Ammeter Design

When an electromechanical meter movement is overpowered, causing the needle to “slam” all the way to one extreme end of motion, it is commonly referred to as “pegging” the meter. I’ve seen meter movements that have been “pegged” so badly that the needles are bent from hitting the stop!

Based on your students knowledge of meter movement design, ask them to tell you what they think might become damaged in a severe over-power incident such as this. Tell them to be specific in their answers.

Beginning students sometimes feel “lost” when trying to answer a question like this. They may know how to apply Ohm’s Law to a circuit, but they do not know how to design a circuit that makes use of Ohm’s Law for a specific purpose. If this is the case, you may direct their understanding through a series of questions such as this:

Why does the meter movement “peg” if directly connected to the battery?

What type of electrical component could be used to direct current “away” from the movement, without limiting the measured current?

How might we connect this component to the meter (series or parallel)? (Draw both configurations and let the student determine for themselves which connection pattern fulfills the goal of limiting current to the meter.)

The follow-up question is quite interesting, and causes students to carefully evaluate the performance of the ammeter they’ve “created”. At root, the problem is similar to that of voltmeter loading, except of course that we’re dealing with ammeters here rather than voltmeters.

Determining the ranges for this ammeter is simply an exercise in Ohm’s Law. It is very important that your students recognize the shunt resistor’s value as being in milliohms and not Megaohms! Yes, there is a difference between a lower-case letter “m” and a capital letter “M”!