| Back in the 60s my dad spent about $25 to buy a small transistor radio. That was a lot of money in those days, but well worth it. The new transistor technology allowed for a much less cumbersome radio to be produced. No more lugging around big radios armed with heavy vacuum tubes. In the years that followed the word transistor became a household word. They were employed in a variety of ways within televisions and other electronic devices, increasing both their reliability and functionality.
So what is a transistor and what does it do? It’s an electronic component, developed in the late 1940s. The first transistor was about as big as a softball and crudely made. As such, it was too impractical for commercial use. Then in the l950s technological advancements made commercial production of smaller, high-quality transistors possible. Transistors enjoyed widespread introduction to the consuming mainstream in the l960s, and since then they’ve been made in many different types, shapes, and sizes. Some are shown in Figure 1 below. Figure 1
A commonly used type of transistor is called a field effect transistor, or FET, one of which is shown in Figure 2. The FET has three metal leads which allow it to be connected into electrical circuits. These leads are referred to as the drain (D), the source (S), and the gate (G).
Figure 2
FET’s control the flow of current within an electronic circuit. A good way to understand what they do is to consider the analogy of water flowing through a faucet.
Figure 3
Figure 3 shows a faucet, complete with valve and handle. With the valve closed the flow of water is completely shut off. If the valve is opened partway by rotation of the handle, a trickle of water emerges. The more the handle is turned and valve is opened, the greater the flow of water. The FET shown in Figure 4 operates a lot like a faucet, but with regard to electrical current. Figure 4
The FET controls the flow of current flowing through its D and S leads, but it does not employ a valve or handle to do it. Rather, flow rate is controlled by application of a small amount of voltage to the G lead. The voltage’s influence on the G lead influences the FET to permit current to flow in through the D lead, then out through the S lead. The amount of voltage applied to the G lead is directly related to how much current will be allowed to flow. In this example the D lead on the FET is connected to a 10 volt direct current (VDC) power supply. The S lead is connected to a flashlight bulb which is connected to electrical ground. If you will remember from previous blogs, electric current naturally wants to flow from the supply source to ground, much like water wants to naturally flow downhill. If the bulb was connected directly to the 10 VDC power supply, current would flow through unimpeded and the bulb would light. However, in Figure 4 the FET acts as a regulating device. It’s connected between the 10 VDC power supply and the bulb. When no voltage is applied to the G lead the FET acts like a closed valve and current is unable to flow. Without current we, of course, have no light. When a low amount of voltage, say one volt, is applied to the G lead, the FET acts like a partially opened valve. It permits a trickle of current to flow from the 10 VDC supply to the bulb, and the bulb glows dimly. As voltage to G increases the FET valve opens further, permitting more current to flow. The bulb glows with increasing brightness. When the voltage applied to G increases to the point the FET valve is opened fully, in our example that is 2 volts, full current is allowed to flow from the 10 VDC supply to the bulb. The bulb glows brightly. Generally speaking, the voltage required to be applied to G for control of current flow through an FET depends on overall design and the particular application within an electrical circuit. FETs are often used within electronic devices to turn things on and off, with no other function in between. Next time we’ll look at some example circuits to see how it’s done. ____________________________________________ |
Posts Tagged ‘ground’
Industrial Control Basics – Ladder Diagrams
Sunday, December 18th, 2011| The other day I pressed the button to activate my electric garage door opener and nothing happened. I pushed again and again, still nothing. Finally, I convinced myself to get out of the car and take a closer look. A wooden board I had propped up against the side of the garage wall had come loose, wedging itself in front of the electric eye, you know, the one that acts as a safety. The board was an obstruction to the clear vision of the eye. It couldn’t see the light emitter on the other side of the door opening and wouldn’t permit the door opener to function.
The basic manual control system we looked at last week operates similarly to the eye on a garage door opener. If you can’t “close the loop,” you won’t get the power. Last week’s example was as basic as things get. Now let’s look at something a bit more complex. Words aren’t always the best vehicle to facilitate understanding, which is why I often use visual aids in my work. In the field of industrial control systems diagrams are often used to illustrate things. Whether it’s by putting pencil to paper or the flow diagram of software logic, illustrations make things easier to interpret. Diagrams such as the one in Figure l are often referred to as “ladder diagrams,” and in a minute we’ll see why.
Figure 1 Figure 1(a) shows a basic manual control system. It consists of wires that connect a power switch and electric motor to a 120 volt alternating current power source. One wire is “hot,” the other “neutral.” The hot side is ungrounded, meaning that it isn’t electrically connected to the Earth. The neutral side is grounded, that’s right, it’s driven into the ground and its energy is dissipated right into the earth, then returned back to the power grid. In Figure 1(a) we see that the power switch is open and an air gap exists. When gaps exist, we don’t have a closed electrical loop, and electricity will not flow. Figure 1(b), our ladder diagram, aka line diagram, shows an easier, more simplified representation of the manual control shown in Figure 1(a). It’s easier to decipher because there’s less going on visually for the brain to interpret. Everything has been reduced to simple lines and symbols. For example, the electric motor is represented by a symbol consisting of a circle with an “M” in it. Now, let’s turn our attention to Figure 2 below to see what happens when the power switch is closed.
Figure 2 The power switch in Figure 2(a) is closed, allowing electric current to flow between hot and neutral wires, then power switch, and finally to the motor. The current flow makes the motor come to life and the motor shaft begins to turn. The line diagram for this circuit is shown in Figure 2(b). You might have noticed that the line diagrams show in Figures 1(b) and 2(b) have a rather peculiar shape. The vertically running lines at either side depict the hot and neutral legs of the system. If you stretch your imagination a bit, they look like the legs of a ladder. Between them run the wires, power switch, and motor, and this horizontal running line represents the rung of the ladder. More complicated line diagrams can have hundreds, or even thousands of rungs, making up one humongous ladder, hence they are commonly referred to as ladder diagrams. Next week we’ll take a look at two key elements in automatic control systems, the push button and electric relay, elements which allow us to do away with the need for human intervention. ____________________________________________ |
GFCI Outlets and The Mighty Robot
Sunday, July 3rd, 2011| Most people aren’t aware of just how important those strange looking wall outlets in our kitchens and bathrooms are, you know, the ones with the little buttons that say Test and Reset. They’re known as GFCI outlets, that is Ground Fault Circuit Interrupters, and given the right set of circumstances they could save your life.
The GFCI equipped wall outlet, like a mighty robot, continuously watches the flow of electrons (electrical current) passing through, always on the lookout for incongruities between the hot and neutral wires, and ready to jump into action when necessary. Say, for example, that one of these GFCI equipped outlets has an appliance plugged into it. While the appliance is in safe use there is nothing for the GFCI robot to do. It simply takes note of the balance of electrons flowing between the hot and neutral conductors, notes that they are equal, and continues to watch for inequalities.
But suppose that there is a problem with the appliance, something that causes a ground fault where the user’s body provides an unintended path to errant electrons flowing from the hot side of the wall outlet. Those errant electrons are supposed to traverse the neutral wire back through the wall outlet from whence they came, but they have become unruly. Not to worry, if you are up to code and have an ever vigilant GFCI on that outlet, the robot will immediately notice the anomaly.
Figure 2 – If a Ground Fault Develops in the Hand Mixer and Some Electric Current Flows Through the User’s Body, Then the Robot Notices a Difference In Current Flowing Through the Hot and Neutral Wires in the GFCI Outlet. The Mighty Robot of the GFCI doesn’t like the fact that the electrons are out of balance, that there are more of them flowing through the hot wire than returning through the outlet via the neutral wire, so within a fraction of a second it will jump into action to correct things. It hits a lever on a spring loaded mechanism that snaps open an electrical switch connecting the appliance to the hot and neutral sides of the outlet, effectively cutting off the flow of electrons to the appliance. Cut off from power, the appliance ceases to function, but more importantly, the flow of electrons through the user’s body has been stopped before their body incurs injury, or death.
Figure 3- In Response to the Ground Fault, the Robot Opens a Switch in the GFCI Outlet to Cut Off The Flow of Electricity to the Hand Mixer. The Person Operating the Hand Mixer is Saved. The GFCI robot, having done its job, now goes into a sleep mode. It will be reactivated, ready again for its vigilant watch of errant electrons, when the faulty appliance is unplugged and the Reset button is pressed. This button does what it says, it resets the spring loaded mechanism in the wall outlet, closing the electrical switch, and making the outlet functional again. The GFCI robot immediately goes back into active monitoring mode. Now it should be noted that as dependable as GFCI outlets are, they can become defective. That’s why they have a Test button. This button should be pressed periodically to see if the robot is still on the job. If all is in order, the Reset button pops out of the outlet, and anything plugged into that outlet will not operate. When you press the Reset button back in, everything should operate again if there are no fault conditions. Could the GFCI’s Mighty Robot have prevented the unfortunate incidents discussed during my tenure on The Discovery Channel? Stay tuned to find out… That’s it for GFCI outlets. Next time we’ll take a look at how an invention developed to defend the allies during World War II later morphed into a space age device that cooks our food. _____________________________________________ |
