APPENDIX B : COMPRESSOR DRIVEN AIRDRYER
20950 Issue 1 April 98 B--11
Airdryer -- Electronics
Note: Removingpowertothecompressorwillremovetheairoutputfrom
the airdryer.
The airdryer circuits perform the following functions:
D Provide power to the Peltier cooling element
D Maintain the Peltier cool side at or above 4 C to prevent icing of
the cooler in low ambient temperatures
D Periodically open the condensate drain solenoid
D Periodically sample the output air pressure and humidity, and if
either goes out of specification, (a) remove power from the
compressor and (b) provide visual and audible alarms
D Provide fault indications.
ThePeltier heat pumpinthecooler is supplied withpowertakenfrom
an unregulated 12V supply generated on the printed circuit board.
With this level of power input, the heat pump can provide a 25 C
temperature difference between its hot and cold faces. When the
ambient temperature is less than 29 C, a thermistor control circuit
interrupts the supply, preventing the cold face falling below 4 C and
avoiding the formation of ice.
Condensate is expelled at regular intervals from the Peltier cooler by
way of a solenoid valve. The solenoid is opened for 1 second by a
monostabletriggeredbya clockgeneratordrivingacounter. LinkLK2
allows selection fromarange of counts giving time intervals of, 6 secs,
12.5 mins, 25 mins and 100 mins.
The same counter provides clock signals to sampling latches at a rate
dependent upon the setting of LK1.
The humidity sensor is supplied with a simulated ac voltage derived
froma low frequencybinaryoscillator. Thehumidityalters the sensor
resistance and controls the input to a sampling latch. The pressure
switch provides a switched dc level into a second sampling latch.
LED’s 1 and 2 show these states. If theinput toeither latch is set,clock
pulses controlled by LK1 switch the latch to:
(a) Operate the alarm and fault indicator.
(b) Operate the relay to cut power to the compressor.
Switch SW1 (see Pg. B--31) allows selection of normal operation, or a
test condition, in which the sounder is stopped, the alarm indicator
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