This week we'll complete our initial discussion of Opens & Shorts in electrical circuits. We'll use some of the concepts developed over the past couple of weeks to figure out how to use a DMM to find opens & shorts in circuits.
There will be a demonstration of what happens in an electrical circuit under the conditions of an open circuit and a short circuit. Using the calculations in class, we determined that under normal operation, PR1 = Power dissipated by R1 is 0.364 Watts and PR2 = 0.017 Watts. Since both resistors had a power rating of 0.5 watts, they should both easily handle normal conditions.
Normal Circuit Operation, PR2 = 0.17 Watts.
Short Circuit Demo Schematic.png (15.33 KiB) Viewed 1601 times
When an open is created, the current becomes zero (stops) and all of the voltage will appear across the open. The resistors will have no voltage across them . According to Ohm's Law, V=I*R, so if I=0, the Voltage, V, will be zero as well. Our measurements in class confirmed this. The resistors, because they had no voltage across them or current through them, were not dissipating any power. Therefore, there was no risk of damage to the resistors.
Open Circuit Fault, PR2 = 0 Watts
Short Circuit Demo Schemtic - Open.png (10.73 KiB) Viewed 1600 times
When a short circuit is created across the 1000 ohm resistor, the total circuit resistance of the circuit dropped (to 47 ohms) causing the total current to increase dramatically. In the demonstration the current jumped to about 0.4 Amps, more than 21 times the normal current. All 20 volts of the DC supply show up across the 47 ohm resistor since there are 0 volts across the short & R1. Therefore, the power dissipated by R2 is now about 8.5 Watts. This is about 17 times it's rated power dissipation. As seen in the class demo, the resistor gets hot, begins to discolor, emits a bit of smoke, continues to heat up with further discoloration and smoke, and eventually the resistor will glow and obviously burn. When allowed to do this to completion, the current through the circuit ultimately became near 0, and when measured afterward, the 47 ohm resistor measured in at 100's of thousands of ohms. The characteristic odor of the burnt component was also indicative of the failure due to the short.
Short Circuit Fault, R1 is shorted by wire, PR2 =8.5 Watts.
Short Circuit Demo Schematic - Short.png (15.79 KiB) Viewed 1601 times