CS25410

1. CS25410 Basic Electricity

2. Basic Electricity • The intention of this lecture is to describe basic electrical characteristics in a qualitative way. • Much of the ‘skill’ is to understand the associated jargon, especially as terminology is often used for things not directly related to its strict definition. • For example “D.C.” literally refers to Direct Current, an electrical current which always flows in the same direction.

3. Charge • Charge is measured in coulombs and is the amount of “electricity” present (or flowing). • Charge can be positive or negative. Like charges repel each other. Unlike charges attract each other. • In the vicinity of a charge there is an electric field. The field points in the direction that a positive charge would move. • Charge moves about and may be stored (e.g. in a capacitor or battery).

4. Current • Current is a flow of charge; the rate of movement of charge through a system. It is analogous to the flow of water in a hydraulic system (in litres/s). It is measured in amps; an amp is quite big. • A.C. is alternating current, where the current flows first one way then the other (repeatedly). • This does not mean it cannot transmit energy. • D.C. is Direct Current where a current flows in the same direction at all times (normally implies constant value as well as direction).

5. Amperes • 1 Ampere is large current in microelectronics!! • 15 Amps can cook a turkey on domestic voltage.

6. Don’t be that turkey!!

7. Milli/micro amps • It’s the volts that jolt • It’s the mills that kill !!

8. Voltage • Voltage is electrical “pressure”. • It is analogous to the pressure of a hydraulic system (say like the height of a reservoir or a waterfall). Voltage is measured in volts; contrary to media opinion, volts do not flow. • 240 volt mains power • 5 volts electronic devices

9. Impedance • Impedance is the ‘resistance’ to current flow. It is a very important concept in any electrical circuit. • Impedance is a general term. For many applications ‘resistance’ is equally applicable, however capacitors and inductors have different properties.

10. Energy and Power • Energy is a measure of work done. Power is the work done per unit time. Thus a battery contains so much energy it can power something for a particular time. • If the power needs of the equipment is reduced then the same energy can power it for longer. (These two terms are not restricted to electrical circuits.)

11. Ohm’s Law • Ohm’s Law • V = I x R • This is only true for resistive loads. Most loads are more complex than this. • In general: • V = I x Z • where Z is the impedance of the load. This may depend (for example) on the frequency of an A.C. signal.

12. Kirchhoff’s Current law • What current goes in, goes out.

13. Capacitors • Capacitors • A capacitor is a charge storage device. • It allows A.C. signals to pass through but blocks D.C. signals. • Everything has some inherent capacitance. • Capacitance is usually the enemy in digital circuits, slowing downsignal edges and therefore circuit operation

14. Resistors • Resistors in series are added together – resistance increases. • In parallel resistance is reduced • In particular if R1 = R2 then R will be half of R1 (or R2). • Measures in Ohms (Kilohms - Megohms)

15. Capacitors • Capacitors in parallel are added together – capacitance increases. • In series capacitance is reduced. • Measured in Farads normally microfarads – μF • Be careful with capacitors which are not in low milli farads. You can get a surprise!

16. Try it out • http://www.yenka.com/en/Yenka_Basic_Circuits/

17. Battery Overview

18. Battery History Since 1946 1946 Neumann: sealed NiCd 1960s Alkaline, rechargeable NiCd 1970s Lithium, sealed lead acid 1990 Nickel metal hydride (NiMH) 1991 Lithium ion 1992 Rechargeable alkaline 1999 Lithium ion polymer

19. Two cells A real battery Another battery More precisely Battery Nomenclature Duracell batteries 9v battery 6v dry cell

20. Battery Characteristics • Size • Physical: button, AAA, AA, C, D, ... • Energy density (watts per kg or cm3) • Longevity • Capacity (Ah) • Number of recharge cycles • Discharge characteristics (voltage drop)

21. Further Characteristics • Cost • Behavioral factors • Temperature range (storage, operation) • Self discharge • Memory effect • Environmental factors • Leakage, gassing, toxicity • Shock resistance

22. Primary (Disposable) Batteries • Zinc carbon (flashlights, toys) • Heavy duty zinc chloride (radios, recorders) • Alkaline (all of the above) • Lithium (photoflash) • Silver, mercury oxide (hearing aid, watches) • Zinc air

23. Standard Zinc Carbon Batteries • Features • Inexpensive, widely available • Inefficient at high current drain • Poor discharge curve (sloping) • Poor performance at low temperatures

24. Heavy Duty Zinc Chloride Batteries • Features (compared to zinc carbon) • Better resistance to leakage • Better at high current drain • Better performance at low temperature

25. Standard Alkaline Batteries • Features • 50-100% more energy than carbon zinc • Low self-discharge (10 year shelf life) • Good for low current (< 400mA), long-life use • Poor discharge curve

26. Alkaline-Manganese Batteries (2)

27. Alkaline Battery Discharge

28. Lithium Manganese Dioxide • Features • High energy density • Long shelf life (20 years at 70°C) • Capable of high rate discharge • Expensive

29. Secondary (Rechargeable) Batteries • Nickel cadmium • Nickel metal hydride • Alkaline • Lithium ion • Lithium ion polymer • Lead acid

30. Nickel Cadmium Batteries • Features • Rugged, long life, economical • Good high discharge rate (for power tools) • Relatively low energy density • Toxic

31. NiCd Recharging • Over 1000 cycles (if properly maintained) • Fast, simple charge (even after long storage) C/3 to 4C with temperature monitoring • Self discharge 10% in first day, then 10%/mo Trickle charge (C/16) will maintain charge • Memory effect Can be overcome by discharges

32. Nickel Metal Hydride Batteries • Features • Higher energy density (40%) than NiCd • Nontoxic • Reduced life, discharge rate (0.2-0.5C) • More expensive (20%) than NiCd

33. NiMH Recharging • Less prone to memory than NiCd • Shallow discharge better than deep Degrades after 200-300 deep cycles Need regular full discharge to avoid crystals • Self discharge 1.5-2.0 more than NiCd • Longer charge time than for NiCd To avoid overheating

34. Lead Acid Batteries • Chemistry Lead Sulfuric acid electrolyte • Features • Least expensive • Durable • Low energy density • Toxic

35. Lead Acid Recharging • Low self-discharge • 40% in one year (three months for NiCd) • No memory • Cannot be stored when discharged • Limited number of full discharges • Danger of overheating during charging

36. Lead Acid Batteries • Ratings CCA: cold cranking amps (Car battery) RC: reserve capacity (minutes at 10.5v, 25amp) • Deep discharge batteries Used in golf carts, solar power systems 2-3x RC, 0.5-0.75 CCA of car batteries Several hundred cycles

37. Lithium Ion Batteries • Features • 40% more capacity than NiCd • Flat discharge (like NiCd) • Self-discharge 50% less than NiCd • Expensive

38. Lithium Ion Recharging • 300 cycles • 50% capacity at 500 cycles

39. Lithium Ion Polymer Batteries • Chemistry Graphite (-), cobalt or manganese (+) Nonaqueous electrolyte • Features • Slim geometry, flexible shape, light weight • Potentially lower cost (but currently expensive) • Lower energy density, fewer cycles than Li-ion