How to use a Multi meter
The multi meter is a measuring device that is amongst the most useful tools in a professional electricians bag of tools. They were for many years analogue instruments but with the modern circuitry available have become digital and now are mass produce at a low enough cost to make them accessible to the do it yourself home owner. However, as with all things connected with electricity care must be taken when using one on live appliances or wiring. The first step is to fully read and understand the instructions.. Simple advice but in practice most multi meters are capable of taking a number of readings including voltage, resistance and so on depending on the complexity of the meter. The most useful check that it can perform for the amateur is to establish that power is reaching an appliance that appears not to work. In the home in the United Kingdom the voltage from the socket is a nominal 240v. In the United States it is a nominal 110v. In both cases the voltage is usually less due to losses in the wiring causing voltage drop. The same is applicable to the DC voltage of cars and trucks but here voltage will fluctuate above and below the 12v or 24v stated. This is because as the battery is being charged the voltage produced by the alternator reaches over 14v and this is the voltage throughout the system.
To check that the appliance or component is receiving power the first thing to do is to plug the probes into the multi meter. This is easy enough with the ground or black probe as there is only one black socket. With the red positive lead there is often a choice of two sockets so check the instructions for voltage measurement before plugging that one in. Next turn the meter on, you will see that the face of the meter is divided into segments around the central switch. Each segment has ranges marked in it for example in the segment for measuring AC voltage you can often select either 300v or 750v. Set the pointer at the rating above but closest to the expected voltage of the wiring that you are testing. In the home the AC voltage is 240v max. So the meter should be set at 300v. This will give a reading that shows tenths of a volt whereas if you set it to 750v the reading will be in whole volts, in this case either will do but for DC volts care is needed as most meters can be set to read in fractions of a volt.
Where possible it is better to touch the probes onto exposed connections rather than use the pointed end to pierce the insulation of the wires. Either way check that the polarity is correct, with most meters it does not matter too much as your reading will simply be negative if the polarity is reversed.
For more on multi meters go to www.lcr-meter.net