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.

• 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).

• 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).

1 Ampere is large current in microelectronics!!

15 Amps can cook a turkey on domestic voltage.

It’s the volts that jolt It’s the mills that kill !!

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

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.

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.)

• 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.

What current goes in, goes out.

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

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)

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!

http://www.yenka.com/en/Yenka_Basic_Circuits/

1946 Neumann: sealed NiCd1960s Alkaline, rechargeable NiCd1970s Lithium, sealed lead acid1990 Nickel metal hydride (NiMH)1991 Lithium ion1992 Rechargeable alkaline1999 Lithium ion polymer

Duracell batteries 6v dry cell9v battery

More precisely

Two cells A real battery Another battery

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)

Cost Behavioral factors

◦ Temperature range (storage, operation)◦ Self discharge◦ Memory effect

Environmental factors◦ Leakage, gassing, toxicity◦ Shock resistance

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

Features+Inexpensive, widely available◦ Inefficient at high current drain◦ Poor discharge curve (sloping)◦ Poor performance at low temperatures

Features (compared to zinc carbon)+Better resistance to leakage+Better at high current drain+Better performance at low temperature

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

Features +High energy density+Long shelf life (20 years at 70°C)+Capable of high rate discharge◦ Expensive

Nickel cadmium Nickel metal hydride Alkaline Lithium ion Lithium ion polymer Lead acid

Features+Rugged, long life, economical+Good high discharge rate (for power tools)◦ Relatively low energy density◦ Toxic

Over 1000 cycles (if properly maintained) Fast, simple charge (even after long

storage)C/3 to 4C with temperature monitoring

Self discharge10% in first day, then 10%/moTrickle charge (C/16) will maintain charge

Memory effectCan be overcome by discharges

Features+Higher energy density (40%) than NiCd+Nontoxic◦ Reduced life, discharge rate (0.2-0.5C)◦ More expensive (20%) than NiCd

Less prone to memory than NiCd Shallow discharge better than deep

Degrades after 200-300 deep cyclesNeed regular full discharge to avoid crystals

Self discharge 1.5-2.0 more than NiCd Longer charge time than for NiCd

To avoid overheating

ChemistryLeadSulfuric acid electrolyte

Features+Least expensive+Durable◦ Low energy density◦ Toxic

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

RatingsCCA: cold cranking amps (Car battery)RC: reserve capacity (minutes at 10.5v, 25amp)

Deep discharge batteriesUsed in golf carts, solar power systems2-3x RC, 0.5-0.75 CCA of car batteriesSeveral hundred cycles

Features+40% more capacity than NiCd+Flat discharge (like NiCd)+Self-discharge 50% less than NiCd◦ Expensive

300 cycles 50% capacity at 500 cycles

ChemistryGraphite (-), 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