More basic electricity Non-Ideal meters, Power supplies PHY 202 (Blum) 1
What makes for ideal voltmeters and ammeters? PHY 202 (Blum) 2
Ideal Meters n 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) 3
Voltmeter Devices in parallel have the same voltage. n Voltmeters are placed in parallel with a circuit element, so they will experience the same voltage as the element. n PHY 202 (Blum) 4
Theoretical calculation 5 V = (1 k + 3. 3 k ) I Without the voltmeter, the two n 5 V = (4. 3 k ) I resistors are in series. n I = 1. 16279 m. A n V 3. 3 = (3. 3 k ) (1. 16279 m. A) n V 3. 3 = 3. 837 V n Slight discrepancy? n PHY 202 (Blum) 5
Non-Ideal Voltmeter Ideally the voltmeter should not affect current in resistor. n Let us focus on the resistance of the voltmeter. n PHY 202 (Blum) 6
RV should be large 1 1 1 = + Req R 3. 3 Rv n If Rv , then 1 1 Req R 3. 3 n 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. PHY 202 (Blum) 7
Ammeter Devices in series have the same current. n Ammeters are placed in series with a circuit element, so they will experience the same current as it. n PHY 202 (Blum) 8
RA should be small ammeter is in series with Req = (RA + R 1 + R 3. 3 ) The 1 - and 3. 3 -k resistors. the n If RA 0 For the ammeter to have a minimal effect on the equivalent resistance, its resistance n Req (R 1 + R 3. 3 ) should be small. n Ammeters should have small resistances n PHY 202 (Blum) 9
Power n Recall n n Voltage = Energy/Charge Current = Charge/Time Voltage Current = Energy/Time n The rate of energy per time is known as power. n It comes in units called watts. n PHY 202 (Blum) 10
Power Formulas P=V*I n P = (I * R) * I = R * I 2 n P = V * ( V / R) = V 2 / R n n n Example, a 5. 2 -kΩ resistor has a 0. 65 m. A current for 3 minutes? What is the corresponding power? The corresponding energy? Power = (5200)*(. 00065)2 = 0. 002197 Watt = 2. 2 m. W Energy = Power * Time (. 002197 Joule/sec)*(3 minutes)*(60 seconds/minute) 0. 39546 Joules PHY 202 (Blum) 11
Power differences for elements in “Equivalent” circuits Same for circuit but different for individual resistors PHY 202 (Blum) Resistor dissipates 25 m. W Resistor dissipates 100 m. W 12
Power supplies n n n 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) 13
A kilowatt-hour is a measure of energy PHY 202 (Blum) 14
Power supply n n n 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 n n PHY 202 (Blum) Full Towers need more The more cards, drives, etc. , the more power needed 15
Ex. Power Supply Spec’s PHY 202 (Blum) 16
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 PHY 202 (Blum) 17
Power Factor PHY 202 (Blum) 18
PFC PHY 202 (Blum) 19
Surge protection Takes off extra voltage if it gets too high (a surge). n Must be able to react quickly and take a large hit of energy. n They are rated by the amount of energy they can handle. n n I read that one wants at least 240 Joules PHY 202 (Blum) 20
UPS n Uninterruptible Power Supply, a power supply that includes a large battery to continue supplying power during a brown-outs and power outages n n Line conditioning A typical UPS keeps a computer running for several minutes after an outage, allowing you to save and shut down properly n Recall the data in RAM is volatile (needs power) PHY 202 (Blum) 21
UPS (Cont. ) n Some UPSs have an automatic backup/shut-down option in case the outage occurs when you're not at the computer. PHY 202 (Blum) 22
SPS n n n 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) PHY 202 (Blum) 23
On-line n An on-line UPS avoids these switching power lapses by constantly providing power from its own inverter, even when the power line is fine. n n Power (AC) Battery (DC) through inverter (back to AC) On-line UPSs are better but much more expensive PHY 202 (Blum) 24
Laser printers and UPS Don’t put a laser printer on a UPS n Laser printers can require a lot of power, especially when starting, they probably exceed the UPS rating n PHY 202 (Blum) 25
References Physics, Paul Tipler n http: //www. pcguide. com n Comp. TIA A+ Certification, Mike Meyers n PHY 202 (Blum) 26
Скачать презентацию More basic electricity Non-Ideal meters Power supplies PHY
30b39b28aaab2cdf07fb3ae780e05ef9.ppt