| 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.
Figure 1Figure 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.
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. ____________________________________________ |
Posts Tagged ‘computer program’
Transistors – Digital Control Interface, Part I
Monday, June 18th, 2012| In the navy, the captain is the brains behind a ship’s operations. He gathers information, makes important decisions, then issues orders. He’s not there to roll up his sleeves and swab the decks. The captain relies on the ship’s officers to act as an interface between himself and the sailors that perform the physical labor required on deck.
In this article we’ll see how the FET, that is, the field effect transistor, performs much the same role as the ship’s officers when it is used within electronic controls. There it acts as an interface between electronic components that issue commands and the electrical devices that carry them out. Last week we became familiar with field effect transistors and how their control of electrical current flow is analogous to how a faucet controls the flow of water. Although FETs can be used to vary the flow of current, they’re usually employed to perform a much simpler task, that of simply turning flow on or off, with no in-between modality. Like the captain of a ship, microprocessor and logic chips are the brains behind the operation in all sorts of industrial and consumer electronics. Figure 1 shows a few of them. Figure 1
The chips, which operate on low voltage, contain entire computer programs within them that gather information, make decisions, then instruct the higher voltage devices like motors, electrical relays, light bulbs, and audible alarms to follow. By “information,” I mean data signals received by the chip from its input connections to sensors, buttons, and other electrical components. This data informs the chip’s computer program of important operational information, like whether buttons have been pressed, switches are activated, and temperatures are normal. Based on this data, “decisions” are made by the chip using the logic contained within its program, then, depending on the decisions made, “commands” are issued by the chip. The commands, in the form of electrical output signals, are put into action by the work horses, the higher voltage devices. They, like a ship’s sailors, perform the actual physical work. There is one problem presented by this scenario, however. The electric output signals from the lower voltage chips are not suited to directly control the higher voltage devices because the signal voltage put out by the chips is too low. Even if the chip was designed to work at a higher voltage, the high level of current drawn by the motors, relays, and bulbs would lead to damage of the delicate circuitry within the chip. The chips must therefore rely on the FET to act as a digital control interface between them and the higher voltage devices, much as the ship’s captain depends on his subordinates to carry out his orders. Next week we’ll look at a real life example of how a digital interface is put into operation within an industrial product.
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