Posts Tagged ‘output lead’

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.