Transcript More basic electricity

More basic electricity
Non-Ideal meters, Power, Power
supplies
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What makes for ideal
voltmeters and ammeters?
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Ideal Meters

Ideally when a voltmeter is added to a
circuit, it should not alter the voltage or
current of any of the circuit elements.
These
circuits
should be
the same.
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Voltmeter
Devices in parallel have the same
voltage.
 Voltmeters are placed in parallel with
a circuit element, so they will
experience the same voltage as the
element.

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Theoretical calculation
5 V = (1 k + 3.3 k ) I
Without the
voltmeter, the two
 5 V = (4.3 k ) I
resistors are in
series.
 I = 1.16279 mA
 V3.3 = (3.3 k ) (1.16279 mA)
 V3.3 = 3.837 V
 Slight discrepancy?
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Non-Ideal Voltmeter
Ideally the voltmeter should not affect
current in resistor.
 Let us focus on the resistance of the
voltmeter.

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RV should be large
1
Req

=
1
R3.3
+
If Rv  , then
1
1

Req
R3.3

1
Rv
The voltmeter is in parallel with
the 3.3-k resistor and has an
equivalent resistance Req.
We want the circuit with and without the
voltmeter to be as close as possible.
Thus we want Req to be close to 3.3 k.
This is accomplished in Rv is very large.
Voltmeters should have large resistances.
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Ammeter
Devices in series have the same
current.
 Ammeters are placed in series with a
circuit element, so they will experience
the same current as it.

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RA should be small
ammeter is in series with
Req = (RA + R1 + R3.3 ) The
the 1- and 3.3-k resistors.
 If RA  0
For the ammeter to have a minimal effect on
the equivalent resistance, its resistance
 Req  (R1 + R3.3 ) should be small.
 Ammeters should have small
resistances

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Power

Recall
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Voltage = Energy/Charge
Current = Charge/Time
Voltage  Current = Energy/Time
 The rate of energy per time is known as
power.
 It comes in units called watts.

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Power Formulas
P=V*I
 P = (I * R) * I = R * I2
 P = V * ( V / R) = V2 / R
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Example, a 5.2-kΩ resistor has a 0.65 mA current for 3 minutes?
What is the corresponding power? The corresponding energy?
Power = (5200)*(.00065)2 = 0.002197 Watt = 2.2 mW
Energy = Power * Time
(.002197 Joule/sec)*(3 minutes)*(60 seconds/minute)
0.39546 Joules
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Power differences for elements in
“Equivalent” circuits
Same for circuit but
different for individual
resistors
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Resistor
dissipates
25 mW
Resistor
dissipates
100 mW
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Power supplies
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Supplies power to a computer
Transforms 120 V (wall socket voltage) down to
voltages used inside computer (12 V, 5 V, 3.3 V).
Converts the AC current to DC current (rectifies).
Regulates the voltage to eliminate spikes and surges
typical of the electricity found in average wall socket.
Sometimes needs help in this last part, especially
with large fluctuations.
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A kilowatt-hour is a measure of energy
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Power supply
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Power supplies are rated by the number of
watts they provide.
The more powerful the power supply, the
more watts it can provide to components.
For standard desktop PC, 200 watts is
enough
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Full Towers need more
The more cards, drives, etc., the more power
needed
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Ex. Power Supply Spec’s
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Ex. Power Supply Spec’s (Cont.)
Newer power supplies have a
SATA connection along with
motherboard, Molex and mini
connectors
Power limit
12 is for drives (harddrive or CD), 5 and
3.3 for motherboard
electronics
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Power Factor
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PFC
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Surge protection
Takes off extra voltage if it gets too
high (a surge).
 Must be able to react quickly and take a
large hit of energy.
 They are rated by the amount of energy
they can handle.

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I read that one wants at least 240 Joules
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UPS

Uninterruptible Power Supply, a power supply
that includes a large battery to continue
supplying power during a brown-outs and
power outages
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Line conditioning
A typical UPS keeps a computer running for
several minutes after an outage, allowing you
to save and shut down properly

Recall the data in RAM is volatile (needs power)
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UPS (Cont.)

Some UPSs have an automatic
backup/shut-down option in case the
outage occurs when you're not at the
computer.
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SPS

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Standby Power System: checks the power
line and switches to battery power if it
detects a problem.
The switch takes time (several milliseconds –
that’s thousands if not millions of clock
cycles) during the switch the computer gets
no power.
A slight improvement on an SPS is the “Lineinteractive UPS” (provides some conditioning)
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On-line

An on-line UPS avoids these switching
power lapses by constantly providing
power from its own inverter, even when
the power line is fine.

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Power (AC) Battery (DC) through
inverter (back to AC)
On-line UPSs are better but much more
expensive
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Laser printers and UPS
Don’t put a laser printer on a UPS
 Laser printers can require a lot of
power, especially when starting, they
probably exceed the UPS rating
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References
Physics, Paul Tipler
 http://www.pcguide.com
 CompTIA A+ Certification, Mike Meyers
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