TECHNICAL INFORMATION
87-900-806-01 (F)
6
VENT CONTROL UNIT DESCRIPTION
The control unit has three main sections: power circuit,
protection circuit, and time circuit (refer to following
figure).
Power Circuit
The mains voltage is stepped down to 12 Vac, which is
rectified by the full wave bridge rectifier smoothed by
capacitor C1 and regulated by U8.
The output voltage of U8 is adjusted by P3 to 6.9 V and
feeds the battery timer circuit, and U7 which steps up
the voltage at 23 V. The output of U7 adjusted by P2
drives the vent valve.
Protection Circuit
If the leads are shorted together or the vent valve coil is
shorted, transistor Q2 detects the condition and drives
Q1 to deenergize relay RL1 and consequently the
battery is disconnected.
To avoid complete discharge of the battery, a sensor
circuit is provided. When the battery voltage reaches 4.5
V, transistor Q4 is cut off.
Subsequently, relay RL1 deenergizes and the battery is
disconnected.
Even though the battery is designed to accept
overcharging, to avoid thermal runaway problems
caused by overcharging, diodes D7 and D9 are
provided. This circuit allows an automatic adjustment of
the charging voltage in relation to the environment
temperature.
Timer Circuit
Two similar timer circuits are provided, where U5, U4
are the clocks and U3, U2 are the step-down counters
presettable by jumpers and rotary switches.
When the mains voltage is supplied, the step-down
counter U3 is inhibited by a high preset enable signal at
pin 1 and U5 is reset by a positive pulse at pin 6 (master
reset).
When mains is switched off, U3 starts to count down
and at the end of delay removes the enable signal from
pin 1 of U2 that starts the venting time and allows the
vent valve to open.