The post Ohmmeter: Working Principle & Circuit Diagram | Series & Shunt Type Ohmmeter appeared first on Electrical A2Z.

]]>The same **meter movement** that was used in the **voltmeter** and **ammeter** can be used for the ohmmeter. A voltage source and a variable resistor are added to the ohmmeter’s circuit.

A **series type ohmmeter** is shown in Figure 1.

**Figure 1.** Circuit diagram of a series type ohmmeter.

A three-volt battery is used as the source for the ohmmeter. The battery is built into the meter case. The **meter movement** permits only 0.1 volts for a current of 0.001 amps for full-scale deflection. Therefore, a multiplier resistor is placed in series with the meter coil to reduce the voltage applied to the meter coil.

\[{{R}_{M}}=\frac{E}{I}=\frac{2.9V}{0.001A}=2900\Omega \]

The 2900 ohm multiplier resistor, plus the meter coil resistance, is equal to 3000 ohms. Part of this resistance is made up of a variable resistor to allow the total resistance to vary.

- Because temperature changes or weak batteries can affect the total resistance of the circuit, the ohmmeter must be calibrated (adjusted for zero resistance) in order to ensure the most accurate reading possible.
- The knob used for adjusting the pointing needle position to zero is usually marked “Zero Adjust,” or with an omega (Ω) symbol near it.
- To use the ohmmeter, first short the test leads together. This applies 0 ohms across the meter. Adjust the ohms adjustment knob until the needle points at zero.
- The needle should deflect from its position at rest on the left to the zero resistance indication on the right side of the scale. If the needle does not deflect, it is possible that the battery is dead or extremely weak.
- After the ohmmeter has been calibrated to read zero ohms when the leads are shorted, you can make a reading of an unknown resistance by placing the unknown resistance between the test leads.

**Caution:**

Before connecting an ohmmeter to any electrical circuit to read an unknown value, be sure that the circuit is not energized.

An energized circuit will damage the meter and can be harmful to you. The electrical energy in a circuit is not needed to operate the meter movement coil as it is when using a

voltmeteror anammeter.The batteries inside the case provide the source of power for the ohmmeter. Connecting the ohmmeter to an energized circuit will apply the circuit voltage directly to the coil and battery, which can result in damage to the meter and possible harm to you.

A shunt ohmmeter is connected as shown in Figure 2. In this circuit, the unknown resistance R_{X} is shunted (connected in parallel) across the meter. Low values of R_{X} cause lower currents through the meter. High values of R_{X} cause high meter currents.

When the ohmmeter is connected in the shunt position, the indicating needle deflects from right to left in the manner of the ammeter and voltmeter. Zero resistance is on the left. The scale increases from left to right.

**Figure 2.** Schematic diagram of a shunt ohmmeter. Note that the meter reads in the opposite direction of other ohmmeters.

The resistance value is indicated on the ohms scale, which is a nonlinear scale. A nonlinear scale has markings that are not evenly spaced.

The nonlinear scale factor increases as the needle travels from zero resistance to infinite resistance. In Figure 3, a typical ohmmeter scale is represented.

On the right side of the scale is zero. On the left side is infinity (∞). An infinity reading means that the resistance value is so high that it exceeds the capabilities of the ohmmeter to read it.

**Figure 3.** The ohmmeter scale is a nonlinear scale. The scale factor varies in value along the scale. The most accurate readings are taken along the shaded area (the middle 1/3) of the scale.

Notice how the scale factor changes along the ohmmeter scale. On the right side, the small marks between the numbers 0 and 2 represent 0.2 ohms each. On the left side, between the 50 and 70-ohm marks, the small marks represent 5 ohms each.

To take accurate readings of unknown resistance values, it is recommended that the range selector switch is changed until the reading falls in the mid-third of the scale.

An ohmmeter comes with a selection of ranges that can be changed by rotating the selector switch. Typical range values are R × 10, R × 100, R × 1k, and R × 10k. These markings mean that the reading indicated on the ohm scale should be multiplied by 10, 100, 1000, and 10,000 respectively.

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]]>The post Voltmeter: Definition and Working Principle appeared first on Electrical A2Z.

]]>The same basic **meter movement** that is used in an **ammeter** is also used to measure voltage. This is providing that the impressed voltage across the coil never exceeds 0.1 volt, as computed, for full-scale deflection.

To arrange the meter to measure higher voltages, multiplier resistors are placed in series with the meter movement coil using a switch. A meter similar to the meter that measured current is used. Refer to Figure 1.

Voltmeters are always connected in parallel with the device being measured.

**Voltmeter Example**

Follow the steps as the multipliers are computed so that the meter can measure voltages from 0–1 V, 0–10 V, 0–100 V, and 0–500 V.

**Step 1.**

Remember that no more than 0.1 volt is allowed across the meter coil at any time. Therefore, a resistor that will cause a voltage drop of 0.9 V must be placed in series with the meter if the meter is used to measure one volt.

Also, the meter only allows 0.001 A for full-scale deflection. This is the highest current allowed in the coil circuit. The multiplier resistor must produce a 0.9 V drop when 0.001 A flows through it.

\[{{R}_{M}}=\frac{E}{I}=\frac{0.9V}{0.001A}=900\Omega \]

**Step 2.**

To convert the 0–10 volt range, a resistor must be selected to produce a 9.9-volt drop.

\[{{R}_{M}}=\frac{E}{I}=\frac{9.9V}{0.001A}=9900\Omega \]

**Step 3.**

To convert the 0–100 volt range, a resistor must be selected to produce a 99.9 volt drop.

\[{{R}_{M}}=\frac{E}{I}=\frac{99.9V}{0.001A}=99,00\Omega \]

**Step 4.**

Finally, to use the 0–500 volt range, the resistor must cause a 499.9 volt drop.

\[{{R}_{M}}=\frac{E}{I}=\frac{499.9V}{0.001A}=499,00\Omega \]

Again, a switching device is used to select the correct multiple resistors for the range in use. Read the scale on the dial that corresponds to the range selected. The dial on a meter is generally referred to as the range selector switch.

**Figure 1.**

**Step 1**—The multiplier causes an IR drop of 0.9 V.

**Step 2**—The multiplier causes an IR drop of 9.9 V.

**Step 3**—The multiplier causes an IR drop of 99.9 V.

**Step 4**—The multiplier causes an IR drop of 499.9 V.

**Bottom**—Basic setup of a voltmeter. A switch is added to select the range.

**Caution**

A voltmeter is always connected in parallel or across the circuit. To measure a voltage, the circuit does not have to be broken. See Figure 2.

As with the ammeter, when measuring an unknown voltage, always start measuring with the meter set on its highest range. Adjust downward to the proper range to avoid damaging the meter.

In addition, be sure that the leads are connected with the correct polarity. The black lead is negative and the red lead is positive

**Figure 2.** A voltmeter is connected in parallel with the device when taking a voltage reading.

The sensitivity of a meter is a sign of quality. Ohms-per-volt is the unit for measuring sensitivity.

In Step 4 of the previous example, the total resistance of the meter and its multiplier resistance is:

\[\frac{499,900\Omega \left( in\text{ }{{R}_{M}} \right)+100\Omega \left( meter\text{ }resistance \right)}{500,000\Omega \left( total\text{ }resistance \right)}\]

The total amount of resistance in the 500 V range is equivalent to the following:

\[\frac{500,000\Omega }{500V}=100{}^{\Omega }/{}_{V}\]

Using Ohm’s law, I = E/R. The reciprocal of I is R/E. This is the same as the meter sensitivity. Therefore, the sensitivity is equal to the reciprocal of the current required for full scale deflection. For the meter used in the above example:

\[Sensitivity=\frac{1}{0.001\Omega \left( coil\text{ }resistance \right)}=1000{}^{\Omega }/{}_{V}\]

The sensitivity of a meter can be used to gauge meter quality. A quality meter has a sensitivity of at least 20,000 ohms/volt. Precision laboratory meters measure as high as 200,000 ohms/volt.

The** accuracy of the meter** is commonly expressed as a percentage, such as 1 percent. This means that the true value will be within one percent of the scale reading.

Another system of rating meters is the accuracy expressed as a percentage of full-scale reading. A meter may have a rating of ±0.05 percent or less. In general, the smaller the percentage, the higher the quality of the meter.

When a voltmeter is connected across a circuit to measure a potential difference, it is in parallel with the load in the circuit. This situation can introduce errors in voltage measurement. In meters with low sensitivity, this is very common. It is very important to keep this in mind.

In Figure 3, two 10,000 ohm resistors form a voltage divider circuit across a ten-volt source. The voltage drops across both R_{1} and R_{2} are 5 volts each.

If a meter with a sensitivity of 1000 ohms/volt on the ten-volt range is used to measure the voltage across R_{1}, the meter resistance will be in parallel with R_{1}. For now, it is enough to know that the addition of this meter cuts the effective resistance of R_{1} in half. The combined resistance of the meter and R_{1} is equal to:

\[\begin{align} & \frac{{{R}_{1}}+{{R}_{M}}}{2}={{\operatorname{R}}_{effective}} \\ & \frac{10,000}{2}=5000\Omega \\\end{align}\]

**Figure 3.** The meter loads the circuit and introduces an error in the voltage reading.

With the meter connected, the total circuit resistance becomes:

\[\begin{align} & {{\operatorname{R}}_{effective}}+{{R}_{2}}={{R}_{total}} \\ & 5000\Omega +10,000\Omega =15,000\Omega \\\end{align}\]

Using **Ohm’s law**, the current can be calculated at approximately 0.00067 amps. Using Ohm’s law again, E_{R1}= 3.35 V and E_{R2}= 6.7 V. The meter has caused an error of more than one volt due to its shunting effect.

To avoid an excess of errors resulting from this effect, a more sensitive meter should be used. In Figure 4, a 5000 ohms/voltmeter is used.

**Figure 4.** A sensitive meter gives more accurate readings.

In this case, the combined resistance of the meter and R_{1} equals 8333 ohms. The total circuit resistance is 18,300 ohms. Using Ohm’s law, I = 0.00055 amps, E_{R1}= 4.6, and E_{R2} = 5.5 volts. An error of 0.4 volts still exists, but the increased sensitivity of the meter has reduced the error.

Even more costly meters, with a sensitivity of 20,000 ohms/volt, can reduce the error to an amount that would be barely noticed.

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]]>The coil in the **meter movement** of an ammeter is wound with many turns of fine wire. If a **large current** is allowed to flow through this coil, the ammeter will quickly burn out.

In order to measure larger currents, a shunt, or alternate path, is provided for current. Most of the current flows through the shunt, leaving only enough current to safely work the meter movement coil.

The shunt is a precision resistor connected in parallel with the meter coil. The use of shunts is illustrated in Figure 1.

**Figure 1**.

**Step 1**—The voltage that causes the full-scale deflection current is computed.

**Step 2**—The shunt carries 9/10th of the current.

**Step 3**—The shunt carries 49/50th of the current.

**Step 4**—The shunt carries 99/100th of the current.

**Bottom**—Basic setup of an ammeter with three shunt resistors. A switch selects the range.

In Figure 2 you see the proper way to connect an ammeter to an electrical circuit. When an ammeter is connected into the circuit, it becomes part of the circuit in order to allow the current to flow through the meter coil.

To connect an ammeter in a circuit, one usually has to make an open by disconnecting some device in the circuit. This allows you to insert the meter into the circuit.

Notice that you are connecting the meter in series with the circuit or device you are trying to measure.

**Figure 2.** An ammeter is always connected in series with the circuit device being measured. The meter must be connected with proper polarity.

The specification of a certain **meter movement** requires 0.001 A, or one milli-ampere of current, for full-scale deflection of the needle. The ohmic resistance of the meter movement coil is 100 ohms.

**Compute** the shunt resistor values for a meter that will measure four different ampere ranges. The ranges are as follows: 0–1 mA, 0–10 mA, 0–50 mA, and 0–100 mA.

**Step 1. **

First, calculate the voltage required for full-scale deflection on the lowest setting which is 0–1 mA.

$\begin{align} & E=I\left( full\text{ }scale\text{ }current \right)\times R\left( resistance\text{ }of\text{ }coil \right) \\ & E=0.001A\times 100 \\ & E=0.1V \\\end{align}$

The meter will read from 0–1 mA without a shunt. For full scale deflection 0.1 volts is required.

**Step 2. **

To convert this same meter to read from 0–10 mA, a shunt must be connected that will carry 9/10 of the current. Thus, 9 mA of current will travel through the shunt, leaving one milli-ampere to operate the meter.

The first step in the calculation determined that 0.1 V is required for full-scale deflection. The shunt is connected in parallel with the coil, so it will also have 0.1 V applied to it.

Since 0.1 V must be applied across the shunt, and the shunt must also account for 9/10 of the current, you can apply Ohm’s law to calculate the shunt’s resistance.

\[{{R}^{s}}=\frac{E}{I}=\frac{0.1V}{0.009A}=11.1\Omega \]

The meter will require a shunt with a resistance value of 11.1 ohms for the 0–10 mA scale.

**Step 3.**

To convert this meter for the 0–50 mA scale, a shunt must be used that will carry 49/50 of the current, or 49 mA. The computation is the same as in Step 2.

\[{{R}_{S}}=\frac{0.1V}{0.049A}=2.04\Omega \]

**Step 4. **

To convert the meter for the 0–100 mA scale, a shunt must be used that will carry 99/100 of the current, or 99 mA.

\[{{R}_{S}}=\frac{0.1V}{0.099A}=1.01\Omega \]

A shunt with an ohmic value of 1.01 is required for the meter to safely use a 0–100 mA range. Look again at Figure 1. Notice the switching device used to change the ranges of the meter. The correct scale on the range dial must be used to correspond to the selected range.

**Caution**

There are **two important things** to remember for the safety of your ammeter.

**First,** an ammeter must always be connected in series with a circuit device or the power supply. Never connect an ammeter in parallel with the power supply or circuit devices, Figure 3.

As you can see through the meter shunt calculations, the applied voltage to the meter movement coil only required 0.1 V for full-scale deflection. If a voltage greater than 0.1 is used, it will cause excessive current to flow through the coil. This will result in damage to the coil.

To make a series connection usually requires breaking the circuit open or disconnecting a device in order to insert the meter. This allows the current to flow through the meter.

**The second thing** to remember is when the current value you are testing is unknown, start at the highest meter range. This way you will not exceed the highest value on the meter scale during the reading of a circuit.

**Figure 3**. Connecting an ammeter. **Top**—an incorrect way. **Bottom**—the correct way.

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