QTech Q23, Owner'S Manual

Page: 38 / 50

TECHNICAL NOTES
Page 38 Q23 Expansion Module
4 to 20mA The 4 to 20mA loop configuration is the same as the 0 to 20mA loop described above, except the
central processor scales the output to 4 to 20mA range. This reduces the DAC resolution to 80%
of the 1024 counts for full scale to 1 part in 819 counts.
Digital to Analog Converter
The Analog outputs are derived from a 10 bit PWM (pulse width modulation) output from the onboard Processor.
The PWM output is integrated by resistor R1 and the 1uF capacitor to produce a 0 to 5 volts DAC output.
Analog Calibration
To compensate for resistor tolerances and variations together with op-amp gain and offset variations the Q23
Module uses software algorithms to individually compensate and calibrate each Analog output channel. The
calibration algorithms correct for voltage offset, zero offset and gain offset. The calibration factors are stored in an
EEPROM. Analog calibration is performed using the Online Configuration & Diagnostic functions.
Power Supply
The Q23 Module is designed to operate with DC power supply voltages of nominally 12 or 24 volt. The Q23
Module will operate on any DC power supply voltage within the range 11 to 28 volts. The Q23 Module draws
approximately 80mA at 12 volts and 60mA at 24.0 volts power supply input voltages.
The DC power supply to the Q23 Module must be adequately smoothed and free from noise and voltage transients.
Where the DC supply is derived from the AC mains, or batteries with an associated AC mains battery charger these
devices must comply with the relevant electrical regulations.
Circuit details of Power Supply Front End.
The above diagram shows the Q23 Module power supply front end circuit. The power supply connector is a 3 way
connector labelled
‘PWR’
on the bottom panel of the Q23 Module. The connections to the power connector are:
G Ground, the negative lead of the power supply.
+ The positive lead of the power supply
C Optional power control input from a master module.
In the above circuit diagram the 30 volt transient suppression diode ZD1 and the 2 amp fuse protect the Q23
Module against reversed supply connection, over voltage supply and transient voltages. The fuse is a plug-in 2
amp TE5 fuse located on the Q23 Module. The fuse is located at the lower right side of the Q23 Module as viewed
with the front display panel to the right.
In many applications the Q23 Module is operated from 12V/24V standby battery power supply systems. In the
event of the main power supply failing and the battery becoming discharged the Q23 Module can be configured to
turn off in harmony with the master device. This facility protects the Q23 Module against unpredictable operation
as the supply voltage sags.
As can be seen in the above circuit diagram the power control input line turns on Q7, which in turn turns on Q6,
thus applying power to the module. Transient suppression diode ZD1 protects the input circuit from over voltage
and transient spikes.