more basic electricity
Post on 01-Jan-2016
30 Views
Preview:
DESCRIPTION
TRANSCRIPT
PHY 202 (Blum) 1
More basic electricity
Non-Ideal meters, Power, Power supplies
PHY 202 (Blum) 2
What makes for ideal voltmeters and
ammeters?
PHY 202 (Blum) 3
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.
PHY 202 (Blum) 4
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.
PHY 202 (Blum) 5
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.
PHY 202 (Blum) 6
Non-Ideal Voltmeter
Ideally the voltmeter should not affect current in resistor.
Let us focus on the resistance of the voltmeter.
PHY 202 (Blum) 7
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.
PHY 202 (Blum) 8
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.
PHY 202 (Blum) 9
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.
PHY 202 (Blum) 10
Simplifying circuits using series and parallel equivalent
resistances
PHY 202 (Blum) 11
Analyzing a combination of resistors circuit
Look for resistors which are in series (the current passing through one must pass through the other) and replace them with the equivalent resistance (Req = R1 + R2).
Look for resistors which are in parallel (both the tops and bottoms are connected by wire and only wire) and replace them with the equivalent resistance (1/Req = 1/R1 + 1/R2).
Repeat as much as possible.
PHY 202 (Blum) 12
Look for series combinations
Req=3k
Req=3.6 k
PHY 202 (Blum) 13
Look for parallel combinations
Req = 1.8947 k
Req = 1.1244 k
PHY 202 (Blum) 14
Look for series combinations
Req = 6.0191 k
PHY 202 (Blum) 15
Look for parallel combinations
Req = 2.1314 k
PHY 202 (Blum) 16
Look for series combinations
Req = 5.1314 k
PHY 202 (Blum) 17
Equivalent Resistance
I = V/R = (5 V)/(5.1314 k) = 0.9744 mA
PHY 202 (Blum) 18
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.
PHY 202 (Blum) 19
Power differences for elements in “Equivalent”
circuits
Resistor dissipates 100 mW
Resistor dissipates 25 mW
Same for circuit but different for individual resistors
PHY 202 (Blum) 20
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.
PHY 202 (Blum) 21
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
PHY 202 (Blum) 22
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
PHY 202 (Blum) 23
Voltage regulator
Most PC’s power supplies deliver 5 V, but most processors need a little less than 3.5 V.
A voltage regulator reduces the voltage going into the microprocessor.
Voltage regulators generate a lot of heat, so they are near the heat sink.
PHY 202 (Blum) 24
VRM/VID
Voltage Regulator Module: a small module that installs on a motherboard to regulate the voltage fed to the microprocessor. It’s replaceable
Voltage ID (VID) regulators are programmable; the microprocessor tells the regulator the correct voltage during power-up.
PHY 202 (Blum) 25
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)
PHY 202 (Blum) 26
UPS (Cont.)
Some UPSs have an automatic backup/shut-down option in case the outage occurs when you're not at the computer.
PHY 202 (Blum) 27
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)
PHY 202 (Blum) 28
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
PHY 202 (Blum) 29
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
top related