the range setting of the switch; for example, when using the multimeter with the switch set to 300 AC VOLTS, read from the scale that has a maximum reading of 300 ac. Simply take the reading directly from either of the digital multimeters.
WARNING Always be alert when taking voltage or amperage measurements if it is necessary to move the meter. If the instrument is moved in a way that causes tension on the test leads, one or both leads may be pulled Tom the jack(s). The leads will be energized just as the circuit to which they are connected, and they can be dangerous.
The positions of the jacks may differ for a particular measurement, from one meter to another. Notice how the jacks are labeled on the instrument you use, and follow the instructions from the manufacturer of the instrument.
It is possible that you may never use a multimeter for amperage measurements. Most multimeters are designed with quite low current ranges. The clamp-on ammeter (discussed earlier) is the most convenient portable instrument for measuring ac amperes.
As mentioned earlier, ohmmeters have their own voltage source. This circumstance is also true of the ohmmeter function of multimeters. The size and number of batteries for different instruments vary. Usually one or more 1 1/2- to 9-volt batteries are used for resistance measurements.
As you must set up the meter to measure voltage accurately, so you must set it up for measuring resistance. If you are to measure a 120-ohms resistor, for example, set the selector switch to ohms at the appropriate range. For the analog instruments, set the switch to the R x 1 or x 1 as appropriate. Read the value from the ohms scale directly. For higher values of resistance like 1,500 ohms, for example, use the R x 100 or x 100 range. In this case, multiply the reading from the ohms scale by 100.
For all critical resistance measurements, always touch the leads together and set the indicator needle to zero with the appropriate adjustment knob. Do not let the leads touch your fingers or anything else while you are zeroing the meter.
Be certain that there is no power on the circuit or component you are to test when measuring resistance. Be sure also to discharge any capacitors associated with the circuit or component to be tested before connecting the instrument to the circuit or component
For critical measurements, make sure that only the circuit or component you are to test touches the leads while you take the reading; otherwise, the reading may be inaccurate, especially on the higher resistance ranges.
Many times you will use the ohmmeter for continuity tests. All you will want to know is whether the circuit is complete or not. You will not have to zero the meter for noncritical continuity tests. You will want to touch the leads together to see where the needle comes to rest. If the needle stops at the same place when you place the leads across the circuit, you will know the path has a low resistance. In other words you will know there is continuity through the circuit.
Construction Electricians also use other instruments for different types of resistance measurements. We will discuss these instruments next.
The megohmmeter is a portable instrument consisting of an indicating ohmmeter and a source of dc voltage. The dc source can be a hand-cranked generator, a motor-driven generator, a battery-supplied power pack, or rectified dc.
The megohmmeter is commonly called a "megger" although Megger is a registered trademark. The megger tester shown in figure 7-25 is an example of a dual-operated megohmmeter. having both a hand- cranked generator and a built-in line power supply in the same module.
Any one of the ohmmeters shown in figure 7-24 will measure several megaohms. You may wonder why they are not called megohmmeters. What is the difference between the megger and the typical ohmmeter? Does not each of them have an indicator and a dc voltage source within the instrument enclosure? The megger is capable of applying a much higher value of dc voltage to the circuit or component under test than is the typical ohmmeter. Meggers that will supply a test potential of 500 volts are common inContinue Reading