First some basic terms:
|Resistance (similar to Impedance)||R||Ohms (Ω)|
Voltage is analogous to mechanical force, and is sometimes called Electro-Motive Force or EMF. It represents potential energy. Like the concept of zero potential energy, the concept of zero volts is an arbitrary designation. Only differences between voltages are significant. Therefore zero volts is only a reference (called Ground) used to measure voltage differences.
Current is analogous to mechanical velocity. In order to transfer power (or even information), some current must flow. Useful current in circuits must always flow through a complete loop, not just one wire.
Resistance is analogous to mechanical friction or damping. Just as friction resists the motion of an object due to a net force acting on it, resistance resists the tendency of current to flow due to a voltage difference.
In a resistive circuit, voltage and current are related by the most important relationship in electrical engineering, Ohm's Law:
|V = IR|
Impedance is a type of resistance to changes in voltage or current. It is defined by differential equations.
Electrical power can be found as the product of voltage and current, P = VI. This is analogous to mechanical power, which is the product of instantaneous force and velocity, P = Fv.
If you subsitute in for Ohm's Law, it becomes
|P = I²R|
|P = V²/R|
These equations only apply when computing the power which is converted to heat in a resistor (or another device which acts like a resistor, such as an electric heater). In other devices such as transistors and diodes, power should be computed as P = VI.
A circuit consists of devices that are connected by wires. This usually includes a power source, passive devices, and active devices.
Parallel and Series terms are sometimes used to refer to how two devices are connected to eachother.
Circuits usually need an external source of power to operate.
Constant Voltage Source: A constant voltage output is maintained by varying the current depending on the circuit resistance (V=IR).
Constant Current Source: A constant current output is maintained by varying the voltage as dictated by Ohm's Law.
Direct Current(DC) Source: The controlled variable (Voltage or Current) is kept constant. DC power is generally more dangerous than AC power.
Alternating Current(AC) Source: The controlled variable (Voltage or Current) alternates or changes between positive and negative, usually in a sine wave.
The most basic components of a circuit are passive devices. Passive Components are components that do not require a power source other than the signals (wires) to which they are connected.
A Resistor provides resistance to current. Higher resistance will decrease the current for a given voltage. Resistance is measured in Ohms (Ω). An equivalent resistance (Req) can be calculated for resistors in series or parallel. Adding Reistors in series increases the resistance; adding reistors in parallel decreases the resistance.
Measured in Farads(F). A capacitor “tries” to keep the voltage the same. It slows any voltage change. The higher the capacitance, the longer the change takes. Capacitors are often used as simple noise filters, usually on lines carrying power. Capacitors will block DC current and pass AC current. For DC analysis a capacitor can be approximated by an open circuit. For AC analysis they can be approximated by a short ciruit.
Measured in Henrys(H). Inductors “try” to keep the current the same. This slows any current change. The higher the inductance, the slower the change. Inductors, like capacitors, are also used in noise filters. For analysis, an inductor can be replaced by a short circuit in DC circuits. They can be approximated by an open ciruit in AC circuits.
This is a special diode whose breakdown voltage is set to a controlled value. Zener diodes are usually used as voltage references. If a zener diode is placed in series with a properly chosen resistor and supplied with a source voltage higher than the breakdown voltage, the voltage across the zener diode will remain at the breakdown voltage.
Zener diodes are sometimes used as simple surge suppressors. Again placed in series with resistors, the zener diode can prevent the voltage input to a FET or integrated circuit from exceeding the diode's breakdown voltage.
An LED is essentially the same as a diode, but with the added property of giving off light (photons) when forward biased. Always use a resistor in series to limit the current.
High-brightness LED's are becoming useful as light sources at the time of this writing (2006). They are more efficient than conventional light bulbs (though not quite as efficient as modern fluorescent lights), and they last longer than both. White LED's, which are actually made by combining blue LED light sources with yellow-green flourescent materials, are available with power ratings up to about 3-5 watts (and they produce as much light as 10 watt halogen bulbs).
A transistor's characteristics are relatively complex, allowing the transistor to be used in many different ways. The simplest use of a transistor is as a switch. When used in this way, it can be used to turn on and off a load. This allows a low current carrying signal line to control a higher current device.
A transformer operates on AC signals only. It is simply two coils of wire with the two coils having no direct electrical connection. The changing voltage/current in one coil is induced into the second coil. Any DC (constant) component of the input voltage/current is removed from the output signal. The ratio of the number of turns of wire between the two coils determines the scale factor of the input voltage/current to the output voltage/current. The voltage/current can be increased or decreased in this way. If the voltage is increased, then the current is decreased.
Used to convert AC voltage to DC voltage. The rectifier is made from a set of diodes.
After being rectified, the power is usually filtered (or “smoothed”) by a circuit containing capacitors and/or inductors. Learn more
Active devices require a power source (usually 5V or 3.3V). This includes any digital logic devices.
An opamp is a very versatile analog active device. When combined with resistors and capacitors, they can be used as voltage adders, integrators, differentiators, as well as anolog amplifiers (aka multipliers). They usually require a symmetric positive and negative dc power source as well a ground reference, although op-amps designed for use with a single voltage supply are available.
|Beginners often forget that no opamp can produce an output voltage which is higher than the positive voltage supply or lower than the negative supply. In fact, if a typical opamp is powered with +15V and -15V supplies, its output is limited to about +13V to -13V.|
Opamps are also useful as a type of buffer, called a voltage follower. A voltage follower presents a high impedance connection for the volatage input, and a low impedance output. This basically means that the input voltage can be set and maintained with low current draw, while the ouput can source or sink a much higher current while maintaining the same voltage.
Voltage regulators are complex devices containing transistors, resistors, opamps, and capacitors. They usually take a higher voltage that can be noisy (varying), and outputs a fairly constant and usually fixed voltage. An example would be using a battery pack (say 6V, 7.2V, or 9V) to power a circuit that runs at 5V. A regulator would be used in between the battery and the circuit to drop the voltage down to 5V and provide a constant stable ouptut. There are various types of regulators.
Analog circuits operate on continously varying voltage or current, ie. voltage that has an infinite number of possible values. Under the right conditions, analog circuits can be more accurate and faster than digital circuits, but analog circuits are more susceptible to noise.
Many sensors give an anolog voltage output proportional to the variable being measured. Usually the analog voltage is fed into a Analog to Digital Converter (ADC) which converts the analog votlage to a binary value (1's and 0's). The ADC may be part of the microcontroller or a separate chip.
Digital logic generally refers to 0's and 1's, or two well defined states (binary). Digital logic is used because it offers far superior tolerance to noise than analog circuits.
Although binary systems have only two possible states, digital circuits/signals can be considered to have 3 output states.
Input pins are always set to a High-Z state to allow another device to control the voltage level and provide the input state.
Binary logic functions are implemented electrically using combinations of transistors and/or diodes. The electrical devices are referred to as gates. The symbols are shown below.
Components such as resistors and capacitors state their values in sometimes crytic methods. Following are some of the ways that are used to label component values.
Most resistors are labeled using 4 color bands. This uses 2 significant digits, one multiplier, and one tolerance value.
Capacitors are usually labeled using three numbers and optionally a letter to designate tolerance. The first two numbers are significant digits, the third is a multiplier and designates the number of zeros to add to the first two digits to get the value in pico Farads (1pF = 10-12 Farads).
See Capacitor Value Codes for detailed information on more capacitor code formats.