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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 5 V = (4.3 k ) I I = 1.16279 mA V3.3 = (3.3 k ) (1.16279 mA)

V3.3 = 3.837 V Slight discrepancy?

Without the voltmeter, the two resistors are in series.

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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

If Rv , then

Voltmeters should have large resistances.

1=

1+

1

Req R3.3 Rv

1

1

Req R3.3

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.

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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

Req = (RA + R1 + R3.3 )

If RA 0

Req (R1 + R3.3 ) Ammeters should have small

resistances

The ammeter is in series with the 1- and 3.3-k resistors.

For the ammeter to have a minimal effect on the equivalent resistance, its resistance should be small.

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Power

Recall 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.

Power Formulas

P = V * I P = (I * R) * I = R * I2

P = V * ( V / R) = V2 / R 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

Resistor dissipates 100 mW

Resistor dissipates 25 mW

Same for circuit but different for individual resistors

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Power supplies

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.

A kilowatt-hour is a measure of energy

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Power supply

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 Full Towers need more The more cards, drives, etc., the more

power needed

Ex. Power Supply Spec’s

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Ex. Power Supply Spec’s (Cont.)

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12 is for drives (hard-drive or CD), 5 and 3.3 for motherboard electronics

Power limit

Newer power supplies have a SATA connection along with motherboard, Molex and mini connectors

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. 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 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

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 “Line-interactive 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. 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

References

Physics, Paul Tipler http://www.pcguide.com CompTIA A+ Certification, Mike

Meyers

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