Watlow QPAC Modular SCR Power Control Service - Manual (Page 16)

16 WATLOW QPAC Service Manual
Test Procedures, Chapter 4
Bias & Gain
These adjustments are necessary for the following control cards:
QBF - Bias and Gain adjustments
QBV - Gain adjustment
QAL - Bias, Gain and Current Limit adjustments
QAF - Bias and Gain adjustments
Bias and/or Gain Adjustments
The bias and gain are factory set for a command signal of 4 - 20mA. However, if
minor or major adjustments are required, follow this adjustment procedure.
1. Set the temperature control for zero output (4mA or 0VDC) by adjusting the set point
below the ambient temperature, or put the control in manual mode at 0% power.
a. If there is an output, turn the bias potentiometer clockwise until the
output just disappears.
b. If there is no output, turn the bias potentiometer counterclockwise until
the output is noted, then turn it back to until it is just OFF.
2. Set the temperature control for full output (20mA or 5VDC) by turning the set point
above the ambient temperature, or put the control in manual mode at 100% power.
a. If the output of the SCR is full ON, turn the gain potentiometer counter
clockwise until a drop in the output is noted. Next, turn the pot back to
until the output is just full ON.
b. If there is no SCR output, or the output is not full ON, turn the gain
potentiometer pot clockwise until the output is just full ON.
3. Repeat Steps 1 and 2 until no further adjustments are necessary.
Current Limit Adjustments
The QAL control card is a phase angle control with the capability to limit the maxi-
mum current to the load. A potentiometer on the QAL card adjusts the current limit
setting. Use the following procedure to adjust the current limit on initial setup. The
purpose of the procedure is to bring the load power up slowly so the maximum cur-
rent to the load is not exceeded before the current limit is adjusted.
❋
Note: If the input signal from the temperature control is abruptly increased,
a short overcurrent through the load may occur as the QAL card circuitry detects the
higher current.
1. Attach a clamp-on ammeter to the load line.
2. Adjust the QAL card current limit potentiometer fully counterclockwise (for mini-
mum current flow).
3. Turn the temperature control ON, and adjust the input signal to the control card
for 0% power.
4. Turn the power to the QPAC ON.
5. Gradually increase the input signal.
6. Adjust the current limit potentiometer clockwise until current to the load is mea-
surable.
7. Gradually increase the input signal to 100% power, then adjust the current limit
potentiometer to obtain the desired maximum current to the load.
❋
NOTE:
Any adjustments or
troubleshooting of
this unit should be
done by a qualified
technician.