11 · 2 GENERAL ELECTRIC ORIGINAL INPUTS
The pilot-device signals which supply the information to the . logic control must each be connected to a device that accepts the signal and converts it to a logic-level ON or OFF signal of the proper magnitude. Electrical contacts are the most frequent inputs and are normally operated at 125 volts d-e with static control. One hundred and fifteen volts a-c is also used as an input to static control.
Electrical contacts do not exhibit reliable circuit continuity or contact fidelity at the low voltages used as direct inputs to the logic elements. At higher voltages such as 115 volts a-c or 125 volts d-e, the voltage will burn away an oxidation film and assure good contact fidelity.
The 125-volt d-e original-input operation is as follows. The LEPS provides a source for 125 volts d-e to be used with this input. This -125 volts d-e is connected to one side of the con tacts of pilot devices such as limit switches, push buttons, and pressure switches, and the other side of the contact is connected to the input terminal P on the d-e original input. The logic output of the device is at the terminal marked L as shown in Fig. 11 ·22. A zero-volt connection must also be made to the device at the terminal marked 0. A built-in monitor light is incorporated in the 125-volt d-e original input, which is on the 125-volt side of the circuit and indicates whether or not voltage is present at the input terminal. Approximately 5 milliamperes (rna) flow through the pilot device when its contacts are closed .
Each input is separate and snaps into a grooved mounting track. The entire unit is encapsulated and uses wire-clamp termi nals. The zero-volt bus must be used to make the connection from the LEPS, in order to minimize voltage drop and provide proper spacing of each unit. One bus accommodates up to six inputs.
Figure 11·22 shows the schematic of the 125-volt d-e original input. With the output terminal L connected to the input of a logic element, -4 volts will be present when the unit is not in the ON condition. The NPN transistor cannot conduct base to emitter or collector to base when the pilot device is open, because no current path to -125 volts d-e exists. When the
pilot device closes, the input terminal P receives -125 volts d-e which turns the neon indicating light on and commences charging capacitor C1 to a voltage slightly above -4 volts. The NPN then commences conduction, since its emitter is then more negative than its base . This allows the PNP transistor to conduct emitter to base and also emitter to collector, which causes the base of the NPN to become even more positive than its emitter.
This results in a rapid turn-on of both transistors and a resultant ON signal at terminal L. With the NPN in full conduction, its emitter decreases in voltage to less than 0.5 volt while still being in full saturation. This also causes capacitor Cl to dis charge its previous charge, so that when the pilot device again opens, it will be essentially discharged and ready again to go through the above sequence. This technique makes this input
device very insensitive to transient electric noise and line dis turbances, and increases the reliability of the system by pro viding the logic with an input only when a valid input signal exists. The logic output of the device, terminal L, can supply up to 12 input signals to the logic elements.
Figures 11·23 and 11·24 are examples of static control panels and show the proper grouping of original inputs along the left side of the cabinet.
The 115-volt a-c original-input device is necessary to convert a 115-volt a-c contact closure to a usable input signal to static control. The 115-volt a-c signal is stepped down through a trans former, rectified, and filtered, causing a transistor to become satnrated; then the output of the transistor in again filtered.Figure 11·25 shows the physical appearance of the a-c original input.
Figure 11·26 shows the schematic of the a-c original input. Essentially, the circuit is identical to the d-e original input but is preceded by a transformer and full-wave rectifier. When the pilot device closes, 115 volts a-c is supplied to the primary of the transformer. This voltage is reduced, rectified, and filtered,
causing point a to rise to slightly above -4 volts d-e. The NPN transistor then commences conduction (since its emitter is then more negative than its base). This allows the PNP transistor to
conduct from emitter to base and also emitter to collector , which causes the base of the NPN to become even more positive than its emitter. This results in a rapid turn on of both transistors and a resultant ON signal at the output terminal.
Approximately 20 rna flows through the pilot device when its contacts are closed. The output terminal can supply up to 12 input signals to the logic circuitry.
The 115 volts a-c for the a-c original input should be supplied from the power-supply side of the incoming a-c line filter de scribed later.
Two resistance-sensitive input devices (Fig. 11·27) are avail- · able to detect a change of resistance, one being a plug-in element for use with photocells and the other a panel-mounted unit for probes or applications requiring greater sensitivity and isolation from system ground potential.
The plug-in module without a built-in monitor light and one with a built-in monitor light have two input terminals 2 and 1 and the resulting output at terminal 4. Terminal 1 is system zero volts, which is ground, and most applications are thereby limited to detecting the change of resistance of a photoconductive cell as light strikes the cell. The device yields an ON signal at terminal 4 upon decreasing resistance and will remain in the ON condition until the resistance raises somewhat above the trip value. The best application is when a considerable resistance change occurs rapidly, as the input is not designed to be an analog-sensitive input. The resistive load must not generate a voltage, as the input device will supply the small potential of a volt or two to the load, and a change of resistance is the parameter being sensed. The photoconductive cell, generally the cadmium sulfide type, is excellent, and the two leads from the cell can be directly connected to the input with shielded wire. This device’s trip point is adjustable by a built-in potentiometer from 50 to 4 kilohms.
The panel-mounted input requires 115 volts a-c to be con nected. It is more sensitive and in addition has a differential adjustment so that the dropout point may be varied with respect to the pickup point. This self-contained unit has a form C reed switch as an output, which should be connected to the standard 125-volt d-e original input to provide an input to static control. Its operating range is adjustable from 300 kilohms to 500 ohms, and the resistive-load terminals (A and X) are isolated from sys tem ground. The wires from the load to the device should be shielded, and the load can be photocells, probes, or other inputs whose resistance changes greatly upon actuation . Again, the voltage must not be generated in the load, but the resistance sensitive input supplies about a volt potential across the load.
Each device is very useful to the control designer in providing a means for taking a solid-state photoconductive cell directly into the static control or for using the resistance of the material between two probes to be the switching means, with the safety of a low voltage at the probes.
The proximity switch combined with a CR115D8 power sup ply can supply signals into static control through the proximity switch input. This solid-state limit switch can detect ferrous or nonferrous metal without physical contact with the material and, without any moving parts, can supply an ON signal into a static unit. The driving capability of the switch is limited to one input, and normally a proximity-switch input is utilized which can itself provide a 50-unit output-driving capability.
The CR 115D proximity switch has four wires colored black, green, red, and white connected to its encapsulated amplifier. These wires connect to the CR115D8 power supply, which sup plies the 30 volt d-e for operating the sensing head. All wires should be connected to the power supply, and the red and white wires are to be of the insulated shielded type with only one end of each shield connected to logic common, 0 volts, on the power supply. The white wire is the direct input to the prox imity-switch-input module. If time delay is required, it must be accomplished in the static control with the DELAY module.
A single CR 115D8 power supply can accommodate up to five proximity switches simultaneously with independent opera tion of each switch. The proximity-switch input must be used when connected to counting or register-type circuits. Two prox imity-switch inputs are packaged in one module. The input ter minals are 1 and 5 with respective output terminals 4 and 8. Each output can drive up to 50 inputs of any logic elements .
Occasionally in counting and register circuits a fixed and ac curate pulse rate is required. The most convenient reference is the 50- or 60-cycle incoming a-c voltage which is available as -+- 12 volts at the LD terminal of the LEPS. With this a-c voltage connected to terminal 1 of the sine- to square-wave con verter, a square-wave output is available at terminal 4. An ON signal and an OFF signal will each exist for approximately 8 msec.
A NOT output is also available at terminal 3, but the standard output and NOT output cannot be connected to an OR to yield twice the frequency. If the two outputs are each connected to a single shot, and those outputs connected to an OR, twice the line frequency of pulses would be available, with the ON signal existing for approximately 100 11-sec for each pulse.
The sine- to square-wave converter output terminals can each drive up to 12 other logic inputs. The element consists of a converter and a unit reset in one module. The unit reset is an independent function.
11 ·3 GENERAL ELECTRIC OUTPUT AMPLIFIERS
The output of any logic function must be amplified to switch the power required by external loads. A variety of voltage and current ratings are available to economically facilitate the ener gizing and deenergizing of typical power devices, depending on the power involved. The voltages commonly utilized are 24 volts d-e, 115 volts a-c, and occasionally 90 to 180 volts d-e. Figure 11 ·28 illustrates the four plug-in module forms and three panel mounted output amplifiers. Amplifiers cannot be paralleled to increase rating, and must have the proper voltage and polarity applied to their terminals.
The 6-watt d-e output amplifier can switch a d-e load whose current does not exceed 0.25 amp and any d-e voltage up to its nominal rating of 24 volts. The load is usually inductive, such as a pilot-operated solenoid valve, and a counter-electro motive-force (cemf) diode is incorporated in the amplifier to reduce the effect of the cemf voltage generated when an induc tive load is deenergized.
Figure 11·29 shows the schematic of the 0.25 amp 24-volt d-e output amplifier, frequently referred to as the 6-watt output. An emitter-follower circuit is employed with the PNP-1 transis tor. When an OFF signal, -4 volts, is at terminal 1, PNP-1 transistor conducts from emitter to collector, which in turn causes sufficient voltage to appear at the base of PNP-2 transistor to cause it to conduct from emitter to collector. With PNP-2 transistor in full saturation and conducting, its collector voltage is essentially zero volts, thereby positive-biasing the base of the
power transistor and assuring it is not in saturation. The diode in series with the emitter of the power transistor causes the emitter to be more negative with respect to the zero-volt bus than the voltage drop through PNP-2, which is the voltage im pressed on the base of the power transistor.
When an ON signal, 0 volts, is impressed at terminal 1, PNP-1 and PNP-2 cease conduction, and the power transistor goes into full saturation. This allows current to flow from the zero-volt
bus P+ through the amplifier to terminal 4, then to the minus voltage power bus, usually -24 volts d-e, and through the power load. The CEMF diode must be connected across the load to protect the power transistor from the CEMF voltage generated by the load upon deenergization. Connecting terminal 2 across the load also connects the optional built-in monitor light in parallel with the load to provide a visual indication in the plug-in module as to its actual output condition. This is very valuable in panel check-out procedures.
The maximum rated current of the 6-watt d-e output amplifier is 0.25 amp at a d-e voltage up to 24 volts d-e. D-e power supplies normally have a higher voltage output at no-load, and the maximum voltage limit of the amplifier is 28 volts d-e. The power supply must be filtered.
Two complete and independent 6-watt amplifiers are packaged in a single plug-in module. The optional built-in indicating lamp, if utilized, draws an additional 20 rna from the 24-volt d-e power supply. Approximately a 1-volt drop can be expected in the power circuit of the output amplifier.
The 36-watt d-e output amplifier can switch a d-e load whose current does not exceed 1.50 amp and any d-e voltage up to its nominal rating of 24 volts. The load is usually inductive, such as a pilot-operated solenoid valve, and a CEMF diode
is incorporated in the amplifier to reduce the effect of the CEMF voltage generated when an inductive load is deenergized.
Figure 11·30 shows the schematic of the 1.50-amp 24-volt d-e output amplifier, frequently referred to as the 36-watt output. Similar in operation to the 6-watt output, an OFF signal at termi nal 1 causes the PNP-1 and PNP-2 transistors to conduct and the power transistor to have a positive bias to assure its noncon duction. An ON signal at terminal 1 results in the PNP-1 transis tor ceasing conduction, and the PNP-2 transistor also ceases conduction. This causes the power transistor to saturate fully and supply power to output terminal 4. The CEMF diode con nection at terminal 2 is commoned as a termination point for both amplifiers for the -24-volt bus.
Two amplifiers are mounted on a common baseplate for sepa rate panel mounting. Approximately a 1-volt drop can be ex pected in the power circuit by the output amplifier. The maxi mum rated current of the 36-watt output amplifier is 1.50 amp at a d-e voltage up to 24 volts d-e. D-e power supplies normally have a higher voltage output at no-load, and the maximum volt age limit of the amplifier is 28 volts d-e. The power supply must be filtered .
The l-amp a-c output amplifier can switch 115-volt a-c power to an inductive load whose inrush is 3.7 amp and holding current is 1 amp. This is a solid-state switch utilizing a silicon controlled rectifier ( SCR) as the basic switching device.
An SCR is a silicon diode which will not let current flow in either direction through the device unless a small signal is applied to the gate lead, and then conduction occurs in only the forward direction, anode to cathode. After the gate is ap plied, the SCR continues to conduct even when the gate pulse is removed. The SCR stops conducting when the current through the device essentially decreases to zero and requires another gate pulse to again commence conducting.
Figure 11·31 shows the 115-volt a-c power circuit with one SCR in a diode-circuit configuration. With the SCR in the con ducting state, one half-cycle of current would flow to the load from Ll to Dl to the SCR to D2 to the load to L2, and the other half-cycle would flow from L2 through the load to D3 to the SCR to D4 to Ll. This causes full a-c power to flow through the load, and unidirectional current to flow through the SCR static switching device. If the SCR receives a gate pulse, it will conduct for the remaining portion of the half-cycle and require another gate pulse to again conduct for the next half cycle of a-c power to the load.
The gate-pulse source is a multivibrator circuit which, when an ON signal is applied, generates approximately 2,000 gate pulses per second which cause the SCR to conduct. Each time current through the SCR passes through zero, the SCR ceases conduction, but soon receives another gate pulse to cause con duction very early in the next half-cycle of a-c power.
The RC in parallel with the SCR provides better turnoff characteristics at low currents. The minimum load current is 100 rna. The normal-inrush rating is considered to be 6 times the continuous-current rating, and the l-amp a-c output amplifier can be suitably protected by a Buss tron KAA-1 fuse or its equivalent.
Figure 11 ·32 shows the schematic of the l-amp a-c output amplifier. The unijunction transistor UJ in this circuit is the key part of the oscillator which continuously provides 2,000 pulses per second to the two-input AND amplifier portion of this SCR-triggering configuration. With an ON signal at terminal 1, PNP-1 will not conduct when the oscillator circuit also provides an ON signal to its input, which allows the power transistor PNP-2 to momentarily conduct. This conduction of PNP-2causes current to flow through the air-core pulse transformer PT, which gates the SCR into conduction. When the oscillator circuit removes its ON-signal input to PNP-1, this transistor again conducts, which causes PNP-2 not to conduct and no gate pulse to exist. The diode in parallel to the PT primary coil is to mini mize the CEMF voltage generated upon deenergizat ion of the coil. The diode in the oscillator circuit is to limit the magnitude of the positive-voltage pulse to zero volts, the pulse being sup
plied by the combination of inductance and capacitance con nected to one of the bases of the unijunction UJ. This output amplifier is packaged in a single plug-in element .
The 4-amp a-c output amplifier is a plug-in unit which can switch 115-volt a-c power to an inductive load whose inrush is 16 amp and holding current 4 amp. This is a solid-state switch utilizing a Triac as the basic switching device. The circuit is the same as in the 1-amp device, except that a Triac takes the place of the SCR and the four diodes. Fuse with a KAA-4 tron fuse.
A 10-amp panel-mounted a-c amplifier is also available; see Fig. 11·33.
Occasionally an independent-contact output is required which can switch remote 115-volt a-c circuits. The relay output ampli fier is a plug-in element containing a transistor-driven relay whose contacts are rated 12-amp inrush, 3-amp carry in 115-volt a-c circuits only, and whose approximate life under rated load is 10 to 20 million operations.
With an ON signal applied to terminal 1 in Fig. 11·34, PNP-1 will not conduct and PNP-2 will go into saturation, energizing the relay coil. With no ON signal applied to terminal 1, PNP-1 does conduct, which makes the base of PNP-2 more positive than its emitter. With PNP-2 not conducting, the CEMF energy in the relay coil is dissipated through the diode in parallel with the coil.
The 180-volt d-e output amplifier is a solid-state amplifier which is utilized in d-e circuits above 24 volts d-e. It uses silicon controlled rectifiers as the switching devices, and is frequently used with small clutch and brake coils. Its maximum current rating is 1 amp. The circuit for this panel-mounted device is shown in Fig. 11 ·35.
With this output amplifier, an ON signal must always be sup plied to either the ON or OFF input termin al, and the NOT of that ON signal must be applied to the other input terminal. With the built-in NOTS conveniently available in most logic functions, this is simple to obtain.
One SCR will always be conducting, since one triggering cir cuit always will have an ON signal as an input. Assume SCR-2 is conducting, which will charge the capacitor so that point a is positive and point b is negative. When the ON signal is re moved from the OFF terminal and applied to the ON terminal , SCR-1 commences conduction and connects the positively charged point a of the capacitor to the minus bus. This momentarily causes SCR-2 to see no voltage difference across it and cease conduction. The capacitor proceeds to charge to the opposite polarity, and the reverse situation will occur when the ON input is moved to the other input terminal. Power flows through the load when SCR-1 conducts.
With the ON signal removed from the ON input terminal and an ON signal applied to the OFF input terminal, SCR-2 com mences conduction, SCR-1 is no longer being triggered, and point b of the capacitor, being charged to a positive polarity , is connected in parallel to SCR-1; this momentarily causes no voltage difference across it, and conduction ceases through SCR-1. The cemf energy in the load is dissipated through the resistor diode in parallel with the load. SCR-2 remains in con duction, current flowing from the plus bus through R 1 to the minus bus. The capacitor simultaneously is being charged, so that point b becomes minus and point a becomes plus. An induc tive load will require a resistor R2, sized to cause 50 rna to flow in parallel. The ohmic size will depend upon the d-e voltage used and will be separately mounted. The SCR triggering circuits are identical to those shown in Fig. 11·32.
Remote-mounted pilot lights are common in operator consoles and stations; the economical manner to display a logic-level sig- · nal is by means of a pilot-light output amplifier. Four separate amplifiers are packaged in a single plug-in element, with each amplifier capable of switching a 40-ma bulb at a voltage up to 24 volts d-e.
Without an ON signal to the input terminal (Fig. 11 ·36) , PNP-1 is in conduction, and the base of PNP-2 is held more positive than its emitter and does not conduct. A few milliam peres do flow through the load, but this has no effect since R 1 is much greater than R2. With an ON signal applied to the input terminal, PNP-1 no longer conducts, PNP-2 goes to full satura tion, and power flows to the lamp’s resistive load. Emitter-to base current flow in PNP-2 goes through the load to its minus power bus, in contrast to the PNP-1 emitter-to-base current flow . The nominal voltage rating of this amplifier is 24 volts d-e, but d-e power supplies normally have a higher voltage output at no-load, and the maximum voltage limit of the amplifier is 28 volts d-e average, if we assume a 120-cps ripple frequency .
The internal lamp driver for monitor lights built into standard logic elements is an optional feature. It affords visual indication
of the output of a logic function and is an excellent aid in check ing the sequence of operation and isolating a circuit malfunction. The built -in monitor light indicates the output condition in the module where the logic is performed.
Since the monitor lights are mounted inside the element with a special transistor-amplifier circuit, they cannot be added later to elements not initially equipped with them. A 6-volt incan descent No. 345 miniature lamp with a flange base was used in early designs but has been replaced by a light-emitting diode (LED) to provide long life.
The typical amplifier circuit that switches each monitor light is shown in Fig. 11·37. The input signal for the lamp driver comes from the resistor network, which provides the same signal to the final output transistor of the logic function.
