USING A MULTIMETER

USING A MULTIMETER
Analogue and digital multimeters have either a rotary selector switch or push buttons
to select the appropriate function and range. Some Digital Multimeter (DMMs) are auto
ranging; they automatically select the correct range of voltage, resistance, or current
when doing a test. However you need to select the function.
Before making any measurement you need to know what you are checking. If you are
measuring voltage, select the AC range (10v, 50v, 250v, or 1000v) or DC range
(0.5v, 2.5v, 10v, 50v, 250v, or 1000v). If you are measuring resistance, select the
Ohms range (x1, x10, x100, x1k, x10k). If you are measuring current, select the
appropriate current range DCmA 0.5mA, 50mA, 500mA. Every multimeter is different
however the photo below shows a low cost meter with the basic ranges.

The most important point to remember is this:

You must select a voltage or current range that is bigger or HIGHER than the

maximum expected value, so the needle does not swing across the scale and hit the

"end stop."

If you are using a DMM (Digital Multi Meter), the meter will indicate if the voltage or

current is higher than the selected scale, by showing "OL" - this means "Overload." If

you are measuring resistance such as 1M on the x10 range the "OL" means "Open

Loop" and you will need to change the range. Some meters show "1' on the display

when the measurement is higher than the display will indicate and some flash a set of

digits to show over-voltage or over-current. A "-1" indicates the leads should be

reversed for a "positive reading."

If it is an AUTO RANGING meter, it will automatically produce a reading, otherwise the

selector switch must be changed to another range.

The black "test lead" plugs into the socket marked "-" "Common", or "Com," and the

red "test lead" plugs into meter socket marked "+" or "V-W-mA." The third banana

socket measures HIGH CURRENT and the positive (red lead) plugs into this. You DO

NOT move the negative "-" lead at any time.

The following two photos show the test leads fitted to a digital meter. The probes and

plugs have "guards" surrounding the probe tips and also the plugs so you can measure

high voltages without getting near the voltage-source.

Analogue meters have an "Ohms Adjustment" to allow for the change in voltage of the

battery inside the meter (as it gets old).