Metek TWRF1 SCR Series, Instruction, Operation And Maintenance Manual

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Theory TWRF1 SCR Power Controller
18 Ametek HDR Power Systems Rev 2
4.6 SPAN and ZERO Controls
The ZERO and SPAN potentiometers are screwdriver adjustable. The center position of the ZERO
control corresponds to zero bias and zero power output. The approximate zero setting can be found
by rotating the control over its range, stopping near the midpoint of rotation. The precise zero setting
can be found by observing the output level while rotating the control near the midpoint of rotation.
From the midpoint to extreme CCW (counterclockwise) is the zero or negative region. Operating the
control in this region, one can zero the output from the TWRF1 at any output level of a temperature
controller or other external control device. From midpoint to extreme CW is the positive control region,
whose extremes correspond to 0% and 100% power output, respectively, as manually adjusted.
The SPAN control is used as a final adjustment to set the TWRF1 output to precisely 100%, when the
principal control device (external controller, external “manual” control or ZERO potentiometer) is set to
demand 100% output.
4.7 Voltage-Squared Feedback
Voltage-squared feedback simulates power feedback. It is used to help maintain constant power
delivered to the load when the line voltage changes and the load resistance remains constant.
4.8 Shut-down
The shutdown circuit provides a quick SCR shutdown signal. This is accomplished via a customer
provided contact across terminal 3 and terminal 4. This removes the input signal to the comparator
that tells the circuit when to gate the SCRs on. It also instantaneously turns off the SCR gating pulses.