Diodes

Diode












When N and P-type semiconductor materials are joined together to form a single crystal a Diode is created. The diode allows electron flow in one direction only.

A diode has a forward bias when the Anode (P-material) is connected to B+ and the Cathode (N-material) is connected to B.
Reversing the source voltage on the diode will result in current flow stoppage. This is called reverse bias.

Diodes are rated for specific voltage and current flow. The diode can not withstand unlimited forward bias and current flow.

Zener Diode







A diode which will allow a specified amount of reverse flow current is called a Zener Diode. If the breakdown voltage of the zener diode is 6 volts, at 6 volts and above the zener diode will allow reverse current flow with no damage to the diode.

Below the breakdown voltage the zener diode will function as a normal diode and allow current flow in only one direction.

Zener diodes are often used in charging systems to rectify or convert AC current to DC. Like the diode, zener diodes are rated for specific voltage, current and reverse current.

Light Emitting Diode







Light-Emitting Diodes (LED) emit visible light when forward biased. As current flows through the diode, electrical energy is converted into visible light that is radiated through the thin positive material layer in the diode.

Transistors

The Transistor is a diode with some additional semiconductor material. The transistor contains two PN junctions, compared to one in a diode.












Transistors can be constructed in two ways: the P section can be sandwiched between two N sections forming a NPN transistor, or the N section sandwiched between two P sections forming a PNP transistor

The three sections of the transistor are called the Emitter, the Base and the Collector.

Current applied to the base will flow through the transistor. Current flows through the NPN in one direction and through the PNP in the opposite direction.

Transistors are used to control current flow, act as a switch or as an amplifier to vary the current output dependent on base voltage variations.

A transistor allows control of large currents with small current signals.

Relays




A Relay is a switch that uses electromagnetism to physically move the contacts.

A small of amount of current flow through a relay coil moves an armature and opens or closes a set of points.

The points control the flow of a larger amount of current through a separate circuit.

Think of the two sides of a relay independently.

- Control side: Which includes the B+(KLS6) and B-(KL85) for the coil that creates the magnetic force. If this side of the relay fails open the work side points will remain in their at rest position.
- Work side: Which includes the B+ input power (KL3O) and the Relay output (KL87). Failure of this side of the relay in the closed position (sticking points) will result in constant current flow.

BMW uses relays with various numbers of pins (3,4,5 pin) and pin configurations (normally open, normally closed and changeover type). Do not substitute relays. Always replace with the same type (e.g. DME Main Power Relay, Secondary Air Pump Relay and Rear Window Defroster Relay.). Refer to ETM 0140.

Solenoids







A Solenoid, like a relay, uses current flow and electromagnetism to produce mechanical movement. Solenoids consist of a coil winding around a spring loaded metallic plunger.

When current flows through the winding, the magnetic field attracts the movable plunger, pulling it against spring pressure into the center of the coil. When current flow stops, the magnetic field collapses and the plunger is moved out of the coil by spring pressure.

Solenoids are commonly used in starter motors, injectors and purge valves.

Switches

A Switch is a mechanical device used to start, stop or redirect current flow. A switch can be installed on the positive side of the circuit or the negative side of the circuit. A switch can be used to to control a load device directly or used to operate a relay which in turn can operate a higher current device. (e.g. Headlight switch, Horn button and Window switch.)

Resistors

Resistors limit the current flow in a circuit. The resistor is used in a circuit to introduce a desired amount of resistance into the circuit.

Resistors are available in fixed resistance or variable resistance.
Fixed resistors are color coded to indicate their resistance.

Resistor color Code Guide