Relay

A relay is an electromagnetic switch. In other words it is activated when a current is applied to it. Normally a relay is used in a circuit as a type of switch (as you will see below). There are different types of relays and they operate at different voltages. When you build your circuit you need to consider the voltage that will trigger it.

EXAMPLE CIRCUIT

This simple circuit activates the relay only when the LDR is dark (covered). This could be used as part of an automatic animal feeder. For instance, if the animal was fed at night the circuit above would activate the relay. A second circuit, connected to the other side of the relay releases food into a dish.

Wirewound

Wirewound resistors are commonly made by winding a metal wire, usually nichrome, around a ceramic, plastic, or fiberglass core. The ends of the wire are soldered or welded to two caps or rings, attached to the ends of the core. The assembly is protected with a layer of paint, molded plastic, or an enamel coating baked at high temperature. Because of the very high surface temperature these resistors can withstand temperatures of up to +450 °C.^[5] Wire leads in low power wirewound resistors are usually between 0.6 and 0.8 mm in diameter and tinned for ease of soldering. For higher power wirewound resistors, either a ceramic outer case or an aluminum outer case on top of an insulating layer is used. The aluminum-cased types are designed to be attached to a heat sink to dissipate the heat; the rated power is dependent on being used with a suitable heat sink, e.g., a 50 W power rated resistor will overheat at a fraction of the power dissipation if not used with a heat sink. Large wirewound resistors may be rated for 1,000 watts or more.

Because wirewound resistors are coils they have more undesirable inductance than other types of resistor, although winding the wire in sections with alternately reversed direction can minimize inductance. Other techniques employ bifilar winding, or a flat thin former (to reduce cross-section area of the coil). For most demanding circuits resistors with Ayrton-Perry winding are used.

Applications of wirewound resistors are similar to those of composition resistors with the exception of the high frequency. The high frequency of wirewound resistors is substantially worse than that of a composition resistor

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RESISTORS IN SERIES AND IN PARALLEL

Resistors can be connected together in two ways to give different overall values. This is especially useful if you do not have a resistor of the correct value and need to make it up from other available ones.

1. Resistors in SERIES - When resistors are connected in series, their values are added together:

R _total=R₁+R₂

For example: 1K+1K+3K9=5K9 (total value)

2. Resistors in PARALLEL -When resistors are connected in parallel, their total resistance is given as:

1/R_total = 1/R₁ + 1/R₂

For example: 1/R_total = 1/1K + 1/1K = 0.5K or 500 ohms

OR= R1 x R2

R1 +R2

= 1 x 1 = 1

1 +1 = 2 = 0.5k

Resistor

Resistors determine the flow of current in an electrical circuit. Where there is high resistance in a circuit the flow of current is small, where the resistance is low the flow of current is large. Resistance, voltage and current are connected in an electrical circuit by Ohm’s Law.

When a resistor is introduced to a circuit the flow of current is reduced. The higher the value of the resistor the smaller/lower the flow of current.

Resistors are used for regulating current and they resist the current flow and the extent to which they do this is measured in ohms (Ω). Resistors are found in almost every electronic circuit.

The value of a resistor can be written in a variety of ways. Some examples are given below:

47R means 47 ohms
5R6 means 5.6 ohms
6k8 means 6800 ohms
1M2 means 1 200 000 ohms