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Measuring Electricity

Measuring Current

In the last section, we worked through a common voltage measurement. Now, we will go over measuring current flow. Just as a water pipe might have a pressure gauge and a billing meter; our electrical circuit has pressure(voltage) and flow rate(amperage).

Measuring current flow is a bit more difficult in most circuits; because we cannot simply touch our meter probes to a couple of terminals. We must instead insert our meter into the the circuit so that it becomes part of the circuit and all the current flows through the meter (you may recognize this as a series circuit, and it is just that).

This causes a couple of issues; the first being that we have to disconnect some part of our circuit in order to create a place to connect the meter. In some cases, that may pose a physical challenge such as unsoldering a component or cutting a wire. The other issue to keep in mind is that our meter itself has some amount of resistance, and thus will actually decrease the current by some amount. This second issue may or may not be a factor for the desired measurement; depending on desired accuracy, meter specifications, and circuit tolerances. In most cases, this won't be significant, but it is worthwhile to keep it in mind.

Range Selection

Selecting the proper range on our meter for measureing current is very similiar to doing so for voltage. You should select a range that is comfortably above the expected reading. Again, if you have any doubt, start out higher and work downwards for your final reading as a meter can be damaged by attempting to measure more than it's range is set for.

Where current ranges differ from voltage ranges is that a meter often has a different 'port' or connection point for higher current ranges. This will often be labeled as "10A" for 10 amps maximum range (you're meter may differ). On the settings dial, you should also see a setting for this range; and the two are meant to be used together. Lower ranges in the mA's often use the same port as voltage measurements. Consult your meter manual for specifics.

Current Connections

Multimeter connections for current measurementAs I stated in the introduction, we must connect our meter in series with our circuit. If you remember from series circuits, the current through each device is the same - so it won't matter where you chose to interrupt the circuit and insert the meter. This is helpful, because it allows us to chose a location that is physically the easiest - possibly an existing plug, the terminals on a switch, or something similiar so that we don't need to cut wires or remove parts from the circuit board.

In order to use existing connectors for current measurements, you will need to unplug the connector. If you have a single wire connector(uncommon but nice), you can simply insert a probe from your meter into each of the two connector halves. You will want the red probe to go to the connector closest to the positive power supply, and the black probe closest to the ground.

In the case of multi-wire connectors, it is necessary to connect the meter to the desired current path; and then restore the remaining path(s) with a jumper wire. Use caution when doing this; avoid restoring part of the circuit at a time and watch out for inadvertently shorting separate paths to each other.

Using a switch for a connection is often the easiest test point; though it may not be immediately obvious how and why. With a bit of luck, our switch will be of a single current path variety (quite common and nice - will have two and only two connection points). To use it, we simply allow the current to pass around the internal contacts through our meter. To do this, we connect a meter probe to each of the terminals on the switch. Again, we want the read to be the terminal connected to or electrically nearest the positive supply, and the black connected to or electrically nearest the ground. Leave the switch turned off, and our meter will complete the circuit instead of the switch - just what we wanted!

I won't get into detail about multi-path switches, called double pole and/or double throw (more than double is also possible). They can also be used, but like with connectors - we must restore the remaining circuits through some other manual means. Often with these type of switches it is easier to remove a wire at the switch for the circuit we want to test rather than try to bypass the other contacts.

In the next section, we will step thru a real world current measurement ...

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