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  k  ) I 5 V = (4.3 k  ) I I = mA V 3.3 = (3.3 k  ) ( mA) V 3.3 = 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 R V should be large If R v  , then Voltmeters should have large resistances. 1 = R eq R 3.3 RvRv 1  1 R eq R 3.3 The voltmeter is in parallel with the 3.3-k  resistor and has an equivalent resistance R eq. We want the circuit with and without the voltmeter to be as close as possible. Thus we want R eq to be close to 3.3 k . This is accomplished in R v 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 R A should be small R eq = (R A + R 1 + R 3.3 ) If R A  0 R eq  (R 1 + R 3.3 ) 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 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 * I 2 P = V * ( V / R) = V 2 / 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 = Watt = 2.2 mW Energy = Power * Time ( Joule/sec)*(3 minutes)*(60 seconds/minute) Joules PHY 202 (Blum)11

PHY 202 (Blum)12 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)13 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 PHY 202 (Blum)14

PHY 202 (Blum)15 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 PHY 202 (Blum)16

Ex. Power Supply Spec’s (Cont.) PHY 202 (Blum)17 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 PHY 202 (Blum)18

PFC PHY 202 (Blum)19

PHY 202 (Blum)20 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)21 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)22 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)23 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)24 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)25 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 CompTIA A+ Certification, Mike Meyers PHY 202 (Blum)26