Chapter 12
Fundamentals of Electricity & Electronics
- Short circuit—an unintentional low resistance path between two components in a circuit or between a
t beam position.
component/conductor and ground. It usually creates high current flow, which burns out or causes damage to the circuit conductor or components.
- Open circuit—a circuit that is not a complete or continuous path. An open circuit represents an infinitely large resistance. Switches are common devices used to open and close a circuit. Sometimes a circuit opens due to a component failure, such as a light bulb or a burned out resistor.
- Continuity—the state of being continuous, uninterrupted or connected together; the opposite of a circuit that is not broken or does not have an open.
- Discontinuity—the opposite of continuity, indicating that a circuit is broken or not continuous.
Voltage Measurement
Voltage is measured across a component with a voltmeter or the voltmeter position on a multimeter. Usually, there is a DC and an AC selection on the meter. Before the meter is used for measurements, make sure that the meter is selected for the correct type of voltage. When placing the probes across a component to take a measurement, take care to ensure that the polarity is correct. [Figure 12-170] Standard practice is for the red meter lead to be installed in the positive (+) jack and the black meter lead to be installed in the negative meter jack (−). Then when placing the probes across or in parallel with a component to measure the voltage, the leads should match the polarity of the component. The red lead is on the positive All meters have some resistance and will shunt some of the current. This has the effect of changing the characteristic of the circuit because of this change in current. This is typically more of a concern with older analog type meters. If there are any questions about the magnitude of the voltage across a component, then the meter should be set to measure on the highest voltage range. This prevents the meter from “pegging” and possible damage. The range should then be selected to low values until the measured voltage is read at the mid-scale deflection. Readings taken at mid-scale are the most accurate.
Current Measurement
Current is measured with the ammeter connected in the current path by opening or breaking the circuit and inserting the meter in series. [Figure 12-170] Standard practice is for the red meter lead to be installed in the positive (+) jack and the black meter lead to be installed in the negative meter jack (−). The positive side of the meter is connected towards the positive voltage source. Ideally, the meter should not alter the current and influence the circuit and the measurements. However, the meter does have some effect because of its internal resistance that is connected with the rest of the circuit in series. The resistance is rather small and for most practical purposes, this can be neglected.
Checking Resistance in a Circuit
The ohmmeter is used to measure the resistance. In its more basic form, the ohmmeter consists of a variable resistor in series with a meter movement and a voltage source. The meter must first be adjusted before use.
Refer to Figure 12-171 for meter configurations during adjustments. When the meter leads are not connected (open), the needle points to the full left-hand position, indicating infinite resistance or and open circuit. With the lead placed together, the circuit is shorted as shown with the meter needle to the full right-hand position. When a connection is made, the internal battery is allowed to produce a current through the movement coil, causing a deflection of the needle in proportion to the value of the external resistance. In this case, the resistance is zero because the leads are shorted.
The purpose of the variable resistor in the meter is to adjust the current so that the pointer reads exactly zero when the leads are shorted. This is needed because as the battery continues to be used, the voltage changes, thus requiring an adjustment. The meter should be “zeroed” before each use.
To check the value of a resistor, the resistor must be disconnected from the circuit. This prevents any possible damage to the ohmmeter, and it prevents the possibility of any inaccurate readings due to the circuit being in parallel with the resistor in question. [Figure 12-172]
Continuity Checks
In many cases, the ohmmeter is not used for measuring the resistance of a component but to simply check the integrity of a connection from one portion of a circuit to another. If there is a good connection, then the ohmmeter reads a near zero resistance or a short. If the circuit is open or has a very poor connection at some point like an over-crimped pin in a connector, then the ohmmeter reads infinity or some very high resistance. Keep in mind that while any measurement is being taken, contact with the circuit or probes should be avoided. Contact can introduce another parallel path and provide misleading indications.