DMM Basics Part 2

  The next meter function we will look at is the proper use of he ammeter. This particular operation of the meter has been known to trip up even the most seasoned technician simply because it is not the most common meter test. The DMM will have two possible inputs for the red lead. One input will be protected by a 10 amp fuse and the other will be protected by a 400 milliamp fuse, therefore, these two inputs can be referred to as the high scale and the low scale respectively (see figure 1). These two inputs are where the red lead will go while the black remains in the common input. By the way, if milli looks like micro to you, and it has been some years since you attended SET class, why not review your basic electrical  skills by attending the Basic Meters Usage (18043.06H) Hands On Class? To see where one is being offered near you, check out the schedules page.   

Figure 1

   The rule of thumb when checking amperage is to start off in the high scale, which will protect the DMM. Remember, DMM fuses are NOT cheap, so work smart when testing amperage. Even eSI is taking this fact into consideration. When performing a parasitic load test, the technician is is directed to hook up a fused jumper before installing the  DMM  on the load tool. This is a good idea as mini fuses are cheaper than DMM fuses and parasitic draw is probably the most common test you will make with an ammeter. Figure 2  below shows the meter fuses marked F1 and F2.                                                                   

  Another important note (and tripping point for some techs) is the lead hook up in the circuit. Most of your testing will have the leads installed on the circuit to be tested in parallel, however when checking amperage, the circuit must be opened and checked in series. Once you have the meter hooked up, turn it on and start out on the 10A scale (don't forget to put the red lead in the 10A input) and operate the circuit. If you have a reading of less then 400 milliamps, now would be a good time to move your leads and switch the meter to the low scale. Doing so will give you a better picture of current flow. Something else to remember when testing amperage, allow the current to flow for a few minutes and let the circuit heat up. Resistance may go down allowing more current to flow, which could be the cause of the problem.

  Trust the meter and know what results to expect before performing any electrical test.

  Good luck with your diagnosing, fix it right the first time and we will see you at the training center.