.4 Speed Indication
The output of VT1 is fed, via a Schmitt trigger, to a monostable circuit (IC3 pins 4,
5, 6, 8, 9 and 10). The pulse width is fixed by a time constant of 22 nf and a 50k
variable resistor RV1. The output pulses are fed to the digital display unit (PCB D)
where they are integrated to give a D.C. voltage proportional to speed.
The digital display unit consists of a 7107 analogue-to-digital converter/driver I.C.
and its associated components, driving two LED's. The two most significant digits
are driven by the 7107 to indicate the speed. The two least significant digits are
connected through twelve 1k resistors (RN1) to give a fixed 00 display.
8.5 Lid Safety Circuit
When the Timer switch is made, contact SW4/2 applies a voltage to charge up
C8. This sets one input of the NOR gate IC3 pins 11 to 13, to ‘1’, thus de-
energising the Lid Solenoid and locking the lid.
When the Timer switches to ‘Off’ the charging current to C8 ceases, but the
discharge time constant is long enough to delay the energising of the solenoid for
about three-and-a-half minutes. This ensures that the lid cannot be opened before
the rotor has stopped.
To allow the lid to be opened as soon as the rotor stops, the circuit comprising:
IC4 pins 1, 2, 3, 4, 5 and 6; IC3 pins 1, 2 and 3, together with VT2, monitors that
the run-down is normal and if so, discharges the capacitor C8. The method
employed uses the outputs of two frequency discriminators: IC4 pins 5 and 6
(directly) and IC4 pins 1, 2, 3 and 4 (after a delay) as inputs to a NOR gate.
If the 60 rev/min discriminator operates sufficiently in advance of the 6 rev/min
circuit, then a pulse is generated by the NOR gate which switches on VT2, to
discharge the capacitor. Under failure conditions no pulse will be generated and
the lid will remain locked for about three-and-a-half minutes.
8.6 Balance Circuit
IC2 pins 4/13 and 5/12 form a bi-stable circuit, which is forced into the required
stable state at switch-on by the capacitor C5. In this stable state pin 13 is near
earth so that Relay A can be energised; and pin 12 is high so that the balance
lamp is off.
If the rotor is unbalanced sufficiently to cause the Balance microswitch to open
momentarily, the bi-stable circuit will switch over to its other stable state. IC2 pin
13 will then be high, de-energising the relay. Pin 12 will be low, switching the
Balance lamp LP2 on and removing the charging current to C8, so that the lid is
unlocked after about three-and-a-half minutes.
The bi-stable can be reset by switching the mains supply off for a few seconds
and then on again.