Author Topic: Digital Multimeter (DMM) - Part 5  (Read 2015 times)

Offline K9DJT

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Digital Multimeter (DMM) - Part 5
« on: May 12, 2014, 01:58:08 PM »
At this point we’ve discussed voltage and resistance measurement using a DMM (Digital Multimeter) and even some convenience features such as Min-Max, Touch-Hold and diode check.  So it’s about time we take a look at the third piece of Ohms law, current, using A as the symbol on the rotary switch to indicate Ampere measurement.  As there is AC and DC voltage selection, there is AC current and DC current selection.  Some instruments might also provide a better resolution by supplying  a milliampere or microampere selection.  Typically, the maximum current which can be measured directly is 10 amps but that is dependent on the model and brand.  Higher currents reaching thousands of amps can be measured using accessories such as current clamps or current shunts.

Current measurements are primarily used to diagnose performance issues with a piece of equipment or process, e.g., the apparatus you are servicing is working, but not working well.  Maybe it’s overheating, running too slow or too fast.  The point being is that it is working and you have a very good idea of what the current should be.  If you do not have a clue of what to expect, do not attempt a measurement.  (This rule holds true to any electrical measurement; if you don’t know what value to expect, why bother?)  If the equipment you are servicing is dead, non-functional, K9DJTyou should make voltage measurements, or resistance measurements once power is removed and caps  discharged.  Yes, you could make a current measurement and maybe discover no current being drawn, but if the problem is a short, the current will be sky high and hopefully you’re using a meter which is fused.  If not, it could blow up or at minimum fry the insides.  Naturally, the severity is dependent on the power you’re working with.  The biggest mistake people make with a DMM setup to measure current, is instead of placing the meter in series with the load, they place the probes across the load or source.  Again, depending on the power you are working with, this situation becomes an extreme safely issue.  The DMM or VOM (Volt-Ohm-Meter) has a very low input impedance in the current mode and thus it’s like placing a short across the potential.  So remember, when measuring current with a DMM/VOM directly…meaning no accessory is being used…place the meter IN SERIES WITH THE LOAD, NOT across it.  This means you need to break the circuit under test in order to attach the test leads.  In addition, you need to place the RED test lead into the appropriate current jack on the DMM relating to the value of current you intend to measure.  In many cases there will be a 10 Amp jack and a 400 mA jack.

73, Gary