MEASURING CURRENT
You will rarely need to take current measurements,
however most multimeters have DC current ranges such as 0.5mA, 50mA, 500mA and
10Amp (via the extra banana socket) and some meters have AC current ranges. Measuring the current of a circuit will tell
you a lot of things. If you know the
normal current, a high or low current can let you know if the circuit is
overloaded or not fully operational.
Current is always measured when the circuit is
working (i.e: with power applied).
It is measured IN SERIES with the circuit or
component under test. The easiest way to measure current is to remove the fuse
and take a reading across the fuse-holder. Or remove one lead of the battery or
turn the project off, and measure across the switch.
If this is not possible, you will need to remove
one end of a component and measure with the two probes in the
"opening."Resistors are the easiest things to desolder, but you may
have to cut a track in some circuits. You have to get an "opening" so
that a current reading can be taken.
The following diagrams show how to connect the
probes to take a CURRENT reading.
Do not measure the current ACROSS a component as
this will create a "short-circuit."
The component is designed to drop a certain
voltage and when you place the probes across this component, you are
effectively adding a "link" or "jumper" and the voltage at
the left-side of the component will appear on the right-side. This voltage may
be too high for the circuit being supplied and the result will be damage
Measuring current through a resistor
Measuring the current of a globe
Do NOT measure the CURRENT of a battery (by
placing the meter directly across the terminals) A
battery will deliver a very HIGH current and damage the meter
Do not measure the "current a battery will
deliver" by placing the probes across the terminals. It will deliver a
very high current and damage the meter instant ly. There are special battery
testing instruments for this purpose.
When measuring across an "opening" or
"cut," place the red probe on the wire that supplies the voltage (and
current) and the black probe on the other wire. This will produce a
"POSITIVE" reading.
A positive reading is an UPSCALE READING and the
pointer will move across the scale- to the right. A "NEGATIVE
READING" will make the pointer hit the "STOP" at the left of the
scale and you will not get a reading. If you are using a Digital Meter, a
negative sign "-" will appear on the screen to indicate the probes
are around the wrong way. No damage will be caused. It just indicates the
probes are connected incorrectly.
If you want an accurate CURRENT MEASUREMENT, use a
digital meter.
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