Friday, August 12, 2016

The Electrical Relay


Thus far we have seen a selection of Input devices that can be used to detect or “sense” a variety of physical variables and signals and are therefore called Sensors.




But there are also a variety of electrical and electronic devices which are classed asOutput devices used to control or operate some external physical process. These output devices are commonly called Actuators.
Actuators convert an electrical signal into a corresponding physical quantity such as movement, force, sound etc. An actuator is also classed as a transducer because it changes one type of physical quantity into another and is usually activated or operated by a low voltage command signal. Actuators can be classed as either binary or continuous devices based upon the number of stable states their output has.
For example, a relay is a binary actuator as it has two stable states, either energised and latched or de-energised and unlatched, while a motor is a continuous actuator because it can rotate through a full 360o motion. The most common types of actuators or output devices are Electrical RelaysLightsMotors and Loudspeakers.
We saw previously that solenoids can be used to electrically open latches, doors, open or close valves, and in a variety of robotic and mechatronic applications, etc. However, if the solenoid plunger is used to operate one or more sets of electrical contacts, we have a device called a relay that is so useful it can be used in an infinite number of different ways and in this tutorial we will look at Electrical Relays.
Electrical Relays can also be divided into mechanical action relays called “Electromechanical Relays” and those which use semiconductor transistors, thyristors, triacs, etc, as their switching device called “Solid State Relays” or SSR’s.
 The Electromechanical RelayThe term Relay generally refers to a device that provides an electrical connection between two or more points in response to the application of a control signal. The most common and widely used type of electrical relay is the electromechanical relay or EMR.
An Electrical RelayThe most fundamental control of any equipment is the ability to turn it “ON” and “OFF”. The easiest way to do this is using switches to interrupt the electrical supply. Although switches can be used to control something, they have their disadvantages. The biggest one is that they have to be manually (physically) turned “ON” or “OFF”. Also, they are relatively large, slow and only switch small electrical currents.Electrical Relays however, are basically electrically operated switches that come in many shapes, sizes and power ratings suitable for all types of applications. Relays can also have single or multiple contacts within a single package with the larger power relays used for mains voltage or high current switching applications being called “Contactors”.In this tutorial about electrical relays we are just concerned with the fundamental operating principles of “light duty” electromechanical relays we can use in motor control or robotic circuits. Such relays are used in general electrical and electronic control or switching circuits either mounted directly onto PCB boards or connected free standing and in which the load currents are normally fractions of an ampere up to 20+ amperes. The relay circuit are common in Electronics applications.As their name implies, electromechanical relays are electro-magnetic devices that convert a magnetic flux generated by the application of a low voltage electrical control signal either AC or DC across the relay terminals, into a pulling mechanical force which operates the electrical contacts within the relay. The most common form of electromechanical relay consist of an energizing coil called the “primary circuit” wound around a permeable iron core.This iron core has both a fixed portion called the yoke, and a moveable spring loaded part called the armature, that completes the magnetic field circuit by closing the air gap between the fixed electrical coil and the moveable armature. The armature is hinged or pivoted allowing it to freely move within the generated magnetic field closing the electrical contacts that are attached to it. Connected between the yoke and armature is normally a spring (or springs) for the return stroke to “reset” the contacts back to their initial rest position when the relay coil is in the “de-energized” condition, i.e. turned “OFF”. 

Electromechanical Relay Construction


In our simple relay above, we have two sets of electrically conductive contacts. Relays may be “Normally Open”, or “Normally Closed”. One pair of contacts are classed as Normally Open, (NO) or make contacts and another set which are classed as Normally Closed, (NC) or break contacts. In the normally open position, the contacts are closed only when the field current is “ON” and the switch contacts are pulled towards the inductive coil.

In the normally closed position, the contacts are permanently closed when the field current is “OFF” as the switch contacts return to their normal position. These terms Normally Open, Normally Closed or Make and Break Contacts refer to the state of the electrical contacts when the relay coil is “de-energized”, i.e, no supply voltage connected to the relay coil. Contact elements may be of single or double make or break designs. An example of this arrangement is given below.
The relays contacts are electrically conductive pieces of metal which touch together completing a circuit and allow the circuit current to flow, just like a switch. When the contacts are open the resistance between the contacts is very high in the Mega-Ohms, producing an open circuit condition and no circuit current flows.

When the contacts are closed the contact resistance should be zero, a short circuit, but this is not always the case. All relay contacts have a certain amount of “contact resistance” when they are closed and this is called the “On-Resistance”, similar to FET’s.

With a new relay and contacts this ON-resistance will be very small, generally less than 0.2Ω’s because the tips are new and clean, but over time the tip resistance will increase.

For example. If the contacts are passing a load current of say 10A, then the voltage drop across the contacts using Ohms Law is 0.2 x 10 = 2 volts, which if the supply voltage is say 12 volts then the load voltage will be only 10 volts (12 – 2). As the contact tips begin to wear, and if they are not properly protected from high inductive or capacitive loads, they will start to show signs of arcing damage as the circuit current still wants to flow as the contacts begin to open when the relay coil is de-energized.

This arcing or sparking across the contacts will cause the contact resistance of the tips to increase further as the contact tips become damaged. If allowed to continue the contact tips may become so burnt and damaged to the point were they are physically closed but do not pass any or very little current.

If this arcing damage becomes to severe the contacts will eventually “weld” together producing a short circuit condition and possible damage to the circuit they are controlling. If now the contact resistance has increased due to arcing to say 1Ω’s the volt drop across the contacts for the same load current increases to 1 x 10 = 10 volts dc. This high voltage drop across the contacts may be unacceptable for the load circuit especially if operating at 12 or even 24 volts, then the faulty relay will have to be replaced.

To reduce the effects of contact arcing and high “On-resistances”, modern contact tips are made of, or coated with, a variety of silver based alloys to extend their life span as given in the following table.

Electrical Relay Contact Tip Materials

Ag (fine silver)
1. Electrical and thermal conductivity are the highest of all the metals.
2. Exhibits low contact resistance, is inexpensive and widely used.
3. Contacts tarnish easily through sulphurisation influence.
AgCu (silver copper)
1. Known as “Hard silver” contacts.
Better wear resistance and less tendency to arc and weld, but slightly higher contact resistance.
AgCdO (silver cadmium oxide)
1. Very little tendency to arc and weld, good wear resistance and arc extinguishing properties.
AgW (silver tungsten)
1. Hardness and melting point are high, arc resistance is excellent.
2. Not a precious metal.
3. High contact pressure is required to reduce resistance.
4. Contact resistance is relatively high, and resistance to corrosion is poor.
AgNi (silver nickel)
1. Equals the electrical conductivity of silver, excellent arc resistance.
AgPd (silver palladium)
1. Low contact wear, greater hardness.
2. Expensive.
Platinum, Gold and Silver Alloys
1. Excellent corrosion resistance, used mainly for low-current circuits.
Relay manufacturers data sheets give maximum contact ratings for resistive DC loads only and this rating is greatly reduced for either AC loads or highly inductive or capacitive loads. In order to achieve long life and high reliability when switching alternating currents with inductive or capacitive loads some form of arc suppression or filtering is required across the relay contacts.

Extending the life of relay tips by reducing the amount of arcing generated as they open is achieved by connecting a Resistor-Capacitor network called an RC Snubber Network electrically in parallel with an electrical relay contact tips. The voltage peak, which occurs at the instant the contacts open, will be safely short circuited by the RC network, thus suppressing any arc generated at the contact tips. For example.

Electrical Relay Snubber Circuit






Electrical Relay Contact Types.

As well as the standard descriptions of Normally Open, (NO) and Normally Closed, (NC) used to describe how the relays contacts are connected, relay contact arrangements can also be classed by their actions. Electrical relays can be made up of one or more individual switch contacts with each “contact” being referred to as a “pole”. Each one of these contacts or poles can be connected or “thrown” together by energizing the relays coil and this gives rise to the description of the contact types as being:
  • SPST – Single Pole Single Throw
  • SPDT – Single Pole Double Throw
  • DPST – Double Pole Single Throw
  • DPDT – Double Pole Double Throw
with the action of the contacts being described as “Make” (M) or “Break” (B). Then a simple relay with one set of contacts as shown above can have a contact description of:
"Single Pole Double Throw – (Break before Make)", or SPDT – (B-M)
Examples of just some of the more common diagrams used for electrical relay contact types to identify relays in circuit or schematic diagrams is given below but there are many more possible configurations.

Electrical Relay Contact Configurations

Where:
    C is the Common terminal
    NO is the Normally Open contact
    NC is the Normally Closed contact
Electromechanical relays are also denoted by the combinations of their contacts or switching elements and the number of contacts combined within a single relay. For example, a contact which is normally open in the de-energised position of the relay is called a “Form A contact” or make contact. Whereas a contact which is normally closed in the de-energised position of the relay is called a “Form B contact” or break contact.

When both a make and a break set of contact elements are present at the same time so that the two contacts are electrically connected to produce a common point (identified by three connections), the set of contacts are referred to as “Form C contacts” or change-over contacts. If no electrical connection exists between the make and break contacts it is referred to as a double change-over contact.

One final point to remember about using electrical relays. It is not advisable at all to connect relay contacts in parallel to handle higher load currents. For example, never attempt to supply a 10A load with two relays in parallel that have 5A contact ratings each, as the mechanically operated relay contacts never close or open at exactly the same instant of time. The result is that one relay contact will always be overloaded even for a brief instant in time resulting in premature failure of the relay over time.

Also, while electrical relays can be used to allow low power electronic or computer type circuits to switch relatively high currents or voltages both “ON” or “OFF”. Never mix different load voltages through adjacent contacts within the same relay such as for example, high voltage AC (240v) and low voltage DC (12v), always use separate relays for safety.

One of the more important parts of any electrical relay is its coil. This converts electrical current into an electromagnetic flux which is used to mechanically operate the relays contacts. The main problem with relay coils is that they are “highly inductive loads” as they are made from coils of wire. Any coil of wire has an impedance value made up of resistance ( R ) and inductance ( L ) in series (LR Series Circuit).

As the current flows through the coil a self induced magnetic field is generated around it. When the current in the coil is turned “OFF”, a large back emf (electromotive force) voltage is produced as the magnetic flux collapses within the coil (transformer theory). This induced reverse voltage value may be very high in comparison to the switching voltage, and may damage any semiconductor device such as a transistor, FET or micro-controller used to operate the relay coil.

One way of preventing damage to the transistor or any switching semiconductor device, is to connect a reverse biased diode across the relay coil.
When the current flowing through the coil is switched “OFF”, an induced back emf is generated as the magnetic flux collapses in the coil.
This reverse voltage forward biases the diode which conducts and dissipates the stored energy preventing any damage to the semiconductor transistor.
When used in this type of application the diode is generally known as a Flywheel Diode, Free-wheeling Diode and even Fly-back Diode, but they all mean the same thing. Other types of inductive loads which require a flywheel diode for protection are solenoids, motors and inductive coils.

As well as using flywheel Diodes for protection of semiconductor components, other devices used for protection include RC Snubber Networks, Metal Oxide Varistors or MOV and Zener Diodes.

The Solid State Relay.

While the electromechanical relay (EMR) are inexpensive, easy to use and allow the switching of a load circuit controlled by a low power, electrically isolated input signal, one of the main disadvantages of an electromechanical relay is that it is a “mechanical device”, that is it has moving parts so their switching speed (response time) due to physically movement of the metal contacts using a magnetic field is slow.

Over a period of time these moving parts will wear out and fail, or that the contact resistance through the constant arcing and erosion may make the relay unusable and shortens its life. Also, they are electrically noisy with the contacts suffering from contact bounce which may affect any electronic circuits to which they are connected.

To overcome these disadvantages of the electrical relay, another type of relay called a Solid State Relay or (SSR) for short was developed which is a solid state contactless, pure electronic relay.

The solid state relay being a purely electronic device has no moving parts within its design as the mechanical contacts have been replaced by power transistors, thyristors or triac’s. The electrical separation between the input control signal and the output load voltage is accomplished with the aid of an opto-coupler type Light Sensor.

The Solid State Relay provides a high degree of reliability, long life and reduced electromagnetic interference (EMI), (no arcing contacts or magnetic fields), together with a much faster almost instant response time, as compared to the conventional electromechanical relay.


Also the input control power requirements of the solid state relay are generally low enough to make them compatible with most IC logic families without the need for additional buffers, drivers or amplifiers. However, being a semiconductor device they must be mounted onto suitable heatsinks to prevent the output switching semiconductor device from over heating.

Solid State Relay
The AC type Solid State Relay turns “ON” at the zero crossing point of the AC sinusoidal waveform, prevents high inrush currents when switching inductive or capacitive loads while the inherent turn “OFF” feature of Thyristors and Triacs provides an improvement over the arcing contacts of the electromechanical relays.

Like the electromechanical relays, a Resistor-Capacitor (RC) snubber network is generally required across the output terminals of the SSR to protect the semiconductor output switching device from noise and voltage transient spikes when used to switch highly inductive or capacitive loads. In most modern SSR’s this RC snubber network is built as standard into the relay itself reducing the need for additional external components.

Non-zero crossing detection switching (instant “ON”) type SSR’s are also available for phase controlled applications such as the dimming or fading of lights at concerts, shows, disco lighting etc, or for motor speed control type applications.

As the output switching device of a solid state relay is a semiconductor device (Transistor for DC switching applications, or a Triac/Thyristor combination for AC switching), the voltage drop across the output terminals of an SSR when “ON” is much higher than that of the electromechanical relay, typically 1.5 – 2.0 volts. If switching large currents for long periods of time an additional heat sink will be required.

Input/Output Interface Modules.

Input/Output Interface Modules, (I/O Modules) are another type of solid state relay designed specifically to interface computers, micro-controller or PIC’s to “real world” loads and switches. There are four basic types of I/O modules available, AC or DC Input voltage to TTL or CMOS logic level output, and TTL or CMOS logic input to an AC or DC Output voltage with each module containing all the necessary circuitry to provide a complete interface and isolation within one small device. They are available as individual solid state modules or integrated into 4, 8 or 16 channel devices.

Modular Input/Output Interface System.
The main disadvantages of solid state relays (SSR’s) compared to that of an equivalent wattage electromechanical relay is their higher costs, the fact that only single pole single throw (SPST) types are available, “OFF”-state leakage currents flow through the switching device, and a high “ON”-state voltage drop and power dissipation resulting in additional heat sinking requirements. Also they can not switch very small load currents or high frequency signals such as audio or video signals although special Solid State Switches are available for this type of application.

In this tutorial about Electrical Relays, we have looked at both the electromechanical relay and the solid state relay which can be used as an output device (actuator) to control a physical process. In the next tutorial we will continue our look at output devices called Actuators and especially one that converts a small electrical signal into a corresponding physical movement using electromagnetism. The output device is called a Solenoid.

Tuesday, August 9, 2016

Relay application considerations


Selection of an appropriate relay for a particular application requires evaluation of many different factors:
  • Number and type of contacts – normally open, normally closed, (double-throw)
  • Contact sequence – "Make before Break" or "Break before Make". For example, the old style telephone exchanges required Make-before-break so that the connection didn't get dropped while dialing the number.
  • Contact current rating – small relays switch a few amperes, large contactors are rated for up to 3000 amperes, alternating or direct current
  • Contact voltage rating – typical control relays rated 300 VAC or 600 VAC, automotive types to 50 VDC, special high-voltage relays to about 15,000 V
  • Operating lifetime, useful life - the number of times the relay can be expected to operate reliably. There is both a mechanical life and a contact life. The contact life is affected by the type of load switched. Breaking load current causes undesired arcing between the contacts, eventually leading to contacts that weld shut or contacts that fail due erosion by the arc.
  • Coil voltage – machine-tool relays usually 24 VDC, 120 or 250 VAC, relays for switchgear may have 125 V or 250 VDC coils,
  • Coil current - Minimum current required for reliable operation and minimum holding current, as well as, effects of power dissipation on coil temperature, at various duty cycles. "Sensitive" relays operate on a few milliamperes
  • Package/enclosure – open, touch-safe, double-voltage for isolation between circuits, explosion proof, outdoor, oil and splash resistant, washable forprinted circuit board assembly
  • Operating environment - minimum and maximum operating temperature and other environmental considerations such as effects of humidity and salt
  • Assembly – Some relays feature a sticker that keeps the enclosure sealed to allow PCB post soldering cleaning, which is removed once assembly is complete.
  • Mounting – sockets, plug board, rail mount, panel mount, through-panel mount, enclosure for mounting on walls or equipment
  • Switching time – where high speed is required
  • "Dry" contacts – when switching very low level signals, special contact materials may be needed such as gold-plated contacts
  • Contact protection – suppress arcing in very inductive circuits
  • Coil protection – suppress the surge voltage produced when switching the coil current
  • Isolation between coil contacts
  • Aerospace or radiation-resistant testing, special quality assurance
  • Expected mechanical loads due to acceleration – some relays used in aerospace applications are designed to function in shock loads of 50 g or more
  • Size - smaller relays often resist mechanical vibration and shock better than larger relays, because of the lower inertia of the moving parts and the higher natural frequencies of smaller parts. Larger relays often handle higher voltage and current than smaller relays.
  • Accessories such as timers, auxiliary contacts, pilot lamps, and test buttons
  • Regulatory approvals
  • Stray magnetic linkage between coils of adjacent relays on a printed circuit board.
There are many considerations involved in the correct selection of a control relay for a particular application. These considerations include factors such as speed of operation, sensitivity, andhysteresis. Although typical control relays operate in the 5 ms to 20 ms range, relays with switching speeds as fast as 100 us are available. Reed relays which are actuated by low currents and switch fast are suitable for controlling small currents.
As with any switch, the contact current (unrelated to the coil current) must not exceed a given value to avoid damage. In high-inductance circuits such as motors, other issues must be addressed. When an inductance is connected to a power source, an input surge current or electromotor starting current larger than the steady-state current exists. When the circuit is broken, the current cannot change instantaneously, which creates a potentially damaging arc across the separating contacts.
Consequently, for relays used to control inductive loads, we must specify the maximum current that may flow through the relay contacts when it actuates, the make rating; the continuous rating; and the break rating. The make rating may be several times larger than the continuous rating, which is itself larger than the break rating.

Derating factors

Type of load% of rated value
Resistive75
Inductive35
Motor20
Filament10
Capacitive75
Control relays should not be operated above rated temperature because of resulting increased degradation and fatigue. Common practice is to derate 20 degrees Celsius from the maximum rated temperature limit. Relays operating at rated load are affected by their environment. Oil vapor may greatly decrease the contact life, and dust or dirt may cause the contacts to burn before the end of normal operating life. Control relay life cycle varies from 50,000 to over one million cycles depending on the electrical loads on the contacts, duty cycle, application, and the extent to which the relay is derated. When a control relay is operating at its derated value, it is controlling a smaller value of current than its maximum make and break ratings. This is often done to extend the operating life of a control relay. The table lists the relay derating factors for typical industrial control applications.

Undesired arcing

Main article: Arc suppression
Switching while "wet" (under load) causes undesired arcing between the contacts, eventually leading to contacts that weld shut or contacts that fail due to a buildup of contact surface damage caused by the destructive arc energy.
Inside the 1ESS switch matrix switch and certain other high-reliability designs, the reed switches are always switched "dry" to avoid that problem, leading to much longer contact life.
Without adequate contact protection, the occurrence of electric current arcing causes significant degradation of the contacts, which suffer significant and visible damage. Every time a relay transitions either from a closed to an open state (break arc) or from an open to a closed state (make arc & bounce arc), under load, an electrical arc can occur between the two contact points (electrodes) of the relay. In many situations, the break arc is more energetic and thus more destructive, in particular with resistive-type loads. However, inductive loads can cause more destructive make arcs. For example, with standard electric motors, the start-up (inrush) current tends to be much greater than the running current. This translates into enormous make arcs.
During an arc event, the heat energy contained in the electrical arc is very high (tens of thousands of degrees Fahrenheit), causing the metal on the contact surfaces to melt, pool and migrate with the current. The extremely high temperature of the arc cracks the surrounding gas molecules creating ozonecarbon monoxide, and other compounds. The arc energy slowly destroys the contact metal, causing some material to escape into the air as fine particulate matter. This action causes the material in the contacts to degrade quickly, resulting in device failure. This contact degradation drastically limits the overall life of a relay to a range of about 10,000 to 100,000 operations, a level far below the mechanical life of the same device, which can be in excess of 20 million operations.

Saturday, August 6, 2016

Power Relay Station

Power Relay Station

The power relay station is the junction box for all systems of the spacecraft. The power relay station enables ad hoc repairs to damaged systems. Power distribution can be configured to reroute power from working systems to systems that are not working. Less important systems can be sacrificed in favor of systems necessary for survival of the ship and crew. Using the power relay station, a skilled engineer can revive an otherwise derelict ship so it can limp back to port for repairs.

WARNING!
Improper use of the power relay station can result in catastrophic system failure and total destruction of the spacecraft.





Power is needed by the systems of a spacecraft. The following diagram graphically depicts the flow of power from the power plant to the gravity drive. Like the gravity drive in this example, all systems consume power according to this process.


The power plant converts hydrogen fuel into electrical energy. Electricity produced by the power plant is stored in the capacitor. Each power plant unit adds to the total energy storage capacity of the capacitor.

The capacitor acts as a ready supply of electrical energy for the systems of the spacecraft. Unlike a battery, which releases energy slowly, the capacitor responds instantly to large sudden demands for power. The amount of energy that can be stored by the capacitor is determined by the number of power plant units in the design.

Each system has a power converter. The power converter directs power from the capacitor to the system equipment through a power relay junction. The power converter for each system is designed to handle the energy load demanded by its system. The gravity drive power converter often routes a lot of power because a large number of gravity drive units is typical in a spacecraft design.

power relay junction connects a power converter to system equipment. Normally, every power converter has one single power relay junction connecting to its corresponding system equipment. The figure depicts the gravity drive power converter connected to the gravity drive equipment, which is the normal state.

System equipment is the final consumer of the electrical power produced by the power plant. System equipment performs the specific function of the system. The gravity drive provides propulsion through space and creates an artificial gravity environment inside the spacecraft.


When a system is damaged, the damage affects the power converter for the system, not the equipment of the system. This is crucial to understanding the role of the power relay station. Damage to a system reduces the output of that system's power converter. The full potential of the system equipment can be restored by connecting other working power converters to the system's equipment. Using the power relay station, multiple power converters can be connected to the equipment of a single system.


The display screen is divided into two sections by red lines. The small upper section displays the power plant shunts. The large lower section displays the power relay junctions.
Controls are activated by clicking on the console using the hand cursor. Controls also have corresponding keyboard equivalents. Help about using the hand cursor and a table showing the key commands for this station can be found on the Keyboard and Mouse Controls page.

Power Relay Junctions

The power relay junction section displays power converters in a row across the top. System equipment is displayed in a row across the bottom. Power junctions are displayed as thick gold traces connecting power converters at the top to system equipment at the bottom.


There may be multiples of some systems, such as weapon bays, turrets, transporter rooms and sick bays. Each instance of those systems will be represented by its own power converter and system equipment. Each different system is assigned a number for identification.




Healthy Power Converter for Gravity DriveThe vertical bar at the left shows the health of the system. This bar corresponds to the damage percentage shown on the Damage Report (F6) window.

The thick vertical bar at the right shows power level. The power level will match the health level unless the system is overclocked.

The gold line protruding from the bottom center is the power relay junction connecting the converter to a system, the Gravity Drive in this case.

Damaged Power Converter for Life Support
Low health bar at the left indicates a power converter that is heavily damaged.

The thick vertical bar at the right shows power level. Damaged power converters provide less power than undamaged power converters. This power converter has been overclocked to restore some of its lost performance. Refer to the Overclocking section for more details.


Disconnected Surgery UnitRed tint indicates the system equipment is not receiving power. Equipment does not function when it is not powered.

Absense of a thick vertical bar at the right shows power level at zero. This example is zero because no power converter is connected to it.


Life Support System with Multiple Connections
Multiple gold power relay junction lines show that the power converters for a turret and a surgery unit have been connected to the life support system to augment the output of the damaged life support power converter.

Even with additional power converters attached, the combined total power to the life support system is less than the maximum that it could be. Systems will suffer a reduction in overall performance that is proportional to the power supplied. A system that is powered at 50% will operate at 50% of its capability.

Navigation

Access to every system's power relay junction is possible, even though only eight junctions can be displayed at once.

The power converter displayed at any position is changed by clicking on the blue arrows on the system type image. Click on the right arrow to switch to the next system's power converter in the ship's internal list of systems. Click on the left arrow to switch to the previous system's power converter. In this manner, each power converter position on the panel can be "scrolled" to show a different power converter among those available.

The system equipment displayed at any position is changed by clicking on the blue arrows on the system type image. Click on the right arrow to switch to the next system's equipment in the ship's internal list of systems. Click on the left arrow to switch to the previous system's equipment. In this manner, each system equipment position on the panel can be "scrolled" to show a different system among those available.Note that the power converters shown at the top row do not have to correspond to the system equipment shown at the bottom row.
When one end of a power relay junction is not visible on the display, the gold junction line is drawn from the visible end to one side of the display screen.


Making Connections

Click on the connector at the power converter end of a gold junction line to disconnect the system. Power converters are shown across the top of the power relay section. 

The last power converter that is clicked becomes the current converter when making a junction to the next equipment connection that is clicked.


Click on the connector above a system equipment image to create a junction to the last converter that was clicked. System equipment is shown across the bottom of the power relay section.

Smaller power converters may not supply all of the power needed by equipment of larger systems. Multiple power converters can be connected to the same system equipment.


Overclocking

Overclocking is a way of restoring lost performance to a damaged system. The degraded performance level of a damaged system can be doubled by overclocking, up to its undamaged design performance. A system cannot be overclocked beyond its original designed performance.

Click on the power level bar next to a power converter to overclock it, resulting in a possible increase in performance of that system.



WARNING!
Overclocked systems will incur damage due to stress over time. This can result in catastrophic system failure and total destruction of the spacecraft.

Power Plant Shunts

The upper section of the power relay station displays power plant output and power plant shunts.



Power plant shunts appear in a row across the bottom. Eight different shunts enable risky overrides inside the power plant. Each shunt that is activated restores 1/8th of the power plant's undamaged output. Shunts will restore power plant output lost due to damage, up to 100%; they will not enable the power plant to exceed its original design capability.



WARNING!
Active shunts will damage the power plant over time. This can result in catastrophic failure of the power plant and total destruction of the 
spacecraft.

The output of the power plant is shown as a horizontal bar across the top. When no shunts are enabled, this bar corresponds to the power plant damage percentage shown on the Damage Report (F6) window. Active shunts will raise the output of the power plant beyond the health level of the power plant.

Wednesday, August 3, 2016

The Pole and throw of Relay

     Since relays are switches, the terminology applied to switches is also applied to relays; a relay switches one or more poles, each of whose contacts can bethrown by energizing the coil.
Normally open (NO) contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive. It is also called a "Form A" contact or "make" contact. NO contacts may also be distinguished as "early-make" or "NOEM", which means that the contacts close before the button or switch is fully engaged.
Normally closed (NC) contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive. It is also called a "Form B" contact or "break" contact. NC contacts may also be distinguished as "late-break" or "NCLB", which means that the contacts stay closed until the button or switch is fully disengaged.
Change-over (CO), or double-throw (DT), contacts control two circuits: one normally open contact and one normally closed contact with a common terminal. It is also called a "Form C" contact or "transfer" contact ("break before make"). If this type of contact has a "make before break" action, then it is called a "Form D" contact.
The following designations are commonly encountered:
SPST – Single Pole Single Throw. These have two terminals which can be connected or disconnected. Including two for the coil, such a relay has four terminals in total. It is ambiguous whether the pole is normally open or normally closed. The terminology "SPNO" and "SPNC" is sometimes used to resolve the ambiguity.
SPDT – Single Pole Double Throw. A common terminal connects to either of two others. Including two for the coil, such a relay has five terminals in total.
DPST – Double Pole Single Throw. These have two pairs of terminals. Equivalent to two SPST switches or relays actuated by a single coil. Including two for the coil, such a relay has six terminals in total. The poles may be Form A or Form B (or one of each).
DPDT – Double Pole Double Throw. These have two rows of change-over terminals. Equivalent to two SPDT switches or relays actuated by a single coil. Such a relay has eight terminals, including the coil.
The "S" or "D" may be replaced with a number, indicating multiple switches connected to a single actuator. For example, 4PDT indicates a four pole double throw relay that has 12 switch terminals.
EN 50005 are among applicable standards for relay terminal numbering; a typical EN 50005-compliant SPDT relay's terminals would be numbered 11, 12, 14, A1 and A2 for the C, NC, NO, and coil connections, respectively.
DIN 72552 defines contact numbers in relays for automotive use;
85 = relay coil -
86 = relay coil +
87 = common contact
87a = normally closed contact
87b = normally open contact

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