Cutler-Hammer, Inc. Direct Static Logic part2

Should the application not require the full input capabilities of the AND switch, the unused terminals must be connected to the +10-volt bus. The maximum capacity of this unit is 5 watts

at 10 volts d-e. This switch should not be used to drive other logic switches, as the voltage drop across D2 makes operation of the driven switches marginal.

12·3 CUTLER-HAMMER OUTPUT AMPLIFIERS

The output of static switches is not sufficient to provide energy to power-consuming devices, such as pilot lights, relays, and solenoids. If these devices are to be used, power amplification is required. The various devices used are detailed in this section.

The first output amplifier to be considered is the 10-volt power AND. Referring to the diagram of Fig. 12·9, we note that the portion of the circuit to the left of the broken line is the input side of the three-input AND switch. The change in circuitry which makes this a power AND is found to the right of the broken line. On this logic board, transistor T2 is the power type, cap

able of driving a variety of loads, including two 10-volt pilot lights connected in parallel. Note, too, that the load resistor is absent; the load device itself serves that function. Another change is found in the addition of two diodes. Diode D 1 provides a discharge path when an induced voltage is created by the deenergization of an inductive load. Diode D2 reduces the leak age current through transistor T2 to essentially zero during the time that transistor T2 is in the nonconducting state.

Should the application not require the full input capabilities of the AND switch, the unused terminals must be connected to the +10-volt bus. The maximum capacity of this unit is 5 watts at 10 volts d-e. This switch should not be used to drive other logic switches, as the voltage drop across D2 makes operation of the driven switches marginal.

The second output amplifier to be considered is the 24-volt power AND. The circuitry to the left of the broken line (Fig. 12·10) is the same as that of the three-input AND switch. The 24-volt power AND differs, however, in the circuitry to the right of the broken line. Transistor T2 is of the power type. Operating at 24 volts, it will drive certain pneumatic solenoid valves. Diode D 1 provides a discharge path for the inductive load, and D2 reduces the leakage current through transistor T2 when T2 is nonconducting. Resistor R drops the 24-volt bus supply to 10 volts to conform with the 10-volt requirement of state-indication lights.

Should the application not require the full input capabilities of the AND switch, the unused terminals must be connected to

the +10-volt bus. The maximum capacity of this unit is 24 watts at 24 volts d-e.

The output relay provides a third type of output amplifier. Where applications require a considerable amplification of both power and voltage, a small panel-mounted relay might be desir able. Two variations are available, differing only in coil voltage. The 10-volt d-e relay utilizes the full capacity of the 10-volt power AND. The 24-volt d-e relay is driven by the 24-volt power AND. The contact structure and rating is SPDT, 10 amp, and 115 to 230 volts.

The fourth type of output amplifier is the silicon controlled rectifier (SCR) amplifier. Because the SCR requires very little current in its control circuit to cause substantial current conduc tion in its power circuit, it functions as a very efficient amplifier. It can, therefore, take the output signal of most logic elements and step up the signal sufficiently to drive power contactors. Except for component ratings, the circuit of Fig . 12·11 is the same as those for the 5-amp and 10-amp SCR amplifiers. It

should be noted that only the 5-amp size has noise-suppression circuits (not shown in the diagram) .

Consider the circuit of Fig. 12·11 with the reed-relay contacts closed by a signal from the driving logic element. If we assume that the first half of the 115-volt ac voltage is the positive half, current will flow in the power circuit as follows: through fuse Fl, diode D1, SCR-2, and fuse F2. The current through the control circuit for this half-cycle will flow as follows: through F1, diode CD1, resistor R3, and closed reed-relay contacts RR, R2, and F2. The small voltage drop across resistor R2 is applied to the gate of the SCR. As a result, a tiny current flows from gate to cathode, causing the SCR to “fire” or conduct in its power circuit.

On the other half -cycle, the current reverses and the power circuit now takes this path: through fuse F2, diode D2, SCR-1, and fuse Fl. The control circuit to turn on SCR-1 takes this path: through fuse F2, diode CD2, reed-relay closed contacts RR, R3, and R1, and fuse Fl. Now resistor R1 provides the gate voltage to trigger SCR-1 to conduct in its power circuit. The purpose of the slow and fast fuses is to make full use of the SCRs while still adequately protecting them.

12 ·4 MUSTS FOR CUTLER-HAMMER SYSTEMS

This section deals with reset circuits, inhibit circuits, undervolt age protection, and fan-out limitations.

Reset circuits require special attention in transistor circuits. Reset is considered to mean returning a switch to a predeter mined state, i.e., resetting a set-reset MEMORY to the no-output condition. The case, however, of resetting a switch in the normal course of a functional sequence is considered routine and wm not be covered here. A second case, unique to transistors, will be discussed.

When the 10-volt power is first turned on , all transistors in the system start turning on. In elements such as AND, OR, and NOT, the input signals originating with limit switches and push buttons insure that the proper transistor ends up conducting. Memory-type circuits, those that are designed to pick up on momentary signals and then latch up, obviously cannot have the self -correcting type of input configurations . In these elements, with the exception of the retentive MEMORY, a resetting action is necessary so that their state is predictable. Where a problem of this type exists, boards are designed with a master reset terminal serving all elements on the board. Such a terminal should be held negative to reset and then positive to permit normal operation.

The reset gate (Fig. 12·12) is a circuit that will, when com bined with a NOT switch and upon the application of power, automatically cause the NOT switch to generate the signal re quired by master reset terminals. It is available as a component for mounting on the rear of the bucket. When the 10 volts d-e first appears, the capacitor starts charging through the base of transistor Tl. Thus T1, as an NPN, conducts, shutting off T2.

As the capacitor approaches full charge, it no longer passes T1 base current. Tl stops conducting, and the delayed +10-volt output signal appears at A.

The 47-ohm resistor and diode provide a discharge path for the capacitor that allows the gate to reset when the 10-volt power is off. The reset gate plus a set-reset MEMORY may be used to provide undervoltage protection (Fig. 12·13). When the power comes on, the reset gate holds the master reset terminal of the set-reset MEMORY negative. This insures that the set-reset assumes the no-output condition. It further will hold it in the no-output state, regardless of the position of the RESET button, until the the reset-gate output signal appears. After the reset-gate output signal appears, pressing the RESET button will turn on the set-reset MEMORY’s output signal. Either the STOP button or loss of the 10-volt supply will shut it off.

Inhibit circuits are sometimes required for gating purposes . The inhibit terminal is not a master reset terminal, nor does

it deal with the “power-on” problem. Inhibit functions are strictly for logic purposes and are found only on memory-type switches. The function they perform is to prevent changing state, i.e., from having an output signal to not having one, or vice versa. The general configuration of these circuits is such that the inhibit terminal must be at + 10 volts in order to permit change. If not required, these circuits should be connected to the +10-volt bus.

Fan-out limitations must always be considered in the system . The fan-out capability of a given switch is its ability to serve as an input signal to a plurality of other static switches. The term comes from the fact that the signal wires fan out from the output terminal of the given switch to a plurality of input terminals. Since the capacity of a transistor is limited, it is only reasonable to assume that the ability of a transistor switch to drive other switches is limited. In fact, two limits exist. One is the ability of the driving switch to provide current. The second relates to controlling the voltage at the input terminal when no output signal exists.

Let us look at the two cases one at a time, using the configura tion of Fig. 12·14. For our purposes the AND is supposed to drive the set-reset MEMORY, the OR, and the NOT . As we take· each case, this circuit will be redrawn to show the critical portion of each element. For the first case we will assume that the AND switch has an output signal. This means that the transistor shown is conducting. In the circuit of Fig. 12 ·15, notice how each additional element added increases the parallel paths. Ob viously this increases the current that the driving transistor must carry. The maximum collector current recommended is 80 rna. For purposes of calculation the input resistance of the NOT switch should be considered as 10 kilohms. This is due to the emitter-followe r configuration.

The circuit of Fig. 12·16 applies to case 2. The problem is simply that the various input configurations form a voltage divider network. This network centers about the output terminal of the driving element. When the driving transistor is not con ducting, this network sets the actual voltage at the output termi nal. It is recommended that the voltage at this point not be allowed to exceed +5 volts. For purposes of calculations the forward

resistance of transistors and diodes should be neglected. It should be noted that the OR unit does not act as a current source in this configuration, because of its input diodes.

12 ·5 CUTLER-HAMMER POWER SUPPLIES

Depending upon the components utilized, one, two, or three different voltages may be used in a static-switching control.

When three are used they consist of 10 volts d-e for all logic plus certain output amplifiers24 volts d-e for 24-volt power AND 48 volts d-e for d-e signal converters Obviously the exclusive u se of a-c signal converters in an appli cation eliminates the need for the 48-volt supply, just as proper selection of output amplifiers can avoid the requirement for 24 volts d-e. This discussion will deal with the most complete case-namely , all three voltages.

Regardless of whether one or three power supplies are used to achieve these voltages, they will be related as shown in Fig. 12·17. The reasons for this selection are: first, the logic supply is set at 10 volts d-e, a desirable voltage for transistor circuits . Second, since a greater voltage and power level than the 10 volts can supply is desired across most contacts, a 48-volt signal converter voltage is used. The 48- and 10-volt supplies have their negative sides made common, as this configuration permits the use of a clamping diode in the signal converter. Third, the 24-volt supply provides an output voltage and power level that is compatible with certain solenoid values , typewriter solenoids, and other devices. Making the positive side of the line common provides the simplest internal circuit for the board which must serve as an interface between two dissimilar voltages.

The proper procedure for selection of power supplies is detailed below. The proper starting place is the 48-volt d-e signal converter supply. The first step is to determine the maximum number of d-e converters that can be conducting at the same time. The second step is to calculate the minimum required capacity based on supplying 45 rna to each converter and to select an adequate power supply. Guard against excessive over capacity. Should it be desired to parallel 48-volt d-e supplies, derate them to 85 percent capacity.

Selection of the proper 10-volt power supply requires eight steps. Step 1: Determine both the total number of 10-volt power-AND boards and the maximum number of 10-volt power-AND output signals that can exist at one time. Step 2: Calculate the required capability based on supplying 0.5 amp to each of the maximum number of 10-volt power-AND output signals. Step 3: Determine the number of logic boards, including both the 10- and 24-volt power ANDS. Step 4: Calculate the power requirements for the above logic boards based on 150 rna per board. Step 5: Determine the total 10-volt power requirements by adding the totals from step 2 and step 4. Step 6: Compare the total 10-volt d-e ampere load from step 5 with the ampere capacity of the 48-volt d-e supply at 48 volts d-e. If the 10-volt load does not equal or exceed the amperage available at 48 volts d-e, select a shunting resistor or increase the 10-volt load. Note that since it is possible to get accidental grounds and since the transistors could be destroyed if the positive side of the 48-volt supply gets in contact with the 10-volt bus , the 48-volt supply is designed to collapse under the above loading. Step 7: Based on the 10-volt d-e load requirements determined in step 6, select a power-supply rating. (NOTE: Should it be desired to parallel 10-volt d-e supplies, derate them to 85 percent capacity.) Step 8: Unless the 10-volt d-e load equals approxi mately 75 percent of the power-supply capacity, reduce the ohmic value of the shunting resistor.

Selection of the proper 24-volt power supply requires four steps. Step 1: Determine the maximum number of 24-volt output signals that are present at one time. Step 2: Calculate the 24- volt load based on 1 amp per signal. Step 3: Select an adequate power supply. (NOTE: Should it be desired to parallel 24-volt power supplies, derate them to 85 percent capacity.) Step 4: The 24-volt power supply should be loaded to approximately I 00 percent capacity when the maximum number of inputs are on. Shunting resistors should be used to achieve this state.

Due to the interface between voltages accomplished by the 24-volt power AND and the signal converter, it is important that the 10 volts d-e is first on and last off. To insure this in power loss situations, two relays are required; these are connected as shown in Fig. 12·18.

12·6 MOUNTING AND WIRING CUTLER-HAMMER DEVICES

All static-switching logic elements are mounted on boards such as those shown in Fig. 12·19. Each board contains several switching elements, and each element is an independent switch. · All boards are the plug-in type, and since all boards have the same dimensions and conform physically, they can be arranged in buckets to suit circuit convenience. Each circuit point on the receptacle provides for two taper pins connected in parallel to facilitate wiring.

The “bucket” is a sort of “electronic bookcase” for the boards. The ones illustrated in Fig. 12·19 have a capacity of 20 boards. Basically, the bucket consists of a steel framework, 20 recepta cles, and the necessary bus work at the rear. This basic configuration is designed to be mounted in a cutout of a steel panel or in a conventional 20-in.-wide relay rack. Because of the sepa rate elements per board, the bucket contains an equivalent of approxi mately 80 relay circuits .The standard bucket includes receptacles and two bus bars. In wiring the buckets, it is first necessary to make connection between each plug and its respective A or V bus bar. A separate wire for each plug is recommended to assure a “solid” voltage supply. The A bus bar also serves as the positive side of the line for the 24-volt circuit. The negative side of the 24-volt supply should be brought m by wire to the affected board s. The bus bars on the rear of the bucket should be connected to the voltage source with No. 12 wire. The recommended wire size for logic wiring is No. 22. These wires should be terminated with taper pins for plug insertion and spade terminals for con nection to bus bar.

SCR amplifiers are available in two sizes. Components for each size are mounted on a fin (heat sink) which is painted red as a warning th at the fin is electrically “hot,” i.e.. carries 115 volts a-c. The insulating standoffs are first mounted to the steel panel with two screws each. The fin is then attached to the standoffs by means of two keyhole slots. A terminal strip mounted at the base of the fin provides for connections to the +10-volt control voltage and the 115 volts a-c. Again, all connections to the device are provid ed by screw terminals.

Assembling a DSL system involves two types of wiring. that applied to panel-mounted devices and that used on the rear of the buckets. The panel-mounted devices all have screw terminals and m ay be used with normal control wiring techniques . The rear of the bucket, however, calls for different wiring techniques. The bus bars are drilled and tapped for 6-32 screws. and spade terminations are suggested here. The 20 receptacles each have provision for taper pins in the form of two rows of 18 sockets. so that two wires may be connected to each point. The pins used are A-MP No. 41663. The recommended insertion tool is A-MP No. 380-31 0-F-395-005; the crimping tool is A-MP No. 48698.

12·7 THE CUTLER-HAMMER CODING AND DIAGRAM SYSTEM

Symbology and identification coding varies in the electrical con trol industry. The Cutler-Hammer company has its own system. one that has proved very successful. Its use is suggested since it is built around the marking already present on the boards. This section is devoted to the interpretation of the symbols that appear on Cutler-Hammer company diagrams and the marks that are printed on Cutler-Hammer company logic boards.

Look at the front of a Cutler-Hammer company logic board and you will see several markin gs (Fig. 12 ·20). At the top appears a symbol that identifies the function of the board-an AND function in this case. At the bottom appears a function number which identifies a specific circuit. Either the function number or the part number can be used to obtain an exact duplicate board. Boards with the same function number can be used interchangeably.

The familiar convention of “from left to right” is used on Cutler-Hammer company diagrams. That is, inputs are on the left and outputs are on the right. Because more than one ele ment is on a board, identification of terminals is necessary. The letters A to V, with omissions, identify the specific terminals on the plug of the board. The A and V terminals of all boards are used for the 10-volt power supply. The symbol for one element of an AND board is shown in Fig. 12·20 just as it would appear on a Cutler-Hammer company diagram . The letters in side the small circles identify the plug markin gs. For example, the circled T in the sketch indicates that this termination goes to the T terminal on the plug. The receptacle on the bucket ha:-, markings identical to those that appear on the rear of the plug.

Speedy identification of state-indication lights on logic boards makes for fast troubleshooting . As can be seen on the sketch of Fig. 12 ·20. lights for each element are identified by the letters A, B, C, and D. The fronts of all buckets have slots numbered left to right, at the top. Slots have corresponding numbers on the rear of buckets, at the top. Note, too, the numbers at the bottom of the panel. If the bucket is marked in this manner. when the control is wired, a speedy eye check instantly reveals any board that is not in its proper slot. Though the buckets are not physically identified, they are nevertheless always arranged in an alphabetical sequence from left to right, viz., A, B, C, D, etc.

Figure 12 ·21 is typical of an AND symbol that might app ar on a Cutler-Hammer company logic diagram. A simple “read ing” of the code is all that is required to systematically trace circuitry and determine the physical location of the portion of control under test. Here, for example, is what the code illustrated above reveals: The AND element can be found in panel 3. bucket C, board 5, and its state-indication light is identified by the letter D. With such complete information. the logic diagram is the only “tool” required to effect fast testing for speedy repair · or replacement.

Summary

The Cutler-Hammer company static system is an English logic system which is applied by means of plug-in logic elements. The logic elements are packaged four to a board and have individual indication lights mounted on the front of the board. Standard mounting provides for 20 boards in each bucket. Connections are made by mea ns of taper pin s which must be installed with the use of a special insertion tool.

The heart of this system is the basic transistor switch, operating from a 10-volt d-e power supply. The ON signal for this system is +l 0 volts d-e, and the OFF signal is 0 volts d-e. Recommended voltages for contact-type sensing devices are 48 volts d-e or 115 volts a -c.

This system uses the English logic symbols. When the com pany makes the control-circuit drawing for a factory-wired con trol system, the symbols are coded with numbers to aid in servic ing the equipment.

Fan-out limitations must be calculated on a basis of a maximum of 80 rna load and a minimum output voltage of 5 volts.

Normal installations will requ ire at least two power supplies, one 10-volt supply for logic elements and one 48-volt supply for signal converters. A third supply of 24 volts will be needed if any 24-volt power AN DS are used . Loading for these power supplies should be carefully figured as part of the reliance and safety factors in this system.

State-indication lights are a standard feature of every input and logic elemen t in this system and provide visual checking of any part of the circu it. w ithout other equipment, for norma l troubles.