Meter Circuits

This article covers the measurement of voltage, current, resistance, and power. This article assumes a basic familiarity with voltage and current dividers.

The D'Arsonval Movement

The d'Arsonval movement is the heart of most analog meters. It consists of a movable coil that is placed within the field of a permenant magnet. When current goes through the coil, it creates torque which turns the coil. A pointer is attached to the coil, and a small restoring spring is added which pushes the pointer back over to the side when no current is applied.

The important thing to remember about d'Arsonval movements is that the deflection of the pointer is directly proportional to the current going through the coil.

Basic Meter Circuits

The Ammeter

d'Arsonval movements are usually specified by the full scale current value, and the corresponding voltage across the coil when this full scale current is reached. The resistance of the coil may be specified instead of the voltage, and Ohm's Law easily lets us determine the missing values.

Suppose, for example, that our particular d'Arsonval movement reads full scale at 1 mA, and that the voltage across the coil at 1 mA is 50 mV. Using Ohm's Law, we can easily determine that the coil has a resistance of 50 ohms (from V=IR).

Next, let's assume that we want to make an ammeter with a full scale reading of 10 mA. This means that at full scale, 1 mA is going through the movement (remember from the above paragraph that this particular meter reads full scale at 1 mA), which leaves us 9 mA that has to be going through the shunt resistor. At full scale, the meter drops 50 mV. In a current divider, both resistors have the same voltage across them, so our shunt resistor also has to have 50 mV across it. Now we know the voltage and current through our shunt resistor, and we can use Ohm's Law again. Plug in I and V and solve for R. You should get 5.55 ohms (approximately).

To use an ammeter, we place it in series with the device being measured. This means that all of the current flowing through the device also must flow through the ammeter. If the device draws too much current, then the meter could be damaged. This is why it is very important for ammeter circuits to be properly fused.

Ideally, we like to assume that the ammeter has a negligable effect on the circuit. In the real world, the ammeter does have some resistance and does effect the circuit. Our example meter above has 5 ohms of resistance (5.55 ohms in parallel with 50 ohms for the meter movement). If you are trying to measure the current through a 1 ohm resistor, the meter has 5 times the resistance of the resistor you want to measure! This could very easily cause the circuit to function improperly.

The Voltmeter

A voltmeter can be constructed by placing a resistor in series with a d'Arsonval movement. This makes a simple voltage divider. If we continue using our example d'Arsonval movement from above, let's now assume that we want to make a voltmeter with a full scale reading of 150 volts. We know that the current through the resistor is 1 mA, since the full scale current of the movement is 1 mA, and any current going through the movement must also be going through the series resistor (where else would it go?). We know the current through our resistor (1 mA) and the voltage across it (150-0.050). Using Ohm's Law, this gives us a resistance of 149,950 ohms.

Voltmeters are connected in parallel with the device being measured. The resistance of the device being measured should be taken into consideration, since the meter's resistance may have a noticable effect on how the circuit operates.

The Ohmmeter

An ohmmeter can be constructed out of a d'Arsonval movement with a series resistor, similar to the above voltmeter, with a battery also in series. The series resistor is chosen so that the meter movement deflects to full scale when the terminals of the meter are shorted together. A variable resistor is often used, so that variations in the battery may be accounted for.

This creates a meter with a scale that is not linear. Lower resistances will have a dominant effect on the current. For higher resistances, the resistance of the meter is more dominant. Therefore, the resistance scale is wider on the low end and narrower on the high end.

The Wheatstone Bridge

A more accurate circuit that can be used to measure resistance is called a Wheatstone Bridge. The variable resistor, R3, is adjusted until there is no current flowing through the meter movement. At this point, we know that Rx (the resistance being measured) is equal to (R2/R1)R3. This circuit can cover a much wider range of resistances if R1 and/or R2 can be varied. In practical Wheatstone Bridges, R3 may consist of several resistors which can be switched into the circuit using decimal type dials (one dial for the thousands, another for the hundreds, another for the tens, etc). A very accurate meter with a very large range (up to about 1,000,000 ohms) may be constructed in this manner.

The Wattmeter

Digital Meters

Digital meters do not use d'Arsonval movements, but they still work on the same basic principles. A very precise analog to digital converter and some sort of display (usually a LCD) is used in place of the d'Arsonval movement.

Calibration

How do you know that a volt is really a volt? How do you know that your meter's resistors are accurate? In the U.S., we have the NIST (National Institute of Standards and Technology) which creates standard voltage sources, etc. that can be used to calibrate a meter. Very high quality meters will have a calibration certificate that is tracable back to NIST standards. This calibration is usually only guaranteed for a certain period of time (such as one year).

Most hobbyists do not need this level of accuracy for home use, and do not need to worry about calibration to NIST standards.