Posts Tagged ‘electronic devices’
Transistors – Voltage Regulation Part XV
Sunday, October 28th, 2012
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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Further Inside the Wall Wart
Sunday, September 11th, 2011| What do wall warts, aka AC wall adapters, and microwave ovens have in common? Well, in previous blogs discussing microwaves, we saw how a microwave oven’s high voltage circuitry uses a transformer, diode, and capacitor to effectively convert AC voltage into DC voltage. Wall warts do much the same thing and in very much the same way.
If you will recall from our discussion of microwave ovens in the past few weeks, the transformer in a high voltage circuit transforms 120 volts into a much higher voltage, say 4000 volts, in order to make things work. The diode and capacitor within both the microwave and the wall wart are key to facilitating this magical act, but in the wall wart it happens at a much lower voltage, about 12 volts. Last week we began exploring the inner workings of the wall wart. We discovered how its transformer converts the 120 volts emanating from your average wall outlet to the 12 volts required by most electronic devices. These voltages are shown at Points A and B in Figure 1 below. The fact that the voltage being put out results in waves of energy which alternate between a positive maximum value, zero, and a negative maximum value, makes it an unacceptable power source for most electronic devices. They require voltage that doesn’t alternate, and this is where the wall wart’s diode bridge and capacitor come into play.
Figure 1 – The Workings of the Wall Wart Transformer The wall wart’s diode bridge consists of four electronic components, namely the diodes, which are connected together. This diode bridge goes a bit further than the single diode present in a microwave oven, because it doesn’t merely eliminate negative aspects of alternating voltage. It actually transforms negative voltage into positive voltage. The result is a series of 12 volt peaks as shown at Point C of Figure 1. In fact, we end up with twice as many voltage peaks, and this is important, as you’ll see below. We still have the problem of zero voltage gaps to address. You see, over time the voltage at Point C of Figure 1 keeps changing between 0 volts and positive 12 volts. This can lead to problems, because many electronic devices require a consistent voltage of greater than zero to operate properly. For example, a light emitting diode (LED) might develop an annoying flicker, or you might end up hearing an irritating hum while listening to the radio. These annoyances are virtually eliminated by feeding voltage from the diode bridge into the capacitor, which gets rid of the zero voltage gaps between the voltage peaks. Like a microwave’s capacitor, the one within a wall wart charges up with electrical energy as the voltage from the diode bridge nears the top of a peak. Then, as voltage begins its dive back to a zero value, the capacitor discharges its electrical energy to fill in the gaps between peaks. The result is the rippled voltage pattern at Point D of Figure 1. With the gaps filled in, the voltage is at, or close enough to, the 12 volts required to keep an electronic device operating properly when it is connected to the wall wart’s low voltage power cord. Well, that’s it for our look at the wall warts that power our myriad of electronic devices. Next time we’ll switch to a totally topic and look at some of the basics of food manufacturing equipment design. ____________________________________________ |
