Square D Company ®Norpak Control System part4

13·4 NORPAK OUTPUT AMPLIFIERS

The universal NOR can be made to function as either a NOR element or an output amplifier. Fig. 13·34a shows both the logic circuit and the output amplifier circuit. When the transistor con ducts, it can be thought of as closing a single-pole single-throw switch from collector to emitter. Note that in the logic circuit the transistor (switch) is in parallel with the load, but in the amplifier circuit it is in series with the load. In either case, a 1 input to the transistor causes it to conduct (switch closed) . In the logic circuit, the load is shorted out, and the load current is effectively zero. Thus a 1 input to a universal NOR used as a logic element deenergizes the load or produces a zero output. However a 1 input to the universal NOR used as an output ampli fier causes current to flow through the load. Therefore, a 1 input to a universal NOR used as an amplifier energizes the load or produces an output. When used as an output amplifier, the universal NOR has a maximum rating of 1.5 watts. Note that as its output rating increases from 0.26 to 1.5 watts , its input load rating increases from 1 to 4 units, since the collector current of a transistor is a function of its base current. Increasing the

input load is accomplished by using combinations of the univer sal NOR’s three inputs. The maximum output rating of 1.5 watts is obtained when the load resistance is 270 ohms and all three of the universal NOR’s inputs are connected together. When a universal NOR is powering a relay or other inductive load, a discharge path of an OR element or diode must be provided as shown in Fig. 13·34b. Incandescent lamps have a low resis tance when cold, which can result in inrush currents of 10 times the nominal va lue. A current-limiting resistor should be placed in series with the lamp as shown in Fig. 13·34b.

The type T03 is a solid-state, epoxy-encapsulated d-e output amplifier. It is designed to operate as a switch in a 24-volt d-e circuit supplied by a class 8851 type A51 or A301 auxiliary

power supply, and will energize the load when a 1 signal is applied to terminal B 12. Since the peak voltage of such supplies is 42 volts, the T03 (actually a 0.5-amp device) is rated at 0.3 amp with a minimum load resistance of 80 ohms. However , if a pure d-e source is used (such as a battery or type P7 power supply), the current rating may be increased to 0.5 amp.

The elementary diagram for a NORPAK d-e output amplifier is shown in Fig. 13·35. This is basically a two-stage transistor amplifier.

A logic I at the input causes transistor 1Tl to conduct through resistor 1RES to common. The voltage across 1RES drives the base of 2Tl negative, causing it to conduct; this provides a path for load current from the -24-volt d-e source to common, energizing the load.

The type T04 d-e amplifier is identical to the T03 except in maximum ratings. The ratings are 28.8 watts at 1.2 amp and 24 volts d-e. Minimum load resistance is 20 ohms at 24 volts d-e. Maximum current with a pure d-e power supply or a type P7 power supply is 2.0 amp.

The elementary diagram for an a-c output amplifier is shown in Fig. 13·36. A logic 1 at the input terminal causes the output

of the NOR to go to 0, charging capacitor 1C. 1C charges to the firing voltage of the unijunction transistor. The unijunction then fires, pulsing the pulse transformer, which in turn triggers . the gate to the SCR. The SCR fires, providing a path for load current from L 1 through the bridge rectifier to L2 on the positive half-cycle of a-c power and from L2 to L 1 on the negative half-cycle.

As long as the input signal remains, the capacitor will con tinue to charge and discharge through the unijunction transistor providing continuous a-c voltage to the load.

The type TO 10 is an output amplifier designed for use in separate 120-volt ( +10 percent, -15 percent) a-c circuits. It has a continuous current rating of 1 amp root mean square,

( rms) with a peak inrush current of 10 amp and may be oper ated from either a 50- or 60-cycle power source. The TO 10 will control any Square D Company relay, contactor, or starter up to a NEMA size 2. This amplifier consists of a silicon con trolled rectifier circuit encapsulated in a molded bakelite case. Logic and power terminals are located at the top of the case along with a pilot light to indicate the on-off state of the ampli fier. The unit is protected internally for line-voltage transients found in supply transformers up to 5KVA. If a larger supply transformer is used, additional surge protection must be added to keep the peak voltage below 400 volts. Leakage current in the output circuit will be 10 rna or less. The type TO 10 has an internal firing pulse of approximately 2,000 cps, and there fore it can easily fire into any power-factor load. The type TO 10 amplifier energizes the load when a 1 signal is applied to termi nal B 1-2 and requires only one unit of logic load from the controlling logic element. The load should be fused if the device to be operated is a starter, contactor, solenoid, or other elec tromechanical device wherein there exists the possibility of stick ing or jamming. Such sticking may result in sustained peak cur rents which could be dangerous to the SCR. A Buss Limitron type KAA2 quick-acting fuse, which is rated at 2 amp and 125 volts, is recommended.

The transistorized relay is a device consisting of a transistor ized amplifier used to control an output relay. The device in cludes a power supply which operates from a 120-volt 50- or 60-cycle source. Both the amplifier and the power supply are pack aged in an epoxy-encapsulated module. Only one unit of input logic load is required for operation from a NORPAK logic sys tem. The type T020 is a transistorized relay (Fig. 13·37) offer ing four different modes of initiation. In all modes of initiation , the 120-volt power is connected to terminals 1 and 2. For opera tion from a standard NOR, terminal 8 is connected to common , and terminal 7 serves as the input connection. An output of 1 from the controlling NOR will energize the relay, and a 0 output will deenergize it.

13·5 NORPAK POWER SUPPLIES

There are two general classifications of power supplies used with NORPAK systems-logic power supplies and accessory power supplies. The former is a necessity in all NORPAK systems; the latter is used when particular components become parts of the system. A variety of each type permits selection of only the necessary capacities and functions for a particular system. All NORPAK systems require logic power supplies to provide the power requirements for logic components. Many systems require only one power supply, although in some instances systerns need more than one power supply because of system size or other factors which present power needs in excess of the capacity of a single basic supply.

The elementary diagram for a NORPAK power supply is shown in Fig. 13·38. This power supply consists of a filtered, full-wave, center-tap bridge circuit for the logic +20-volt and -20-volt d-e voltages. The -130-volt d-e voltage for use with pilot devices is from a filtered, full-wave bridge circuit. All voltages are regulated by means of a regulating-type transformer.

The OFF signal for use with off-return MEMORY functions is developed by transistor 1TI and its associated resistor-capacitor input circuit. It functions as a single-shot multivibrator pro viding a short-duration, logic 1 signal to all MEMORYs to insure their return to the OFF state upon loss and reapplication of a-c power.

The pulse signal is a 6-volt 60-cycle a-c voltage for use with retentive MEMORYS.

The selection of the type and number of power supplies in a NORPAK system obviously depends upon the power require-

ments of that system. In general, the -20-volt , and sometimes the +20-volt, system power requirements dictate the particular supply or supplies to be used. Although it is unusual to find a system that requires additional pulse, off, or -130-volt power, a cursory inspection of these requirement s should be made. If the results of this investigation reveal requirements which are close to the capacity of one type P 1 power supply , an accurate tabulation of the requirements should be completed . More frequently in large systems, it will be found that additional -20-volt power is required. In this case, the use of a type P7 power supply will provide an additional 1,000 units of -20-volt power. Experience has shown that the addition of this supply is sufficient for all but very large NORPAK systems or those having unusual -20-volt power requirements. Although infre quently, a system may require more than the 1,000 units of +20-volt power to be supplied by the type Pl supply. Such

a system could be one which uses a very large number of the type T03 or type T04 output amplifiers. The type P7 supply will furnish an additional 20,000 units of +20-volt power. Under these circumstances, the -20-volt terminal of the type P7 would be connected to the system common, and the com mon terminal would serve as the +20-volt output. When ascer taining the total -20-volt and +20-volt power requirements of the system, one should remember that all NOR elements in a PAK are connected to the supply. Therefore, all NORs, includ ing unused NORS, should be considered in determining the size and number of power supplies. Do not parallel power supplies. If multiple power supplies are required for a system, the common terminals are tied together. The logic load should then be divided proportionately among the applicable outputs of the system power supplies.

13 ·6 WIRING NORPAK CIRCUITS

Good wiring practice not only means ease of as embly but also is the basis of trouble-free operation. Some circuits are sensitive to external interference, but if the practices discussed here are followed, this problem can be eliminated. The layout of the components cannot be fully planned until the system design has been completed, so that every element has been taken into ac count. The schematic must then be studied to determine the best layout that will minimize wire runs. The various logic com ponents should therefore be grouped to conform with the circuit functions. One should consider not only interlogic wiring but also wiring from signal-converter outputs to logic inputs, from logic outputs to monitor lights, and from logic outputs to users’ terminals and output devices. Grouping the components according to circuit function will frequently result in a few spare logic elements, but this may prove to be an advantage if circuit changes are made or an element must be replaced. If possible, one or two spare NORs should be allowed for in every 20 Pak to facilitate any future changes. It is best to locate such spare NORs at the middle and end of the 20 Pak. Each unused NOR of any type must have one of its inputs connected to common.

Because an OR Pak (L3) contains 21 diodes which are frequently dispersed throughout the system, a central location is most advantageous. Time DELAYS (L7, LlO) and retentive MEMORYS (L5) use few connecting wires and can therefore be placed on the perimeter of the logic area. Transfer Paks (L8) should be mounted adjacent to the MEMORYs with which they are used. Counting or shift-register circuits made up of transfer MEMORY Paks (Lll) and BCD counter Paks (L12) should be grouped together, with the input NOR to the counter or shift circuit placed as close to that circuit as possible . Signal convert ers are usually located along one side of the panel adjacent to the logic elements to which they connect. Power supplies are mounted above the logic components to provide a chimney effect for the heat generated. The output devices are usually mounted to the side or at the bottom of the logic components. If they happen to be magnetic devices (relays, starters, etc.), they are to be kept at least 6 in. from the logic Paks to prevent any possible interference from induced voltages due to magnetic fields.

Logic wiring requires care and thought; because the NOR has a high-impedance input and is switched very rapidly, it is sucep tible to induced high-frequency transients. These transients or stray pickup signals are especially effective in circuits using transfer elements in one form or another or where a flip-flop MEMORY happens to be in a position subject to these stray input signals. The effect of stray signals can be eliminated if the layout of the logic components described above is followed. In addition, the following wiring practices should be adhered to. A flip-flop MEMORY circuit should only be made up of NORs of one type (standard NORS, power NORs, or universal NORS) and should be contained in one Pak. Neither of the NORs making up the MEMORY should be used as an amplifier to drive relays or lights. Diodes are not to be included as part of the feedback loop of the MEMORY circuit.

The output of a transfer element is a diode; this allows direct paralleling with other outputs. Each output is to be connected directly to pin 7 or 8 or a NOR, but no more than two transfer elements should be paralleled in such a manner. To accommo date more than two transfer elements, only two outputs are con nected to pins 7 and 8 of a NOR with one of its standard inputs connected to -20 volts. W.hen either transfer element pulses, the result will be a -20-volt pulse at the output of the NOR. The outputs of several such NORs are then connected together by means of the usual OR circuit.

When logic-level signals are to interconnect to another remotely located NORPAK panel, these signals should not be taken directly from OR circuits or from MEMORYs but should instead be isolated with NORs (preferably power or universal NORS).

When neon monitor lights are to be used, they should be restricted to use only on or within the NORPAK control cabinet. If the metal case of the neon light is not in firm contact with its mounting surface, the lights may tend to flicker because of slight voltage differences that develop. Incandescent lights should be used whenever a remote indicator is needed.

The +20- and -20-volt power connections to the logic Paks can be made with No. 20 standard wire in series strings “jumper ing” from one Pak to the next. The common run of wire, how ever, should be kept as direct and short as possible. A No. 14 wire should be connected to common on the power supply and then directed down the center of the group of logic components to a series of terminal posts with standoff insulators that arc mounted on the corners of the logic Paks. Each of these points then serves as a local common point with a direct return to the power supply. The common connections of no more than three logic Paks should be “jumpered” in series and brought to the local common terminal via the usual No. 20 logic wire. The common terminal at the power supply should then be connected to the chassis with a No. 14 wire under a power-supply mounting screw. The common terminal of a d-e to d-e signal converter (Nl) should also be connected to the chassis at the mounting screw. Wherever common is brought out of the logic circuit as a users’ terminal, it should be connected to the chassis.

Stray interference can be fairly well controlled by following the practices discussed above, but to help in eliminating the paths of this interference, all logic input wires should be sepa rated from the same channel duct conduit or harnesses that carry any power or load wiring. Stray pickup signals are usually the result of transients that are generated by the dropout of a magnet coil. When the coil is being driven by an output amplifier of the type T03 or T04, there is a diode built into the device that shunts out any circulating currents resulting from the tran sient and in so doing provides protection for the transistor and damps out the transient. This protection is lost, however, if any contacts are interposed between the load and the -24-volt ter minal; if the contact is opened, the shunt path around the load is broken, and a very high transient could develop. To maintain this protection, terminal A 3-4 on the static switch should be connected in directly behind the load instead of at the -24-volt terminal.

Whenever the application includes any type of counting circuit, alternator, or shift register that is driven by contact-making devices, the problem of contact bounce must be considered. Cir cuits designed to eliminate bounce are shown in Fig. 13·32.

Summary

The NORPAK system uses NOR logic and is built around a basic NOR element which is connected in the control panel to make up ANDS and MEMORYS, even ORS.

The logic diagrams used with this system are different from those used in English logic systems but can be developed from English logic diagrams.

An ON signal for NORPAK is -10 to -20 volts d-e, while an OFF signal is 0 volts. The basic power supply provides +20 volts d-e from the base-to-collector circuit of the transistor. In the event that there are some unused NORS, the -20-volt collec tor voltage and the +20-volt bias voltage would create a collector-to-base voltage of 40 volts which could damage the transis tor. Unused NORS should have one input connected to common.

The NORS in NORPAK are encapsulated in standard modules which are then connected into the circuit by jumpers.

The system also includes a test unit which should always be used in servicing the logic section of the panel.