Resistors & Diodes

1K resistor 

Welcome to my first blog about autotronics, this topic is Electrical components used in the automotive industry.

Resistors, are used to reduce or control the current flow, of which there are many different types, colour(see image) and sizes these include; fusible NFR, Standard film SFR, Power metal film PRO1, Metal glazed VR, Metal film MRS16T, and Metal Film MRS25.

 The Coloured bands represent the resistance; the first 2 or 3 bands are the values, the next coloured band is the multiplier which lets the user no how many zeros to add and the last band on the right is the tolerance value eg; gold is 5%, silver 10%. 

The colour codes that can be used to calculate the value of the resistor are; black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9.

 An example of this is a 1K ohm carbon film resistor which has brown, black, red, and gold bands.

All resistors have preferred values these range from this list of numbers and increase by multiplies of 10; 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, 100.

 A value of 82 ohms is written as 82R and a value of 2200 ohms, 2K2. Series resistors (end to end) have their values added together to increase the total value of the resistance but in parallel (side by side) the resistance is less than the value of the smallest resistor.

Diodes are silicon (Si) components which act as an insulator in the reverse bias direction but in the forward bias direction they conduct the current with a forward bias voltage (knee) of 0.6 to 0.7V. They can be thought of as an electric one way valve.

 Diodes have an anode “A” (+) and Cathode “K” (-) identified by a grey or white band on the standard diodes like the 1N4007. 

Here’s an example of a circuit with the 1N4007 diode, a 1K resistor and a 5V power supply; the calculated current through the diode is 5mA and the measured value is close to the expected with 4.4 mA this slight difference is due to the variance in the resistor (+-5%) the measured forward bias voltage was 0.747 V (close to 0.7 volts of a standard diode), the maximum current that could flow through this diode is 1 A, the maximum voltage with a 1K ohm resistor would be 1000V or 1 KV. Next the diode is replaced with an LED, the Current drops down to 3.55mA which is only slightly more than the expected 3.2mA this difference is because of the increased forward bias voltage of the LED (1.8V). Light emitting diodes (LED) also have an anode and cathode (identified by the flat side and a shorter lead). The more expensive LED’s are the white or blue. The colour is determined by the semiconductor material, not by the colouring of the plastic body. They are used as a simple indicator to show when current is flowing in the circuit all LED’s are used with a resistor in series the required resistor value is calculated by the formula: R = (V_S - V_Led) / I 

By varying the construction of the P – N junction special purpose diodes can be made such as; tunnel diodes to generate radio frequency oscillation, zener diodes are made that are used as voltage regulators and varactor diodes to electronically tune radio and TV receivers.

Diodes functions and applications are;

Voltage regulators are made from po­wer diodes and rectify AC to DC by using either; half wave, full wave or a bridge rectifier. This DC is then smoothed by capacitors or choke transformers.

A Free Wheeling diode, or protection diode, is placed in reverse bias across the relay coil to protect the opened switch (ECU) fro­­m back EMF of the stored energy collapsing and arcing across the contacts or damaging the ECU. This works because when the switch is opened the back EMF follows the path created by the now forward bias through the diode in a loop.