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### Chapter 10

Basic Measuring Instruments

There are 3 types of instruments we use in 50A:

- Ammeter
- Voltmeter
- Ohmmeter

To understand loading effects

- To understand a practical use for series and parallel circuits
- To what you are doing when you use them

Ammeter

- What is the max current we could put through this ammeter?

- Add a shunt resistor

Ammeter

- If the shunt resistor was 50Ω, how much more current would go through the shunt resistor than the coil in the ammeter?

Ammeter

- What size shunt resistor would we need to make the FSD 10mA?

- So the shunt resistor needs to be 9 times smaller than the coil. = 500/9= 55.6Ω

Ammeter

- What could we do if we want the FSD to be 1000mA?

- So the shunt resistor needs to be 999 times smaller than the coil. = 500/999= .5005Ω

Ammeter

- When you turn the knob to change scales on an ammeter, you are changing the shunt resistor.

Above is a multi-range ammeter whose coil resistance is 100ΩIf we want the FSD to be 10mA we need a 11.11Ω shunt resistor. (Since 11.11 is 9 times less than 100Ω)

- If we want the FSD to be 100mA we need a .11Ω shunt resistor. (Since 11.11 is 9 times less than 100Ω)

What is the total resistance of the ammeter when set to the 10mA scale?1.11Ω||100Ω = 1.098Ω

- Would we want the resistance in an ammeter to be small or large?

Would we want the resistance in an ammeter to be small or large?

- Suppose we placed an ammeter in series to measure current.

- Would the circuit be more effected with a high internal ammeter resistance or low internal resistance?

- AN IDEAL AMMETER HAS 0 OHMS OF INTERNAL RESISTANCE!!

- Since no ammeter is perfect, the result is called loading effects.

What about Voltmeters?

Voltmeter

- The internal resistance of the coil here is 500Ω

- What can we add to the voltmeter to measure more voltage?

- Add a resistor in series to absorb some of the voltage.

Voltmeter

- A voltmeter is an ammeter except instead of a shunt resistor (parallel resistor), a series resistor is used.

Voltmeter

- If we wanted the FSD of the voltmeter to be 100V, how many volts need to be across the series resistor?

- 99.5V

- So the series resistor needs to be 199 times larger than the coil resistance, or 99,500Ω, so that it absorbs 199 times more voltage than the coil.

Multi-range Voltmeter

- What is the total resistance for a 1V FSD?

- What is the total resistance for a 100V FSD?

- What is the total resistance for a 1000V FSD?

Would we want the resistance in an voltmeter to be small or large?

- Suppose we placed a voltmeter across R2 to measure voltage

- Would the circuit be more effected with a high internal voltmeter resistance or low internal resistance?

- AN IDEAL VOLTMETER HAS ∞ OHMS OF INTERNAL RESISTANCE!!

- Since no voltmeter is perfect, the result is called loading effects.

Multi-range Voltmeter

Voltmeter sensitivity is expressed in ohms/volt (Ω/V). A voltmeter is considered more sensitive if it draws less current from the circuit.

The sensitivity of a voltmeter that draws 50uA is the inverse of this, 20,000 ohm per volt.

- You can determine the internal resistance of a voltmeter using the voltmeter sensitivity?

- The sensitivity of our VOM is 20kΩ/V. This is written on the front of the VOM.

- If the VOM range is set to 100V, the internal resistance is 100Vx20kΩ/V = 2,000,000Ω

Multi-range Voltmeter

- What is the internal resistance of the Voltmeter when set to the 10V scale?

- The internal resistance is 10Vx20kΩ/V = 200,000Ω

Multi-range Voltmeter

- What is the internal resistance of the Voltmeter when set to the 1000V scale and you are measurement reads 230V?

- The internal resistance is 1000Vx20kΩ/V = 20MΩ

- The internal resistance of our DMM is always 10MΩ no matter range is selected. (That’s because there is no range to select.

- The internal resistance of other types of DMM’s are not necessarily 10MΩ, but it is always fixed.

51.3V

48.7V

95.2kΩ

- In figure (a), the voltage across each resistor is 50V

- If we wanted to measure the voltage across R2, we would set the VOM to the 100V scale.

- So the internal resistance is 100Vx20kΩ/V = 2,000,000Ω

- When the voltmeter is placed across R2, this changes the resistance. This is called Loading Effects of the VOM

- R2 is now effectively 100kΩ||2MΩ = 95.2KΩ

- So because of loading effects, V2 = 48.7V instead of 50V

What about Ohmmeters?

- An ohmmeter forces a current to flow through an unknown resistance and then measures the resulting current. For a given voltage, the current is determined by the unknown resistance. That is, the amount of current measured by the meter is an indication of the unknown resistance.

What about Ohmmeters?

- The purpose of the battery is to force current through the unknown resistance. This is why you cannot measure resistance in a circuit when it is turned on. Because then you would have to voltage sources battling each other.

- The variable resistor is called the ZERO OHMS adjustment. Its purpose is to compensate for battery aging.

What about Ohmmeters?

Ammeter

- Thus the more current the less resistant, that is why scales are inverted on VOM.
- Ammeter 0 scale is ohmmeter infinity scale.

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