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Basic Ammeter Use
11 questions By Tony R. Kuphaldt
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Question 1 of 11
Show how this ammeter would be connected to the light bulb circuit to measure the circuit’s electric current:
Also, draw a schematic diagram of this same circuit (with the ammeter connected).
Reveal answer
(Note: the meter’s indication of 3.09 amps is arbitrary, and not important to the question).
Notes:The important thing for students to understand in this question is that the ammeter must become part of the path that the electric current flows through. Explain how this fact contains an element of danger, as contrasted against the relatively “non-invasive” connection of a voltmeter to a circuit.
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Question 2 of 11
Shown here is a circuit constructed on a PCB (a “Printed Circuit Board”), with copper “traces” serving as wires to connect the components together:
How would the multimeter be used to measure the current through the component labeled “R1” when energized? Include these important points in your answer:
- • The configuration of the multimeter (selector switch position, test lead jacks)
- • The connections of the meter test leads to the circuit
- • The state of the switch on the PCB (open or closed)
Reveal answer
In order to measure current through resistor R1, one of its leads must be de-soldered from the circuit board so that the meter may be connected directly in-line (in series) with it.
Notes:Many multimeters use “international” symbols to label DC and AC selector switch positions. It is important for students to understand what these symbols mean.
As you can see in this answer, measuring current through components is generally more difficult than measuring voltage across components, and involves greater risk because the meter must conduct the component’s full current (which in some cases may be significant). For this reason, technicians need to learn troubleshooting techniques prioritizing voltage measurements over current measurements.
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Question 3 of 11
Why is it a very bad idea to connect an ammeter directly across a voltage source, like this?
Reveal answerDue to the ammeter’s very low resistance, it will “draw” a lot of current from the voltage source. In effect, the ammeter will form a short circuit with the voltage source, potentially damaging the meter and/or the source.
In applications where the voltage source possesses very little internal resistance of its own, the current surge resulting from such a short-circuit may be huge. Very large surges of electric current are capable of heating wires to the point where their insulation bursts into flames, as well as causing super-heated blasts of plasma (electrically ionized gas) to form at any point of electrical contact where there is a spark. Either of these high-temperature conditions are hazardous to the person holding the meter and test leads!
Notes:An important point to discuss is how electrical safety encompasses more than just shock hazard. In particular, arc blasts caused by high-current “faults” such as this may be just as dangerous as electric shock. At the very least, placing an ammeter directly across the terminals of a voltage source will likely result in the ammeter’s fuse being blown.
In some cases, ammeter fuses are more expensive than one might think. Safety-rated ammeters often use expensive fast-action fuses with significant current interruption ratings. In the case of the Fluke 187 and 189 multimeters, these fuses cost around $8 each (American dollars, 2004)!
