| Back in the early 1970s my dad, a notorious tightwad, coughed up several hundred dollars to buy his first portable color television. That was a small fortune back then. The TV was massive, standing at 24 inches wide, 18 inches high, and 24 inches deep, and weighing in at about 50 pounds. I think the only thing that made this behemoth “portable” was the fact that it had a carrying handle on top.
A major reason for our old TV being so big and clunky was of course due to limitations in technology of the time. Many large, heavy, and expensive electronic components were needed to make it work, requiring a lot of space for the circuitry. By comparison, modern flat screen televisions and other electronic devices are small and compact because advances in technology enable them to work with far fewer electronic components. These components are also smaller, lighter, and cheaper. Last time we looked at the components of a simple unregulated power supply to see how it converts 120 volts alternating current (VAC) to 12 volts direct current (VDC). We discovered that the output voltage of the supply is totally dependent on the design of the transformer, because the transformer in our example can only produce one voltage, 12 VDC. This of course limits the supply’s usefulness in that it is unable to power multiple electronic devices requiring two or more voltages, such as we’ll be discussing a bit further down. Now let’s illustrate this power supply limitation by revisiting our microprocessor control circuit example which we introduced in a previous article in this series on transistors.
Figure 1
In Figure 1 we have to decide what kind of power to supply to the circuit, but we have a problem. Sure, the unregulated power supply that we just discussed is up to the task of providing the 12 VDC needed to supply power for the buzzer, light, and electric relay. But let’s not forget about powering the microprocessor chip. It needs only 5 VDC to operate and will get damaged and malfunction on the higher 12 VDC the current power supply provides. Our power supply just isn’t equipped to provide the two voltages required by the circuit. We could try and get around this problem by adding a second unregulated power supply with a transformer designed to convert 120 VAC to 5 VAC. But, reminiscent of the circuitry in my dad’s clunky old portable color TV, the second power supply would require substantially more space in order to accommodate an additional transformer, diode bridge, and capacitor. Another thing to consider is that transformers aren’t cheap, and they tend to have some heft to them due to their iron cores, so more cost and weight would be added to the circuit as well. For these reasons the use of a second power supply is a poor option. Next time we’ll look at how adding a transistor voltage regulator circuit to the supply results in cost, size, and weight savings. It also results in a more flexible and dependable output voltage. ____________________________________________ |
Posts Tagged ‘5 VDC’
Transistors – Voltage Regulation Part VIII
Sunday, September 9th, 2012
Transistors – Digital Control Interface, Part III
Sunday, July 1st, 2012| When I was in engineering school in the mid 1970s microprocessor chips were still a fairly new concept. Scientific calculators were the size of a brick back then, and they weighed almost as much, and there were no personal computers.
I remember doing homework on the UNIVAC 1108 mainframe computer at school. To program it I had to sit at a monster of a keypunching machine for which I punched an endless array of holes into paper cards. These holes acted as the programming logic to instruct the computer what functions to perform. The 1108 computer’s mainframe was so huge it was housed in an adjoining room the size of a house. Since the 1980s advances in microprocessor technology have increased computing power and dramatically reduced the size of components, making things like laptops, smart phones, and sophisticated electronic products possible. Last time we began looking at my design solution for the control of a machine which developed medical x-ray film and made use of a microprocessor chip to automate its operation. A field effect transistor (FET) acts as a digital control interface between its 5 volt direct current (VDC) microprocessor and a 12 VDC buzzer. Figure 1 shows what happens when someone presses the button to put everything into action and the microprocessor starts timing. Figure 1
With the button depressed the chip senses 5 VDC from the power supply on its input lead. This in turn signals the computer program to turn the product on. The program then begins counting down the minutes, all the while maintaining a 0 voltage output from the chip’s output lead. With no voltage present on its G lead, the FET does not permit electrical current to flow from the 12 VDC supply, through the buzzer, through D and S, and down to electrical ground. The buzzer remains silent.
Figure 2Figure 2 shows what happens when the program begins its 40-minute warming sequence. The chip raises the output lead voltage to 5 VDC and applies it to G, then the FET permits electric current to flow through it to ground from the 12 VDC supply and the buzzer. Now supplied with power, the buzzer sounds. Then, per programming instructions, after 2 seconds the program shuts off the voltage in the chip’s output lead, current is cut off, and the buzzer goes silent. Next time we’ll see how an FET can be used as an interface between a microprocessor and another higher powered device, that of a 120 VAC motor that’s used to move x-ray film through a series of processes within the developer.
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Transistors – Digital Control Interface, Part II
Sunday, June 24th, 2012| Not too long ago I was retained as an engineering expert to testify on behalf of a plaintiff who owned a sports bar. The place was filled with flat screen televisions that were plugged into 120 volt alternating current (VAC) wall outlets. To make a long story short, the electric utility wires that fed power to the bar were hit by a passing vehicle, causing the voltage in the outlets to increase well beyond what the electronics in the televisions could handle. The delicate electronics were not suited to be connected with the high voltage that suddenly surged through them as a result of the hit, and they overloaded and failed.
Similarly, lower voltage microprocessor and digital logic chips are also not suited to directly connect with higher voltage devices like motors, electrical relays, and light bulbs. An interface between the two is needed to keep the delicate electronic circuits in the chips from overloading and failing like the ill fated televisions in my client’s sports bar. Let’s look now at how a field effect transistor (FET) acts as the interface between low and high voltages when put into operation within an industrial product. I was once asked to design an industrial product, a machine which developed medical x-ray films, utilizing a microprocessor chip to automate its operation. The design requirements stated that the product be powered by a 120 VAC, such as that available through the nearest wall outlet. In terms of functionality, upon startup the microprocessor chip was to be programmed to first perform a 40-minute warmup of the machine, then activate a 12 volt direct current (VDC) buzzer for two seconds, signaling that it was ready for use. This sequence was to be initiated by a human operator depressing an activation button. The problem presented by this scenario was that the microprocessor chip manufacturer designed it to operate on a mere 5 VDC. In additional, it was equipped with a digital output lead that was limited in functionality to either “on” or “off” and capable of only supplying either extreme of 0 VDC or 5 VDC, not the 12 VDC required by the buzzer. Figure 1 illustrates my solution to this voltage problem, although the diagram shown presents a highly simplified version of the end solution.
Figure 1The illustration shows the initial power supplied at the upper left to be 120 VAC. This then is converted down to 5 VDC and 12 VDC respectively by a power supply circuit. The 5 VDC powers the microprocessor chip and the 12 VDC powers the buzzer. The conversion from high 120 VAC voltage to low 5 and 12 VDC voltage is accomplished through the use of a transformer, a diode bridge, and special transistors that regulate voltage. Since this article is about FETs, we’ll discuss transistor power supplies in more depth in a future article. To make things a little easier to follow, the diagram in Figure 1 shows the microprocessor chip with only one input lead and one output lead. In actuality a microprocessor chip can have dozens of input and output leads, as was the case in my solution. The input leads collect information from sensors, switches, and other electrical components for processing and decision making by the computer program contained within the chip. Output leads then send out commands in the form of digital signals that are either 0 VDC or 5 VDC. In other words, off or on. The net result is that these signals are turned off or on by the program’s decision making process. Figure 1 shows the input lead is connected to a pushbutton activated by a human. The output lead is connected to the gate (G) of the FET. The FET is shown in symbolic form in green. The FET drain (D) lead is connected to the buzzer and its source (S) lead terminates in connection to electrical ground to complete the electrical circuit. Remember, electric current naturally likes to flow from the supply source to electrical ground within circuits, and our scenario is no exception. Next time we’ll see what happens when someone presses the button to put everything into action.
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