Posts Tagged ‘pushbutton’

Transistors – Digital Control Interface, Part V

Sunday, July 15th, 2012
     ­­­­­Last time we looked at my electric relay solution to a problem presented by a 120 volt alternating current (VAC) drive motor operating within an x-ray film processing machine.  Now let’s see what happens when we press the button to set the microprocessor into operation. 

 electronic control

Figure 1


     Figure 1 shows that when the button is depressed, the computer program contained within the microprocessor chip goes into action, signaling the start of the control initiative.  5 volts direct current (VDC) is supplied to Output Lead 2, and FET 2 (Field Effect Transistor 2) becomes activated, which allows electric current from the 12 VDC supply to course into the 12 VDC electric relay, through the relay’s wire coil, then conclude its travel into electrical ground.

     The electric relay components, including a wire coil, steel armature, spring, and normally open (N.O.) contact, are shown within a blue box in our illustration.  Current flow is represented by red lines.  The control initiative passes from the microprocessor to FET 2, and then to the 12 VDC electric relay, just as the Olympic Torch is relayed through a system of runners.

     We learned in one of my previous articles on industrial control that when an electric relay coil is energized, electromagnetic attraction pulls its steel armature towards the wire coil and the N.O. electrical contact.  In Figure 1 this attraction is represented by a blue arrow.  With the N.O. contact closed the drive motor is connected to the 120 VAC input, and the motor is activated.

microprocessor control

Figure 2


     Figure 2 shows what happens after the button is depressed.  The computer program is activated, directing the microprocessor chip to keep 5 VDC on Output Lead 2 and FET 2 while the prerequisite 40 minutes elapses.  Thus the relay remains energized and the motor remains on during this time.


Figure 3


     In Figure 3, at the end of the 40 minute countdown, the computer program applies 0 VDC to Output Lead 2.  FET 2 then turns off the current flow to the relay and it begins to de-energize, causing the spring to pull the steel armature away from the N.O. contact and the 120 VAC power supply to be interrupted.  The motor is deactivated.

     At the same time, the computer program applies 5 VDC to Output Lead 1 and FET 1 for 2 seconds.  FET 1 turns on the flow of current through the buzzer, causing it to sound off and signal that the x-ray film processing machine is ready for use.

     Next time we’ll look at how transistors are used to regulate voltage within direct current power supplies like the one shown in Figure 3 above.


Transistors – Digital Control Interface, Part IV

Monday, July 9th, 2012
     The Olympic Torch relay, soon to culminate in London, is the grand daddy of all relays, starting in one country, traversing many others, then ending its journey at the site of the Olympic Games.  It passes through many athletes’ hands while on its journey, its final purpose being to light the Olympic Flame.  Less glamorous, though still useful, is the relay race that often takes place within digital controls.

      Last time we looked at my design solution for the control of a microprocessor controlled medical x-ray film developing machine, where a field effect transistor (FET) acted as a digital control interface between a 5 volt direct current (VDC) microprocessor and a 12 VDC buzzer.  Well, controlling the buzzer wasn’t the only function of the microprocessor.  It also had to control a 120 volt alternating current (VAC) drive motor, the purpose of which was to move x-ray film through a series of processes within the machine.  Yet another requirement was that the machine’s drive motor run 40 minutes upon activation by a start button, then shut off.

     One of the challenges presented by this specification was that an FET standing alone is only suited to control direct current devices like the buzzer, but not alternating current devices like electric motors.  Direct current flows in one direction only when traveling through wire, and since an FET can only pass current in one direction it is the perfect match for those applications.  

     Now, as the name would imply, alternating current flow alternates, that is, it reverses direction and varies in intensity many times each second.  This is a scenario that FETs are not equipped to handle because they can’t deal with reverse flow.  This meant that, for the purpose of my developing machine, I could not use an FET to directly control the 120 VAC motor.  Now let’s take a look at Figure 1 to get a basic look at how I solved the problem.

microprocessor electric relay control

Figure 1 


     Figure 1 shows not one, but two green FET’s, each branching off from the microprocessor chip.  We’ll call them FET 1 and FET 2.  If you recall from last time, the buzzer works on 12 VDC, so FET 1 works well as a direct interface between it and the microprocessor chip.  But in the case of FET 2 we need an intermediary device to handle the alternating current motor.  That device is a 12 VDC electric relay.

     In an earlier blog series on industrial controls I discussed how electric relays use electromagnets to power light bulbs and motors on and off in response to someone pressing a button on a control panel.  We have very much the same mechanics at play in our developing machine.  The relay will power the motor on and off in response to the computer program running within the 5 VDC microprocessor, a programming sequence that is initiated by someone pressing a button. 

     To get the motor control to work in the machine, the gate (G) of FET 2 is connected to another output lead on the microprocessor.  We’ll call that Output Lead 2.  Then, the source (S) of FET 2 is connected to the wire coil in the relay.  To tap into the power source for the relay, the drain (D) of FET 2 is connected to the 12 VDC supply.   Finally, the other end of the relay coil is connected to electrical ground.

     Next time we’ll activate the pushbutton and see how the control initiative passes along a path in a manner reminiscent of the flame in an Olympic Torch relay, but here it passes between the microprocessor, the FET and electrical relay, culminating in power to the drive motor.


Transistors – Digital Control Interface, Part III

Sunday, July 1st, 2012
     When I was in engineering school in the mid 1970s microprocessor chips were still a fairly new concept.  Scientific calculators were the size of a brick back then, and they weighed almost as much, and there were no personal computers.

     I remember doing homework on the UNIVAC 1108 mainframe computer at school.  To program it I had to sit at a monster of a keypunching machine for which I punched an endless array of holes into paper cards.  These holes acted as the programming logic to instruct the computer what functions to perform.  The 1108 computer’s mainframe was so huge it was housed in an adjoining room the size of a house.  Since the 1980s advances in microprocessor technology have increased computing power and dramatically reduced the size of components, making things like laptops, smart phones, and sophisticated electronic products possible.

     Last time we began looking at my design solution for the control of a machine which developed medical x-ray film and made use of a microprocessor chip to automate its operation.  A field effect transistor (FET) acts as a digital control interface between its 5 volt direct current (VDC) microprocessor and a 12 VDC buzzer.  Figure 1 shows what happens when someone presses the button to put everything into action and the microprocessor starts timing. 

 microprocessor control using a MOSFET

Figure 1


     With the button depressed the chip senses 5 VDC from the power supply on its input lead.  This in turn signals the computer program to turn the product on.  The program then begins counting down the minutes, all the while maintaining a 0 voltage output from the chip’s output lead.  With no voltage present on its G lead, the FET does not permit electrical current to flow from the 12 VDC supply, through the buzzer, through D and S, and down to electrical ground.  The buzzer remains silent.

Field Effect Transistor

Figure 2


     Figure 2 shows what happens when the program begins its 40-minute warming sequence.   The chip raises the output lead voltage to 5 VDC and applies it to G, then the FET permits electric current to flow through it to ground from the 12 VDC supply and the buzzer.  Now supplied with power, the buzzer sounds.  Then, per programming instructions, after 2 seconds the program shuts off the voltage in the chip’s output lead, current is cut off, and the buzzer goes silent.

     Next time we’ll see how an FET can be used as an interface between a microprocessor and another higher powered device, that of a 120 VAC motor that’s used to move x-ray film through a series of processes within the developer.


Transistors – Digital Control Interface, Part II

Sunday, June 24th, 2012
     Not too long ago I was retained as an engineering expert to testify on behalf of a plaintiff who owned a sports bar.  The place was filled with flat screen televisions that were plugged into 120 volt alternating current (VAC) wall outlets.  To make a long story short, the electric utility wires that fed power to the bar were hit by a passing vehicle, causing the voltage in the outlets to increase well beyond what the electronics in the televisions could handle.  The delicate electronics were not suited to be connected with the high voltage that suddenly surged through them as a result of the hit, and they overloaded and failed.

     Similarly, lower voltage microprocessor and digital logic chips are also not suited to directly connect with higher voltage devices like motors, electrical relays, and light bulbs.  An interface between the two is needed to keep the delicate electronic circuits in the chips from overloading and failing like the ill fated televisions in my client’s sports bar.  Let’s look now at how a field effect transistor (FET) acts as the interface between low and high voltages when put into operation within an industrial product.

     I was once asked to design an industrial product, a machine which developed medical x-ray films, utilizing a microprocessor chip to automate its operation.  The design requirements stated that the product be powered by a 120 VAC, such as that available through the nearest wall outlet.  In terms of functionality, upon startup the microprocessor chip was to be programmed to first perform a 40-minute warmup of the machine, then activate a 12 volt direct current (VDC) buzzer for two seconds, signaling that it was ready for use.  This sequence was to be initiated by a human operator depressing an activation button.

     The problem presented by this scenario was that the microprocessor chip manufacturer designed it to operate on a mere 5 VDC.  In additional, it was equipped with a digital output lead that was limited in functionality to either “on” or “off” and capable of only supplying either extreme of 0 VDC or 5 VDC, not the 12 VDC required by the buzzer.

     Figure 1 illustrates my solution to this voltage problem, although the diagram shown presents a highly simplified version of the end solution.

microprocessor control

Figure 1

     The illustration shows the initial power supplied at the upper left to be 120 VAC.  This then is converted down to 5 VDC and 12 VDC respectively by a power supply circuit. The 5 VDC powers the microprocessor chip and the 12 VDC powers the buzzer.  The conversion from high 120 VAC voltage to low 5 and 12 VDC voltage is accomplished through the use of a transformer, a diode bridge, and special transistors that regulate voltage.  Since this article is about FETs, we’ll discuss transistor power supplies in more depth in a future article.

     To make things a little easier to follow, the diagram in Figure 1 shows the microprocessor chip with only one input lead and one output lead.  In actuality a microprocessor chip can have dozens of input and output leads, as was the case in my solution.  The input leads collect information from sensors, switches, and other electrical components for processing and decision making by the computer program contained within the chip.  Output leads then send out commands in the form of digital signals that are either 0 VDC or 5 VDC.  In other words, off or on.  The net result is that these signals are turned off or on by the program’s decision making process.

     Figure 1 shows the input lead is connected to a pushbutton activated by a human.  The output lead is connected to the gate (G) of the FET.  The FET is shown in symbolic form in green. The FET drain (D) lead is connected to the buzzer and its source (S) lead terminates in connection to electrical ground to complete the electrical circuit.  Remember, electric current naturally likes to flow from the supply source to electrical ground within circuits, and our scenario is no exception.

     Next time we’ll see what happens when someone presses the button to put everything into action.


Industrial Control Basics – Disconnect Switches

Sunday, March 25th, 2012
     Last week our kitchen ceiling fan and light combo decided to stop working.  We don’t like eating in the dark, so I was compelled to do some immediate troubleshooting.  As an engineer with training in the workings of electricity I have a great respect for it.  I’m well aware of potential hazards, and I took a necessary precaution before taking things apart and disconnecting wires.  I made the long haul down the stairs to the basement, opened the circuit breaker in the electrical panel, and disabled the flow of electricity to the kitchen.  My fears of potential electrocution having been eliminated, my only remaining fear was of tumbling off the ladder while servicing the fan.

     Just as I took the precaution to disconnect the power supply before performing electrical maintenance in my home, workers in industrial settings must do the same, and a chief player in those scenarios is the motor overload relay discussed last week.  It automatically shuts down electric motors when they become overheated.  Let’s revisit that example now.

Industrial Control System

Figure 1


     Our diagram in Figure 1 shows electric current flowing through the circuit by way of the red path.  Even if this line were shut down, current would continue to flow along the path, because there is no means to disconnect the entire control system from the hot and neutral lines supplying power to it, that is, it is missing disconnect switches.  Electric current will continue to pose a threat to workers were they to attempt a repair to the system.  Now let’s see how we can eliminate potential hazards on the line.

Disconnect Switches

Figure 2


     In Figure 2 there is an obvious absence of the color red, indicating the lack of current within the system.  We accomplished this with the addition of disconnect switches capable of isolating the motor control circuitry, thereby cutting off the hot and neutral lines of the electrical power supply and along with it the unencumbered flow of electricity.

    These switches are basically the same as those seen in earlier diagrams in our series on industrial controls, the difference here is that the two switches are tied together by an insulated mechanical link.  This link causes them to open and close at the same time.  The switches are opened and closed manually via a handle.  When the disconnect switches are both open electricity can’t flow and nothing can operate.  Under these conditions there is no risk of a worker coming along and accidentally starting the conveyor motor.

     To add yet another level of safety, disconnect switches are often tagged and locked once de-energized.  This prevents workers from mistakenly closing them and starting the conveyor while maintenance is being performed.  Brightly colored tags alert everyone that maintenance is taking place and the switches must not be closed.  The lock that performs this safety function is actually a padlock.  It’s inserted through a hole in the switch handle, making it impossible for anyone to flip the switch.  Tags and locks are usually placed on switches by maintenance personnel before repairs begin and are removed when work is completed.

     Now let’s see how our example control system looks in ladder diagram format.

Control System Ladder Diagram

Figure 3


     Figure 3 shows a ladder diagram that includes disconnect switches, an emergency stop button, and the motor overload relay contacts.  The insulated mechanical link between the two switches is represented by a dashed line.  Oddly enough, engineering convention has it that the motor overload relay heater is typically not shown in a ladder diagram, therefore it is not represented here.

     This wraps up our series on industrial control.  Next time we’ll begin a discussion on mechanical clutches and how they’re used to transmit power from gasoline engines to tools like chainsaws and grass trimmers.



Industrial Control Basics – Motor Overload Relay In Action

Sunday, March 18th, 2012

    Last week we explored the topic of thermal expansion, and we learned how the bimetal contacts in a motor overload relay distort when heated.  We also discussed how the overload relay comes into play to prevent overheating in electric motor circuits.  Now let’s see what happens when an overload situation occurs.

motor overload relay

Figure 1


     Figure 1 shows a motor becoming overloaded, as it draws in abnormally high amounts of electric current.  Since this current also flows through the electric heater in the overload relay, the heater starts producing more heat than it would if the motor were running normally.  This abnormally high heat is directed towards the bimetal switch contacts, causing them to curl up tightly until they no longer touch each other and open up.  They will only close again when the overload condition is cleared up and the heater cools back down to normal operating temperature.

     Let’s now take a look at Figure 2 to see how the motor overload relay fits into our example of a conveyor belt motor control circuit.  Once again, the path of electric current flow is denoted by red lines.

motor overload relay

Figure 2


     The circuit in Figure 2 represents what happens after Button 1 is depressed.  That is, the electric relay has become latched and current flows between hot and neutral sides through one of the N.O. contacts along the path of the green indicator bulb, the motor overload relay heater, and the conveyor belt motor.  The current also flows through the other N.O. contact, the Emergency Stop button, Button 2, the electric relay’s wire coil, and the motor overload relay bimetal contacts.  The motor becomes overloaded, causing the overload relay heater to produce abnormally high heat.  This heat is directed towards the bimetal contacts, also causing them to heat up.

industrial control

Figure 3


     In Figure 3 the bimetal contacts have heated to the point that they have curled away from each other until they no longer touch.  With the bimetal contacts open, electric current is unable to flow through to the electric relay’s wire coil.  This in turn ends the magnetic attraction which formerly held the relay armatures against the N.O. contacts.  The spring in the electric relay has pulled the armatures up, causing the N.O. contacts to open, simultaneously closing the N.C. contact. 

     These actions have resulted in a loss of current to the green indicator bulb and electric motor.  The red indicator bulb is now activated, and the conveyor motor is caused to automatically shut down to prevent damage and possible fire due to overheating.  This means that even if the conveyor operator were to immediately press Button 1 in an attempt to restart the line, he would be prevented from doing so.  Under these conditions the electric relay is prevented from latching, and the motor remains shut down because the bimetal contacts have been separated, preventing current from flowing through to the wire coil. 

     The bimetal contacts will remain open until they once again cool to normal operating temperature.  Once cooled, they will once again close, and the motor can be restarted.  If the cause of the motor overload is not diagnosed and its ability to recur eliminated, the automatic shutdown process will repeat this cycle. 

     Next time we’ll see how the overload relay is represented in a ladder diagram.  We’ll also see how switches can be added to the circuit to allow maintenance staff to safely work.



Industrial Control Basics – Motor Overload

Sunday, March 4th, 2012

     Last summer my wife and I did a lot of work in the garden.  Many holes were dug, bags of garden soil lifted, and plants planted.  It’s a new garden, and my wife has very big plans for it, so needless to say there was a lot of work to be done.  On more than one occasion we would end the day moaning about our body aches and how we had overdone it.  The next day we would hurt even worse, and we’d end up taking time off to recuperate.  Pain is your body’s way of telling you that it needs attention, and you’d better listen to it or you may have an even heavier price to pay down the road. 

      Electric motors can get overworked, just like our bodies.  Motors are often placed into situations where they are expected to perform tasks beyond their capability.  Sometimes this happens through poor planning, sometimes due to wishful thinking on the user’s part.  Motors can sustain damage when stressed in this way, but they don’t have a pain system to tell them to stop.  Instead, motors benefit by a specific type of electric relay known as an overload relay.  But before we get into how an overload relay works, let’s get a better understanding of how overloads happen.

     Suppose we’re back in the telephone factory discussed in previous blogs, watching a conveyor belt move phones through the manufacturing process.  An electric motor drives the conveyor belt by converting electrical energy into mechanical energy.  Everything is moving along normally when all of a sudden a machine malfunctions.  Telephones start piling up on a belt, and the pile up gets so bad the belt eventually gets jammed and its motor overloaded.  If the electricity flow to the motor isn’t shut down promptly by means of a nearby emergency stop button or an astute operator sitting in central control, then an even bigger problem is in the making, that of a potential fire. 

     When electricity is applied to motors they begin to operate, and their natural tendency is to want to keep operating.  They do so by continuously drawing energy from the electric current being supplied to them.  The greater the workload demand on the motor, the more current it requires to operate. 

     When motors become overloaded as in the scenario presented above, they continue to draw energy unless forced to a stop.  The result is an overabundance of current flowing through the motor and no outlet for its task of converting electrical energy into mechanical energy.  And where is all that pent up energy to go?  It becomes heat energy trapped inside the motor itself, and this heat can build up to the point where the motor becomes damaged or even bursts into flames.

     Next time we’ll look at how overload relays work to keep electric motors from overheating, just as our body’s pain sensors protect us from overdoing it. 



Industrial Control Basics – Emergency Stops

Sunday, February 26th, 2012
     Ever been in the basement when you heard a loud thud followed by a scream by a family member upstairs?  You run up the stairs to see what manner of calamity has happened, the climb seeming to take an eternity.  Imagine a similar scenario taking place in an industrial setting, where distances to be covered are potentially far greater and the dangerous scenarios numerous.

     Suppose an employee working near a conveyor system notices that a coworker’s gotten caught in the mechanism.  The conveyor has to be shut down fast, but the button to stop the line is located far away in the central control room.  This is when emergency stop buttons come to the rescue, like the colorful example shown in Figure 1.

emergency stop pushbutton

Figure 1


     Emergency stop buttons are mounted near potentially dangerous equipment in industrial settings, allowing workers in the area to quickly de-energize equipment should a dangerous situation arise.  These buttons are typically much larger than your standard operational button, and they tend to be very brightly colored, making them stick out like a sore thumb.  This type of notoriety is desirable when a high stress situation requiring immediate attention takes place.  They’re easy to spot, and their shape makes them easy to activate with the smack of a nearby hand, broom, or whatever else is convenient. 

     Figure 2 shows how an emergency stop button can be incorporated into a typical motor control circuit such as the one we’ve been working with in previous articles.

emergency stop button in motor control circuit

Figure 2


     An emergency stop button has been incorporated into the circuit in Figure 2.  It depicts what happens when someone depresses Button 1 on the conveyor control panel.  The N.C. contact opens, and the two N.O. contacts close.  The motor starts, and the lit green bulb indicates it is running.  The electric relay is latched because its wire coil remains energized through one N.O. contact.  It will only become unlatched when the flow of current is interrupted to the wire coil, as is outlined in the following paragraph.  The red lines denote areas with current flowing through them.

     Both Button 2 and the emergency stop button typically reside in normally closed positions.  As such electricity will flow through them on a continuous basis, so long as neither one of them is re-engaged.  If either of them becomes engaged, the same outcome will result, an interruption in current on the line.  The relay wire coil will then become de-energized and the N.O. contacts will stay open, preventing the wire coil from becoming energized again after Button 2 or the emergency stop are disengaged.  Under these conditions the conveyor motor stops, the green indicator bulb goes dark, the N.C. contact closes, and the red light comes on, indicating that the motor is not running.  This sequence, as it results from hitting the emergency stop button, is illustrated in Figure 3.

emergency stop button unlatches electric relay

Figure 3


     We now have the means to manually control the conveyor from a convenient, at-the-site-of-occurrence location, which allows for a quick shut down of operations should the need arise.

     So what if something else happens, like the conveyor motor overheats and catches on fire and no one is around to notice and hit the emergency stop?  Unfortunately, in our circuit as illustrated thus far the line will continue to operate and the motor will continue to run unless we incorporate an additional safeguard, the motor overload relay.  We’ll see how that’s done next time. 


Industrial Control Basics – Unlatching the Latching Circuit

Sunday, February 5th, 2012
     When I had the misfortune of getting stuck in my Uncle Jake’s outhouse as a kid, I would allow my hysteria to get the best of me and forget my uncle’s instructions on how to get out.  It was a series of raps and a single kick that would prove to be the magic formula, and once I had calmed myself down enough to employ them I would succeed in working the door’s rusty latch open.  Our relay circuit below has a much less challenging system to effectively unlatch the pattern of electric current.

      Figure 1 shows our latched circuit, where red lines denote the flow of current.

Latched Electric Relay Circuit

Figure 1


     If you recall, the relay in this circuit was latched by pressing Pushbutton 1.  When in the latched state, the magnetic attraction maintained by the wire coil and steel core won’t allow the relay armatures to release from their N.O. contacts.  The relay’s wire coil stays energized via Button 2, the red bulb goes dark while the green bulb remains lit, even though Button 1 is no longer actively depressed.

     Now let’s take a look at Figure 2 to see how to get the circuit back to its unlatched state.

Unlatching An Electric Relay

Figure 2


     With Button 2 depressed the flow of current is interrupted and the relay’s wire coil becomes de-energized.  In this state the coil and steel core are no longer magnetized, causing them to release their grip on the steel armatures.  The spring will now pull them back until one of them makes contact with the N.C. contact.  The red bulb lights again, although Button 2 is not being actively depressed.  At this point the electric relay has become unlatched.  It can be re-latched by depressing Button 1 again.

     Let’s see how we can simplify Figure 2’s representation with a ladder diagram, as shown in Figure 3.

Electric Relay Latching Circuit Ladder Diagram

Figure 3


        We’ve seen how this latching circuit activates and deactivates bulbs.  Next time we’ll see how it controls an electric motor and conveyor belt inside a factory.


Industrial Control Basics – Latching Circuit

Sunday, January 29th, 2012
     When I think of latches the first thing that comes to mind is my Uncle Jake’s outhouse and how I got stuck in it as a kid.  Its door was outfitted with a rusty old latch that had a nasty habit of locking up when someone entered, and it would take a tricky set of raps and bangs to loosen.  One day it was being particularly unresponsive to my repeated attempts to open it, and the scene became like something out of a horror movie.  There was a lot of screaming.

     When latches operate well, they’re indispensable.  Let’s take our example circuit from last time a bit further by adding more components and wires.  We’ll see how a latch can be applied to take the place of pressure exerted by an index finger.  See Figure 1.

Figure 1


     Our relay now contains an additional pivoting steel armature connected by a mechanical link to the original steel armature and spring.  The relay still has one N.C. contact, but it now has two N.O. contacts.  When the relay is in its normal state the spring holds both armatures away from the N.O. contacts so that no electric current will flow through them.  One armature touches the N.C. contact, and this is the point at which current will flow between hot and neutral sides, lighting the red bulb.  The parts of the circuit diagram with electric current flowing through them are denoted by red lines.

     Figure 1 reveals that there are now two pushbuttons instead of one.  Now let’s go to Figure 2 to see what happens when someone presses on Button 1.

Figure 2


     Again, the parts of the circuit diagram with current flowing through them are denoted by red lines.  From this diagram you can see that when Button 1 is depressed, current flows through the wire coil, making it and its steel core magnetic.  This electromagnet in turn attracts both steel armatures in our relay, causing them to pivot and touch their respective N.O. contacts.  Electric current now flows between hot and neutral sides, lighting up the green bulb.  Current no longer flows through the N.C. contact and the red bulb, making it go dark.

     If you look closely at Figure 2, you’ll notice that current can flow to the wire coil along two paths, either that of Button 1 or Button 2.  It will also flow through both N.O. contact points, as well as the additional armature.

     So how is this scenario different from last week’s blog discussion?  That becomes evident in Figure 3, when Button 1 is no longer depressed.

Figure 3


     In Figure 3 Button 1 is not depressed, and electric current does not flow through it.  The red bulb remains dark, and the green bulb lit.  How can this state exist without the human intervention of a finger depressing the button?  Because although one path for current flow was broken by releasing Button 1, the other path through Button 2 remains intact, allowing current to continue to flow through the wire coil.

     This situation exists because Button 2’s path  is “latched.”  Latching results in the relay’s wire coil keeping itself energized by maintaining armature contact at the N.O. contact points, even after Button 1 is released.  When in the latched state, the magnetic attraction maintained by the wire coil and steel core won’t allow the armature to release from the N.O. contacts.  This keeps current flowing through the wire coil and on to the green bulb.  Under these conditions the relay will remain latched.  But, just like my Uncle’s outhouse door, the relay can be unlatched if you know the trick to it. 

     Relays may be latched or unlatched, and next week we’ll see how Button 2 comes into play to create an unlatched condition in which the green bulb is dark and the red bulb lit.  We’ll also see how it is all represented in a ladder diagram.