ENTC 4350 BIOMEDICAL INSTRUMENTATION I

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ENTC 4350BIOMEDICAL INSTRUMENTATION I

• POWER DISTRIBUTION

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(No Transcript)
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• The power company, in providing electricity to
  the home, hospital, or laboratory, uses
  alternating current (AC) instead of direct
  current (DC) because of the ease with which AC
  can be transformed from one voltage to another,
• For example, from the high voltage of the
  transmission lines to the 115 volts of the home.

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• This is done with a device called a transformer.
• Note that the power company uses high voltages at
  low currents for the long-distance transmission
  of electricity in order to hold down IR losses.
• Whereas lower voltages115 or sometimes 230
  voltsare used for safety reasons in places where
  people live and work.

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• At this point, you are probably dying with
  curiosity to know what the differences are
  between AC and DC power.

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• The following voltage-versus-time signals in are
  all considered DC or direct current, because the
  voltage does not cross the zero axis.

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• In general, all signals that do not cross the
  zero axis are considered DC,
• even though they might vary somewhat with time.
• Where we have a regularly varying AC voltage
  superimposed on a DC voltage, we will call it DC
  with an AC component.

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• In an AC or alternating current signal, the
  voltage is first positive and then negative.
• For every positive peak, there is a negative one
  of the same size and shape.
• It is the type of signal that is most
  conveniently transformed to higher or lower
  voltages.

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Voltage Indicators
Vp
Vrms
Vpp
Vrms Vp .707 (Sine wave)
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• For AC waveforms, we speak of its frequency, f.
• This frequency is just the number of cycles the
  voltage makes each second
• Each cycle is composed of one complete positive
  peak and one complete negative one,
• That is, the voltage must go up, come down, and
  come back up to zero again to complete one cycle.

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Frequency and Period
Period, T
f1
(
)
t
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• The unit most commonly used for frequency is the
  hertz (Hz).
• Named after Heinrich Rudolph Hertz, a 19th
  century German physicist.
• One hertz is simply one cycle per second,
• 60 Hz is 60 cycles per second.

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• We will spare you any funny stories about how EEs
  came to call cycles per second hertz, because
  it only hertz when we laugh.

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• There might be lots of other AC frequencies
  about, but 60 Hz is what is used for power in the
  United States, and thus it is the most common
  frequency.
• It takes exactly 1/60 of a second for one
  complete cycle, which is the same as saying that
  there are 60 cycles per second.

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• An apparent problem arises in talking about AC.
• Namely, what voltage value should we use?
• It is positive at one time, negative at another
  time, and zero in between.
• This was a real hassle for Thomas Edison, and he
  never quite got used to the idea of AC power.

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• We now have a simple answer to this problem,
  though the mathematics by which it was obtained
  are complex.

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• We begin by noting that it would be convenient to
  have our equations such as W I2R and I VR
  yield the same results with either DC or AC.
• With DC, there is no question about what values
  to use for V or I, because when they are given,
  they are either constant or vary so slowly that
  they may be assumed constant.
• With AC, however, this is not the case, and after
  some mathematical manipulations, it was decided
  that the so-called root-mean-square or RMS values
  would be used.

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• The RMS value is obtained by multiplying the peak
  value of the voltage or current by 0.707.
• If the peak voltage is 163 volts, for example,
  the RMS value is 115 volts

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• The next question may be, how does the use of RMS
  values make our electrical equations come out all
  right?
• This RMS business may seem like some kind of a
  trick, and it is, but it makes our equations
  work.

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• Consider two 40-watt light bulbs, one operating
  on AC and the other on DC.
• The brightness B of each bulb is a function of
  the current B kI2,
• where k is a constant.

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• When the bulbs are equally bright, we can assume
  that the AC and the DC currents are equal.
• The power output of the DC bulb is
• W IV.
• If W 40 watts and V 115 volts, I must he
  0.348 amp.

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• If this is the current through the DC bulb, what
  is the current through the AC bulb?
• Since the bulbs are equal in brightness, they
  must have equal effective currents.
• It follows that I(DC) must equal I(AC).

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• In this case, the value of I(AC) is 0.348 amp
  RMS.
• The peak value of the current is about 0.5 amp,
  but this is academic.
• The RMS value of 0.348 amp works in our equation
  because 0.348 amp times 115 volts (RMS) equals 40
  watts (of any kind).
• The peak values of 0.5 amp and 163 volts will not
  work, because the product of these would give
  about 82 watts, but the bulb in fact would burn
  no brighter.

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• Wattage or power is the same, then. whether AC or
  DC is used, so long as the RMS values of the AC
  voltages or currents are used in its computation.
  
• Note, too, that the concept of resistance, when
  applied to resistors, stays the same in either AC
  or DC.
• The point is that the AC resistance of a resistor
  is the same as its DC resistance. and there is no
  such thing as RRMS.

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RMS is a measure of a signal's average power.
Instantaneous power delivered to aresistor is
P v(t) /R. To get average power, integrate
and divide by the period

• RMS Root-Mean-Square

t0T
2
2
2
t0T
Pavg 1 1 v (t)dt (Vrms)
Solving for Vrms
2
Vrms 1 v (t)dt
R T R
T
t0
t0
An AC voltage with a given RMS value has the
same heating (power) effect as a DC voltage with
that same value.
All the following voltage waveforms have the
same RMS value, and should indicate 1.000 VAC on
an rms meter
1
1.733 v
1.414 v
1 v
1 v
Waveform Vpeak Vrms
Sine 1.414 1
Triangle 1.733 1
Square 1 1
DC 1 1
All 1 WATT
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ENTC 4350

• POWER DISTRIBUTION

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The Generator or Electricity Pump

• The power plant has a big coil of wire built into
  a complex device called a generator.
• When the coil is rotated through a magnetic
  field, the electrons in the coil move in response
  to the magnetic force.
• We could say that the electron pressure is
  increased at one end of the wire and reduced at
  the other.
• If the coil is attached through wires to a light
  bulb. which we call a load, the current flows out
  of one end of the wire (the high-pressure side)
  and back to the low-pressure side through the
  load.

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• We may note that the same type of situation
  exists with the heart.
• The heart does not produce blood
• The heart just raises the pressure so that the
  blood can flow through the body.

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• Similarly, our friendly electric company does not
  produce electricity
• The copper wires are already full of electrons.
• The power company raises the electron pressure so
  that the electrons will flow through the wires to
  the hospital to provide
• power,
• heat, and
• light.

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• Moving the wire through the magnetic field raises
  the electrical pressure (voltage) at one end of
  the wire and reduces it at the other.
• Notice that all the electricity always comes back
  to the generator.
• The power company sells it at high pressure and
  gets it back at low pressure.
• All the coal, oil, or water power they use is
  needed just to push the coil through the magnetic
  field.

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• The power company uses not one, but three wires
  or coils that follow each other through the
  magnetic field.
• At any given instant, when one coil is at high
  pressure, the other two coils will be at a lower
  pressure.
• At every instant of time, then, there will be at
  least two coils that are not at zero voltage.

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• The output from all three coils is carried
  through the city as three phase power.

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Ground and Neutral

• The generator has three output wires (one from
  each coil) plus a fourth wire called ground.

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• Ground is exactly that.
• It is a big, copper plate buried in the earth at
  the power plant.
• One end of each of the three coils is connected
  to the copper plate.
• This will insure that all the electricity that
  the power company sends out comes back home
  again.

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• The points to remember are
• First, the power company distributes electricity
  by means of this system of three hot wires plus
  one ground wire, and
• Second, each of the three hot wires can deliver
  power directly to a load.

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• Most homes or hospitals use, the three-wire
  system converted into the more familiar hot,
  neutral, and ground configuration.

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• The ground wire is an important part of the
  hospital safety program.
• It is the return pathway for any electricity that
  might leak out in a defective appliance.

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• You may be curious as to how the three hot wires
  and one ground wire turn into the one hot, one
  neutral, and one ground wire.
• But simply note that
• the neutral is the normal path by which
  electricity moves back toward the power plant,
  and
• the hot and ground wires are effectively the same
  as shown before.

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• We note that eventually all four wires arrive at
  the hospital via power poles or underground
  cables.
• On the power poles, the ground wire is carried at
  the top of the pole in the hope that if lightning
  strikes, it will hit the ground wire and go into
  the earth.

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• Power is moved across country through
  transmission lines at a higher voltage than that
  used in the home or hospital, which is done to
  save money.
• Delivering electricity at high pressure is
  economical because the loss in the wires is
  reduced.
• Nature discovered the idea and used it first.

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• The advantage of a high P and a low Q lies in the
  fact that the pressure drop P QR.
• Thus, for a given resistance R, we can reduce the
  pressure drop by using a small value of Q.
• This effect appears again in the heat production
  equation, W Q2R.
• Once again, for a given R, we can hold down the
  value of W by going to a smaller value of Q.

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• Nature has taken advantage of the previous
  relationship by making the arteries of a strong,
  tough material that can withstand high pressures.
  
• Because the blood flow in the arteries is quite
  fast, only a limited number of arteries

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• In the vascular system, the number of veins is
  far greater than the number of arteries, and the
  venous area is greater than the arterial area.
• This makes sense if we note that the power output
  of the heart is given by the equation
• Wo PQ.
• This means that we can reduce the flow Q as long
  as we increase the pressure P to keep the product
  PQ the same. Q. are needed.

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• With the veins, however, it is another story.
• The pressure is low and the flow is slow, and
• a large venous area is required.
• The veins do not have to be as strong as the
  arteries,
• which is why nurses prefer to stick a vein
  instead of an artery.

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• The power company, then, transmits power between
  cities at high voltage (300,000 volts) to save
  money on copper wire.
• At the hospital, these voltages are reduced to
  230 and 115 volts for safer application.

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• Electricity comes in through the panel board
  (which is usually in the utility closet), then it
  goes through fuses or circuit breakers, and it
  finally passes through wires behind the walls or
  ceilings to the receptacles.
• The receptacles are where you actually plug in to
  get the electricity.
• Notice that the power comes in through the hot
  lead and goes back to the power company via the
  neutral lead.
• Of course, the electricity only flows if
  something is plugged in and turned on.

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• It may help if you think of the hot wires as
  arteries and the neutral wires as the veins.
• Blood flows from the heart to the arteries and
  returns to the heart via the veins.
• The situation with electricity is much the same,
  only the names are different.

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• A three-wire, female receptacle is depicted, into
  which we are going to plug a two-wire appliance
  (a heater).

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• We can think of the electricity as flowing out of
  the hot side of the receptacle, through the
  appliance, and back to the power plant via the
  neutral lead.
• If there are one or more appliances connected to
  the various outlets, there will be a significant
  current flow in the neutral wire.

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• This in turn means that a voltage must exist
• no flow occurs without a pressure to push it
  along.
• From this, we may conclude that the neutral wire
  is not the same as ground, electrically speaking.
  
• Neutral is the line through which the
  low-pressure electricity flows back to the power
  plant.
• The ground wire is at the potential of the
  earth.

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• In modern buildings, you will find three-wire or
  grounded outlets.
• The openings are arranged as shown.
• Note the unusual shape of the ground connections
• This enables us to identify the ground connection.

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• The ground wire is what makes the difference
  between shock or no shock if the appliance is
  defective.
• If all home or hospital appliances were
  guaranteed to be always perfect, no ground wire
  would be needed.

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• The defective heater has been plugged into a
  grounded receptacle and a person has one hand on
  the heater and the other on a metal sink.
• The sink is connected to earth ground by means of
  the water pipes.

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What happens to the person?

• Nothing!
• Some of the electricity flows out of the hot side
  of the receptacle, through the appliance, and
  back to the power plant via the neutral wire, but
  the electricity that leaks off into the defective
  appliance goes back through the ground wire.
• The nurse is quite safe, but if there were no
  ground wire, the result might have been much
  different.

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• The electricity that leaked out through the
  defect follows the path of least resistance to
  whatever ground it could find.
• Without an instrument ground, the person with a
  hand on the well-grounded sink might experience
  the shocking revelation that she was part of the
  pathprovided she lived to realize that fact.