Transistors – Digital Control Interface, Part I

     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.

Digital Chips

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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