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Magtrol Hysteresis Dynamometers
5.3
TORQUE SIGNAL AMPLIFICATION
A printed circuit board mounted on the dynamometer rear panel, identified as part number 234-
401-101-xxx, contains the torque signal amplifiers IC1, IC2 and IC3, load cell excitation supply
consisting of voltage reference DZ1, amplifier IC4 and Transistors Q1-Q4.
Voltage reference DZ1 is a precision voltage source, and along with IC4 and Q1–Q4, provides
+5.00 and -5.00 volts DC excitation to the load cell bridge. Transistors Q1–Q4 are NPN and PNP
transistors, serving as series pass elements to boost the current output drive capability to over 30
mA. Bipolar bridge excitation is used to improve the noise immunity of the low millivolt range
bridge output signal. See B.1 – Torque Amplification Signal in Appendix B.
Amplifier IC1 is a precision, differential input instrumentation amplifier providing amplification
necessary to calibrate the torque signal. IC2 and IC3 provide additional amplification and scaling,
along with the counterclockwise calibration (CCW CAL) control.
The torque signal is calibrated by adjusting the rear panel trim potentiometers P1 and P2, labeled "CW
CAL" and "CCW CAL". Torque signal zero is adjusted by using the rear panel trim potentiometer
P3, labeled "ZERO", when no torque is applied.
The load cell on all dynamometers except the HD-106, produce an output signal of 15 mV at full
torque. The HD-106 output is 7.5 mV full scale. Amplifiers IC1-IC3 calibrate this so the output in
millivolts equals the dynamometer full scale torque.
The torque signal zero may be affected thermally and may need periodic readjustment. Therefore,
the "ZERO" adjustment can be readjusted as needed.
The clockwise calibration (CW CAL) adjustments should not typically need readjustment. In the
event that a "CW CAL" adjustment is needed, a precision calibration beam and weight must be used.
The calibration beam may be purchased from Magtrol.
Note:
Use only an insulated screwdriver or a plastic trim pot adjustment tool when adjusting
For example, the HD-400-6 Dynamometer lists the full scale torque at 40.0 oz·in. Therefore, the
amplifier gain will be set to produce 0.400 VDC output, when the load cell input is ≅0.011 VDC.
5.4
DECIMAL POINT CONTROL
Two solder links on the circuit board indicate to the digital readout instrument where the decimal
point belongs. The chart below shows how this is accomplished by jumping the appropriate link on
the board. Digital output "XXX" represents the result of the analog output (in millivolts) with the
decimal point properly located.
the "CW CAL," "CCW CAL" or "ZERO" trim potentiometers.
Digital Output
SL13
X.XX
Closed
XX.X
Open
XXX.
Closed
53
Chapter 5 – Operating Principles
SL12
Open
Closed
Closed
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