| 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.
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.
Posts Tagged ‘switch’
| I’ve always considered science to be cool. Back in the 5th grade I remember fondly leafing through my science textbook, eagerly anticipating our class performing the experiments, but we never did. For some reason my teacher never took the time to demonstrate any. Undeterred, I proceeded on my own.
I remember one experiment particularly well where I took a big steel nail and coiled wire around it. When I hooked a battery up to the wires, as shown in Figure 1 below, electric current flowed from the battery through the wire coil. This set up a magnetic field in the steel nail, thereby creating an electromagnet. My electromagnet was strong enough to pick up paper clips, and I took great pleasure in repeatedly picking them up, then watching them unattach and fall quickly away when the wires were disconnected from the battery.
Little did I know then that the electromagnet I had created was similar to an important part found within electrical relays used in many industrial control systems. An example of one of these relays is shown in Figure 2.
So, what’s in the little plastic cube? Well, a relay is basically an electric switch, similar to the ones we’ve discussed in the past few weeks, the major difference being that it is not operated directly by human hands. Rather, it’s operated by an electromagnet. Let’s see how this works by examining a basic electrical relay, as shown in Figure 3.
The diagram in Figure 3 shows a basic electric relay constructed of a steel core with a wire coil wrapped around it, similar to the electromagnet I constructed in my 5th grade experiment. If the coil’s wires are not hooked up to a power source, a battery for example, no electric current will flow through it. When there is no current the coil and steel core are not magnetic. For purposes of our illustration and in accordance with industrial control parlance, this is said to be this relay’s “normal state.”
Next to the steel core there is a movable steel armature, a kind of lever, which is attached to a spring. On one end of the armature is a pivot point, on the other end is a set of electrical switch contacts. When the relay is in its normal state, the spring’s tension holds the armature against the “normally closed,” or N.C., contact. If electric current is applied to the wire leading to the pivot point on the armature while in this state, it will be caused to flow on a continuous path through the armature and the N.C. contact, then out through the wire leading from the N.C. contact. In our illustration, since the armature does not touch the N.O. contact, an air gap is created that prevents electric current from traveling through the contact from the armature.
Next week we’ll see how these parts come into play within a relay when electric current flows through the coil, turning it into an electromagnet.