We’ve been discussing the Zener diode voltage regulator circuit, its advantages and disadvantages. We learned that the limiting resistor, RLimiting, creates a major disadvantage in the operation of the circuit, effectively acting as a roadblock to restrict current flow. Let’s see how to improve on that.
Figure 1 illustrates a transistor series voltage regulator circuit.
In this circuit the transistor is known as a bipolar transistor. Like the FET we discussed earlier, it has three electrical connections, however on the bipolar transistor the connections are referred to as the collector, base, and emitter. These are labeled C, B, and E in Figure 1.
The bipolar transistor acts as a valve, resting within the main path of current flow. That is, it controls the flow of electric current traveling from the collector to the emitter, as well as the voltage available at the emitter. The transistor is designed so that current flows in one direction only, from collector to emitter. We’ll talk more about that in our next article.
The limiting resistor, RLimiting, is located on a branch of the circuit leading to the Zener diode and the transistor base. Next time we’ll connect an unregulated power supply and external supply circuit to our transistor series voltage regulator. This will enable us to see how placing RLimiting on the branch, rather than along the main current path, results in a major advantage over using the Zener diode voltage regulator alone.
Posts Tagged ‘field effect transistor’
| I joined the Boy Scouts of America as a high schooler, mainly so I could participate in their Explorer Scout program and learn about electronics. I will forever be grateful to the Western Electric engineers who volunteered their personal time to stay after work and help me and my fellow Scouts build electronic projects. The neatest part of the whole experience was when I built my first regulated power supply with their assistance inside their lab. But in order to appreciate the beauty of a regulated power supply we must first understand the shortcomings of an unregulated one, which we’ll begin to do here.
Last time we began to discuss how the output voltage of an unregulated power supply can vary in response to power demand, just as when sprinklers don’t have sufficient water flow to cover a section of lawn. Let’s explore this concept further.
Figure 1 shows a very basic representation of a microprocessor control system that operates three components, an electric relay (shown in the blue box), buzzer, and light. These three components have a certain degree of internal electrical resistance, annotated as RR, RB, and RL respectively. This is because they are made of materials with inherent imperfections which tend to resist the flow of electric current. Imperfections such as these are unavoidable in any electronic device made by humans, due to impurities within metals and irregularities in molecular structure. When the three components are activated by the microprocessor chip via field effect transistors, denoted as FET 1, 2 and 3 in the diagram, their resistances are connected to the supply circuit.
In other words, RR, RB, and RL create a combined level of resistance in the supply circuit by their connectivity to it. If a single component were to be removed from the circuit, its internal resistance would also be removed, resulting in a commensurate decrease in total resistance. The greater the total resistance, the more restriction there is to current flow, denoted as I. The greater the resistance, the more I is caused to decrease. In contrast, if there is less total resistance, I increases.
The result of changing current flow resistance is that it causes the unregulated power supply output voltage to change. This is all due to an interesting phenomenon known as Ohm’s Law, represented as this within engineering circles:
V = I × R
where, V is the voltage supplied to a circuit, I is the electrical current flowing through the circuit, and R is the total electrical resistance of the circuit. So, according to Ohm’s Law, when I and R change, then V changes.
Next time we’ll apply Ohm’s Law to a simplified unregulated power supply circuit schematic. In so doing we’ll discover the mathematical explanation to the change in current flow and accompanying change in power supply output voltage we’ve been discussing.
| 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.