basic ammeter use

8/13/2019 Basic Ammeter Use

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Basic ammeter use

Question 1:

Show how this ammeter would be connected to the light bulb circuit to measure the circuit'selectric current:

Also, draw a schematic diagram of this same circuit (with the ammeter connected).

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(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 mustbecome part of the path that the electric current flows through. Explain how this fact contains anelement of danger, as contrasted against the relatively "non-invasive" connection of a voltmeterto a circuit.

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Question 2:

Shown here is a circuit constructed on a PCB (a "Printed Circuit Board"), with copper "traces"
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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 circuitThe state of the switch on the PCB (open or closed)

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In order to measure current through resistor R1, one of itsleads must be de-soldered from the circuit board so that themeter may be connected directly in-line (in series) with it.

Notes:Many multimeters use nternational" symbols to label DC andAC selector switch positions. It is important for students tounderstand what these symbols mean.As you can see in this answer, measuring current throughcomponents is generally more difficult than measuringvoltage across components, and involves greater risk becausethe meter must conduct the component's full current (whichin some cases may be significant). For this reason,technicians need to learn troubleshooting techniquesprioritizing voltage measurements over currentmeasurements.

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Question 3:

Why is it a very bad idea to connect an ammeter directly acrossa voltage source, like this?

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Due to the ammeter's very low resistance, it will "draw" a lotof current from the voltage source. In effect, the ammeter willform a short circuit with the voltage source, potentiallydamaging the meter and/or the source.In applications where the voltage source possesses very littleinternal resistance of its own, the current surge resulting fromsuch a short-circuit may be huge. Very large surges of electriccurrent are capable of heating wires to the point where theirinsulation bursts into flames, as well as causing super-heatedblasts ofplasma(electrically ionized gas) to form at any pointof electrical contact where there is a spark. Either of thesehigh-temperature conditions are hazardous to the personholding the meter and test leads!

Notes:An important point to discuss is how electrical safetyencompasses more than just shock hazard. In particular, arcblastscaused by high-current "faults" such as this may be

just as dangerous as electric shock. At the very least, placingan ammeter directly across the terminals of a voltage sourcewill likely result in the ammeter's fuse being blown.In some cases, ammeter fuses are more expensive than onemight think. Safety-rated ammeters often use expensive fast-action fuses with significant current interruption ratings. Inthe case of the Fluke 187 and 189 multimeters, these fusescost around $8 each (American dollars, 2004)!

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Question 4:

What would happen if a multimeter were connected across thecomponent labeled "C1" on this printed circuit board, asshown?


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Reveal Answer

The meter would create a short-circuit with the battery.Determine what damage this short-circuit might cause, to allcomponents involved.

Notes:Even though it might not appear that the meter is horting"the battery in this example, it most certainly is. In askingstudents to determine the resulting damage from such anaction, it is important for them to trace the path of "faultcurrent" through the circuit. Those components within thepath of fault current are in risk of damage, while thosecomponents not within the path of fault current are not atrisk.

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Question 5:

If we were to connect an ammeter directly to an ohmmeter,what would you expect to see the ohmmeter register, forresistance between its test leads?
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Reveal Answer

The ohmmeter should register a very low resistance.

Notes:Ask your students whyan ammeter should have a very lowresistance (fractions of an ohm) between its test leads. Howdoes this property of all ammeters relate to how they areused to measured current in real circuits?

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Question 6:

Most ammeters contain fusesinside to provide protection forthe person using the ammeter, as well as for the ammetermechanism itself. Voltmeters generally do notcontain fusesinside, because they are unnecessary.Explain why ammeters use fuses and voltmeters do not? Whatis it about the nature of an ammeter and how it is used thatmakes fuse-protection necessary?

Reveal Answer

The purpose of the fuse inside the ammeter is to "blow"(open-circuit) in the event of excessive current through themeter.
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Notes:This is a very important question for several reasons. First, italerts students that ammeters are generally fuse-protected,and therefore they must be aware that this fuse could "blow"and render the ammeter inoperative. Secondly, it highlightsone of the major differences between voltmeters and

ammeters, and that is the amount of current typically handledby the respective meter types.

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Question 7:

In this circuit, is the light bulb lit? Why or why not?Also, compare the relative indications of the two ammeters(which ammeter registers the greatest amount of current, andwhich ammeter registers the least amount of current, or dothey both register the same amount of current?).

Reveal Answer

The bulb is not lit, and the two ammeters both register 0amperes of current.

Notes:It is vitally important for students to understand thesignificance of continuity throughout the entire circuit, not

just at one or more points in the circuit.
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Question 8:

In this circuit, is the light bulb lit? Why or why not?Also, compare the relative indications of the two ammeters(which ammeter registers the greatest amount of current, andwhich ammeter registers the least amount of current, or dothey both register the same amount of current?).

Reveal Answer

The bulb is lit, and the two ammeters register equal amountsof current.

Notes:This is a good question to engage students' thinking on meterproperties. A voltmeter does not function the same as anammeter, and the two different types of meters will impactcircuits differently when connected! Incidentally, ammeterstend to get you into more trouble than voltmeters whentroubleshooting circuits, which is why students should be

encouraged to use a voltmeter whenever possible rather thanan ammeter. Of course, clamp-on ammeters are not asunsafe as ammeters requiring direct connection with circuitconductors.

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Question 9:

In this circuit, is the light bulb lit? Why or why not?Also, compare the relative indications of the two ammeters(which ammeter registers the greatest amount of current, and

which ammeter registers the least amount of current, or dothey both register the same amount of current?).

Reveal Answer

The bulb is lit, but ammeter " registers much more currentthan ammeter "B".

Notes:Exactly what ammeter "B" will register (aside from itsindication being less then ammeter "), is an interestingquestion.

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Question 10:

Generally speaking, is an ammeter supposed to be connectedin seriesor inparallelwith the component it's measuringcurrent through?

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Series.

Notes:If your students don't already know, this question is anexcellent opportunity for them to learn about what a eries"

circuit is and how it differs from a "parallel" circuit. Ask yourstudents whyeries" is the correct answer. What is it about aseries connection that makes it the best type of connection touse an ammeter in? What would happen if we connected anammeter inparallelwith a circuit component?

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Question 11:

Ammeters must be connected in serieswith the current to bemeasured, to ensure that all the current moves through themeter:

In order to practically function, an ammeter must have someinternal resistance. It is usually a very small amount, but itdoes exist. It should be apparent to you that the presence ofthis resistance will have some effect on the circuit current,when compared to the amount of current in the circuit withoutany meter connected:

Explain why it is usually safe to ignore the internal resistance of


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an ammeter, though, when it is in a circuit. A common termused in electrical engineering to describe this intentionaloversight is swamping. In this particular circuit an engineerwould say, "The resistance of the light bulb swampstheinternal resistance of the ammeter."

Reveal Answer

When one quantity "swamps" another, we mean that its effectis huge compared to the effect of the other, so much so thatwe may safely ignore it in our calculations and still arrive at areasonably accurate result.

Notes:I have found that the concept of "swamping" is extremelyuseful when making estimations. To be able to ignore thevalues of some components allows one to simplify a great

many circuits, enabling easier calculations to be performed.

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basic ammeter use

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