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Chapter 10. Basic Measuring Instruments. There are 3 types of instruments we use in 50A: Ammeter Voltmeter Ohmmeter. Voltmeters and Ohmmeters are made using ammeters. To understand loading effects To understand a practical use for series and parallel circuits

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Chapter 10

Chapter 10

Basic Measuring Instruments


Voltmeters and ohmmeters are made using ammeters

Voltmeters and Ohmmeters are made using ammeters


Why is it important to know how they work

Why is it important to know how they work?



What can we add to allow more current to flow through ammeter

Ammeter

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

What can we add to allow more current to flow through ammeter?

  • Add a shunt resistor


So what is the full scale deflection fsd with the added 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?

So what is the Full Scale Deflection (FSD) with the added shunt resistor?


We want 9ma to go through the shunt and 1ma to go through the coil

Ammeter

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

(We want 9mA to go through the shunt and 1mA to go through the coil)

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


We want 999ma to go through the shunt and 1ma to go through the coil

Ammeter

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

(We want 999mA to go through the shunt and 1mA to go through the coil)

  • 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.


If we want the fsd to be 10ma we need a 11 11 shunt resistor since 11 11 is 9 times less than 100

If 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)


1 11 100 1 098

1.11Ω||100Ω = 1.098Ω

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


We do not want to affect the circuit when using an ammeter

  • 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!!

We do not want to affect the circuit when using an ammeter.

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


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RVOTD large?http://www.youtube.com/watch?v=lkwh4ZaxHIA&feature=email


Since the max current through this is 1ma the fsd if measuring voltage is 500 x1ma 5v tiny range

What about Voltmeters? large?

Voltmeter

  • The internal resistance of the coil here is 500Ω

Since the max current through this is 1mA, the FSD if measuring voltage is 500Ωx1mA = .5V (Tiny range)

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

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


Voltmeter large?

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


Voltmeter large?

  • 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 large?

  • 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?


We do not want to affect the circuit when using a voltmeter

  • 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!!

We do not want to affect the circuit when using a voltmeter.

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


Multi-range Voltmeter large?

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 large?

  • 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 large?

  • 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 large?

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? large?

  • 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? large?

  • 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? large?

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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