The controller is designed for use with ISA type T (coppgr vs constantan) thermocouples, with a measuring range of -125F to +375F. Electronic cold junction compensation is provided, and the input is unaffected by sensor leadwire resistance. In the event of an open sensor circuit, the controller will produce a blank display, and a flashing SENSOR warning. A special failsafe circuit also disables both the heating and cooling outputs, assuring that both systems will remain inoperative until protection is restored. The indicating accuracy of the controller is .25%. The switching accuracy is mainly a function of its setability; about 1%.

The outputs of the controller consist of three mechanical 1P2T relay contacts, rated 10-amps at 250vac maximum. These relays are energized during all periods of normal operation, and drop out whenever a limit is exceeded, or when power is removed from the equipment.

Input power requirements are 3.5VA max at 110/120vac or 208/240vac, 50/60 Hz. All connections are made at terminal blocks on the back of the unit.

Install the controller at a location which is convenient to system wiring, reasonably free of vibration and temperature extremes, and accessible to equipment operators. The controller is normally panel-mounted in a 3-11/16" x 8-3/16" cutout, using special mounting clamps (p/n 352052). As shown on Drawing No. 470684, the controller normally protrudes 7-3/16" behind the face of the mounting panel.

Connect an ISA type T thermocouple to the SENSOR terminal block in the upper right-hand corner of the controller, observing the RED (-) and BLU (+) color code. Use only type "XT" (copper vs constantan) thermocouple extension wire between the sensor and this terminal block. The controller is not affected by leadwire resistances less than 1000-ohms.

Connect the input power to the terminal block in the lower right-hand corner, as follows:

An indicator LED is provided on the back of the unit adjacent to each output relay. These lights will be on whenever their associated relay is energized. The relays are energized during all periods of normal operation (exception for K3 noted below), opening when limits are exceeded, or when power is removed from the unit. Whenever these relays are energized, their normally open contact set will connect COM to NORM at their associated terminal block.

The output connections are marked HIGH/COM/NORM, LOW/COM/NORM and ALARM/COM/NORM. During normal operation, COM will be connected to NORM. When a high or low limit is exceeded, COM switches to HIGH or LOW, as the case may be. Control power for the heating and cooling systems will therefore normally be wired through the COM and NORM terminals.

The operation of the auxiliary "ALARM" contact varies with the mode selected. In the slaved mode, COM will be connected to NORM until either limit is tripped, at which time COM switches to ALARM. Control power to live loads will therefore normally be wired through the COM and NORM terminals. In the alarm mode, this relay is normally off (COM to ALARM), and is pulsed on (COM to NORM) and off whenever a limit has been exceeded and the SILENCE button on the front of the unit has not been pressed (problem having not been attended to). Power to remote alarm devices will therefore also normally be wired through the COM and NORM terminals.

The controller's temperature measuring circuit, the first half of U8 and its associated circuitry, converts the low level thermocouple input to a scaled and compensated high level analog voltage. The sensor, "mj" (measuring junction) is connected to U8-11, the input of this amplifier. C17 provides low pass filtering for this input. Standard units are designed to measure temperatures over the range of -125F to +375F, using a type T (copper vs constantan) thermocouple. Over this temperature range, this sensor produces emf's ranging from -3.006mV to +8.787mV. This input is amplified and scaled to provide a 0v to -5.0v analog output at U8-6. To achieve this, the gain of the amplifier is set by R17, R18 and R19 at 424.

This non-inverting amplifier will always settle at the point where the potential at its inverting input, U8-10, is exactly equal to its input, U8-11. This condition is established by a feedback current, passed from the output, U8-6, through R18 and R17 to the amplifier's inverting input. For a given input, the magnitude of the feedback current required to establish this balance is a function of R19. Having established that, the magnitude of the amplifier's output voltage then becomes a function of the feedback resistors, R17 and R18. The resistor values used permit the gain to be set exactly at 423.98, with a calibration range of about 2% to accommodate all tolerances.

Thermocouple circuits necessarily involve a second junction, sometimes called the "reference junction". This junction, shown as "rj" on the schematic, has the same "emf vs temperature" characteristic as the sensor. It occurs in series with the sensor's emf, with the opposite polarity. The emf produced by the "rj" junction is algebraically summed with the sensor emf, so any variation of the ambient temperature directly affects the measurement.

U5, R20 and R21 provide reference junction compensation, which minimizes errors due to variations in ambient temperature. U5, which is located at the (-) end of the SENSOR terminal block, is an integrated circuit temperature sensor, which produces an output proportional to its absolute temperature. This output varies 10mV/C, and is scaled by R21 to produce a feedback current through R17 and R18 which will be approximately equal and opposite to that being caused by the influence of the reference junction emf. At room temperature, the emf produced at rj varies about 40.7uV/C which, in turn, produces a feedback current of about 40.7nA through R21. A 1C ambient temperature change will also result in a 10mV variation at U5. This will produce a -41nA change in the total feedback current, which approximately cancels out the shift produced by rj.

U4, R14, R15 R_zt and R16 provide a means of shifting the amplifier's output to zero with a -125F input at "mj". U4 is a precision voltage reference integrated circuit, which provides a stable +6.9v at its junction with R14 and R15. At -125F, the mj input will be -3.006mV. Assuming a 25C ambient temperature, the emf produced by rj will be about +0.992mV, so the net input will be about +3.998mV, producing an offset of about +1.7 volts at U8-6.

Meanwhile, the voltage at the U5, R20 node varies with absolute temperature by 10mV/K, and will therefore be about +2.98v (0C = 273K). This produces a compensator-related offset at U8-6 of about -5.2 volts.

U8 is a precision op amp, with negligible offsets, so the total value of the offsets is therefore about -3.5 volts. To shift the output level back to zero, R16 is adjusted so that the zero network injects a current into the feedback node which is about equal and opposite to that caused by the total offsets; about -3.5v/422.98K = -8.3uA. R_zt is a "zero trim" adjustment, and is located on the back of the unit next to the sensor input terminals.

The indicator A/D converter is basically a digital millivolt meter. The 0/+5.00v output provided at U8-13, and the limit set point analog voltages from R67 and R68, must therefore be converted to -125mV/+375mV to provide a "-125F" to "+375F" display. The scaling circuit also converts these analog voltages to equivalent "C" values.

Analog switch U10 multiplexes the analogs into buffer input U11-3. The output of this buffer is connected to two voltage dividers. The R36 - R38 divider converts the 5-volt analog to a 0/500mV analog, representing the 500F span. The R39 - R41 divider converts the 5-volt analog to a 0/278mV analog, representing the equivalent 278C span. Either the "F" or the "C" analog is selected by U12, is buffered, then applied to R42, an input to a unity gain inverter. The inverted output, U11-8, is then applied to R44, one input of a unity gain summing circuit.

At this circuit, the analog is summed with an offsetting voltage selected by two additional sections of U12. In the "F" display mode, R48 and R49 injects +12.5uA into the feedback node, which shifts the output at U11-14 in the negative direction by exactly 125mV. This provides a "-125" read-out when the measuring circuit analog is at zero volts. In the "C" display mode, an offsetting current of +8.72uA is provided by R50 and R51, to provide -87.2mV at U11-14 (-125F = -87.2C).

The "display blank" input to R46 is taken from the CR15/CR16 junction at U18, and is normally open since the diodes are normally reverse biased. Certain sensor faults cause U18 to forward bias one of these diodes, imposing a -13.3v potential on R46. This forces U11-14 to at least +2.5v, which activates the over-range function of the A/D converter, producing a blank display.

The scaled and level-shifted analog voltage at U11-14 is applied to the input of DVM circuit, U14, through low pass filter R52 and C31. U14 is a dual-slope 3-1/2 digit A/D converter which provides 7-segment outputs to operate the digital display system. The most significant digit is not used in this application, and zero-blanking is not provided.

U14 does the A/D conversion in three phases. First, the converter is internally zeroed. Next, the input voltage is applied to an integrator, the output of which increases from zero at a linear rate until stopped at the end of this precisely timed period. Since the integrator's output slope is a function of its input voltage, its output at the end of this (integrate) phase is directly proportional to the input voltage. In the third and final (de-integrate) phase, the precision +1.000v reference voltage is applied to the integrator to drive its output back to zero at a known rate, while a counter system measures the time required to do so. The resulting count is therefore proportional to the magnitude of the input voltage, and is converted from binary to 7-segment code for display purposes.

Peripheral circuitry includes U15, which provides a +1.000v reference voltage, and U16, which provides a -5v supply for the DVM chip. The DVM chip handles all other necessary

functions internally. A jumper on the back of the unit permits the selection of either a"F" or "C" character for display purposes. Two gates of U26 buffer the choice to analog switch U12 in the scaling circuitry.

U18 provides a system of comparators which monitors the measured temperature with respect to high and low limit settings, and for "reasonableness". U17 provides a precision +6.9v reference for the limit set points, R67 and R68. These set point outputs provide one input to their respective comparator circuits, while the 0/+5.00v temperature analog voltage is connected to the other inputs. When the analog voltage is less than the high limit set point, and greater than the low limit set point, both U18-14 and U18-8 will be low. U19-3 and U19-4 will therefore be high.

Furthermore, when the analog voltage is greater than -50mV, but less than +5.07v, failsafe detector outputs U18-1 and U18-7 will both be high, and U19-10 will therefore be low.

Pressing both reset buttons, S1 and S2, forces U20-3 and U20-4 high. With all three inputs high, U21-6 and U21-9 go low. This disables both the high limit and low limit indicators, enables the output transistors, Q2 and Q4,and latches U20-3 and U20-4 at the high level. With both of its inputs high, U24-4 will be low, which turns off Q7 and DS-8 and turns on Q9 (in the "slave" mode). This is the normal operating status.

If the measured temperature strays outside of the set range, either the high limit or low limit will be tripped, and the above status for that limit will be reversed; the indicator will be on and output transistor off. The indicators are gated by a flasher signal provided from U23-3.

When neither limit is tripped, U24-4 will be low. U23-4 and U22-8 will therefore both be low, holding both the audible warning device (beeper) and the "SILENCED" indicator off. When either one(or both) of the limits are tripped, U24-4 goes high. This enables the audible warning device, but not the "SILENCED" indicator. Pressing S3 forces U24-10 high. U24-11 then goes low, which latches the beeper off, and the "SILENCED" indicator on. Both the beeper and the "SILENCED" indicator are also modulated by the flasher signal.

At power-on, U18-7 is initially held low as C41 begins to charge. This trips both limits and toggles the sensor fault indicator. A momentary loss of power will therefore trip both limits. An open sensor connection will cause the measuring circuit to provide abnormal analog voltage levels and toggle either U18-1 or U18-7. A flashing "SENSOR" warning signal via U22-6, and both limits will also be tripped. U24-3, Q5 and Q6 comprise a power-up reset timer.

Power for the controller can be taken from either 110/120vac or 208/240vac, 50/60 Hz lines. The controller circuitry uses +15v, -15v and +5.1v dc voltages. The +/-15v levels are provided by T2 and its associated bilateral full-wave rectifier circuit. Regulators U2 and U3 stabilize these supply voltages at the required +15v and -15v levels. A separate +5.1v supply is provided for the controller's logic and display circuitry by T1 and regulator U1. All three of these supplies are adjustable, and are set precisely at the 15v and 5.1v levels. The 15v potentials are re-stabilized at +/-12v for use at the low level measuring circuit, U8.

Separate 1P2T contacts capable of switching 10-amps ant 250vac are provided for each of the three outputs. A LED indicator is provided near, and operates with, each relay. The relays are de-energized by abnormal operating conditions (COM to HIGH or COM to LOW and COM to ALARM).

Since the manufacturer's name and part number are often printed on purchased parts, this information is included in the parts lists to help you properly identify the item in question. Purchased parts are often provided by alternate sources however, so this information should be taken as representative, rather than absolute.