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As we’ve come to know through this series of blogs, all electronic components pose some degree of internal resistance to the electric current flowing through them. This resistance results in electrical energy being converted into heat energy, heat which poses potential problems to sensitive components like electronic circuit boards. If things get hot enough, components fail and fires may ignite. To address these issues engineers design circuits with resistors whose job it is to limit the current flowing to electrical components. In this article we’ll see how a limiting resistor protects a Zener diode from this fate, allowing it to continue doing its job of regulating voltage. In our last blog we applied Ohm’s Law to our regulated power supply circuit, which makes use of a Zener diode. See Figure 1. Figure 1
Ohm’s Law gave us the following equation to determine the amount of current, IPS, flowing from the unregulated power supply portion, through the current limiting resistor RLimiting, and making its way into the rest of the circuit: IPS = (VUnregulated – VZener) ÷ RLimiting We learned last week that for the circuit to work, the voltage of the unregulated power supply portion of the circuit, VUnregulated, must be greater than the Zener voltage, VZener. Looking at the equation above, we see that the voltage difference is divided by RLimiting, the value of the limiting resistor in the circuit. This limiting resistor is there to constrain the current flowing to the Zener diode, allowing the diode to keep things under control within the circuit. Basic mathematical principles hold that if a smaller number is divided by a bigger number, the resulting answer is an even smaller number. Applying this principle to the equation above, if RLimiting is a big number, then IPS must be a smaller number. On the other hand the smaller RLimiting gets, the bigger IPS becomes. So what does it take for our circuit to fail? Remove the limiting resistor as shown in Figure 2 and the value for RLimiting disappears. In other words, RLimiting becomes zero.
Figure 2
In this case our Ohm’s Law equation becomes: IPS = (VUnregulated – VZener) ÷ 0 = ∞ The resulting answer is said to go to infinity, or ∞, as it is represented mathematically. In other words, without a limiting resistor being employed within our circuit, IPS will become so large it will overwhelm the diode’s current handling capacity and lead to circuit failure. Next time we’ll go over some advantages and disadvantages of this Zener diode voltage regulating circuit, and why the disadvantages outweigh the advantages for many applications. ____________________________________________ |
Posts Tagged ‘electric current flow’
Transistors – Voltage Regulation Part XIV
Monday, October 22nd, 2012
Industrial Control Basics – Electric Motor Control
Sunday, February 19th, 2012| Electric motors are everywhere, from driving the conveyor belts, tools, and machines found in factories, to putting our household appliances in motion. The first electric motors appeared in the 1820s. They were little more than lab experiments and curiosities then, as their useful potential had not yet been discovered. The first commercially successful electric motors didn’t appear until the early 1870s, and they could be found driving industrial devices such as pumps, blowers, and conveyor belts.
In our last blog we learned how a latched electric relay was unlatched at the push of a button, using red and green light bulbs to illustrate the control circuit. Now let’s see in Figure 1 how that circuit can be modified to include the control of an electric motor that drives, say, a conveyor belt inside a factory.
Figure 1
Again, red lines in the diagram indicate parts of the circuit where electrical current is flowing. The relay is in its normal state, as discussed in a previous article, so the N.O. contacts are open and the N.C. contact is closed. No electric current can flow through the conveyor motor in this state, so it isn’t operating. Our green indicator bulb also does not operate because it is part of this circuit. However current does flow through the red indicator bulb via the closed N.C. contact, causing the red bulb to light. The red and green bulbs are particularly useful as indicators of the action taking place in the electric relay circuit. They’re located in the conveyor control panel along with Buttons 1 and 2, and together they keep the conveyor belt operator informed as to what’s taking place on the line, such as, is the belt running or stopped? When the red bulb is lit the operator can tell at a glance that the conveyor is stopped. When the green bulb is lit the conveyor is running. So why not just take a look at the belt itself to see what’s happening? Sometimes that just isn’t possible. Control panels are often located in central control rooms within large factories, which makes it more efficient for operators to monitor and control all operating equipment from one place. When this is the case, the bulbs act as beacons of the activity taking place on the line. Now, let’s go to Figure 2 to see what happens when Button 1 is pushed.
Figure 2
The relay’s wire coil becomes energized, causing the relay armatures to move. The N.C. contact opens and the N.O. contacts close, making the red indicator bulb go dark, the green indicator bulb to light, and the conveyor belt motor to start. With these conditions in place the conveyor belt starts up. Now, let’s look at Figure 3 to see what happens when we release Button 1.
Figure 3
With Button 1 released the relay is said to be “latched” because current will continue to flow through the wire coil via one of the closed N.O. contacts. In this condition the red bulb remains unlit, the green bulb lit, and the conveyor motor continues to run without further human interaction. Now, let’s go to Figure 4 to see how we can stop the motor.
Figure 4
When Button 2 is depressed current flow through the relay coil interrupted. The relay is said to be unlatched and it returns to its normal state where both N.O. contacts are open. With these conditions in place the conveyor motor stops, and the green indicator bulb goes dark, while the N.C. contact closes and the red indicator bulb lights. Since the relay is unlatched and current no longer flows through its wire coil, the motor remains stopped even after releasing Button 2. At this point we have a return to the conditions first presented in Figure 1. The ladder diagram shown in Figure 5 represents this circuit.
Figure 5
Next time we’ll introduce safety elements to our circuit by introducing emergency buttons and motor overload switches. ____________________________________________ |
