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Boiler Fault Finding - Resistors and Capacitors

 

I think that we can safely say that there isn't a single component on this British Gas PCB that would cost more than £5 to replace, and yet the cost of a replacement PCB is over £200.british gas boiler PCB

Capacitors

What are they?

Capacitors contain conducting plates, usually, a thin metal film, separated by an insulator called the dielectric. They are able to store an electric charge - the larger the capacitance the greater the charge at a given voltage.

Identifying Capacitors

Electrolytic capacitors usually come in an aluminium can with a plastic sleeve. The illustration gives an idea of the shape of the capacitor but some types (axial leads) have a lead leaving each end. These capacitors are polarised - they must be connected the correct way in a circuit.

The important information is written on the can and includes:

  • the value of capacitance in μF

  • the maximum voltage it should withstand

  • the polarity - a negative or positive sign will be drawn

Non - electrolytic capacitors may be connected either way in a circuit because they are not polarised.

Some types used to have a colour coding printed onto the outside of the case of the capacitor. Polyester types had a flattish rectangular shape but even older versions used to be tubular.

You may still come across colour coded capacitors in older equipment and the chart on the separate sheet will help in their identification.

Current non-electrolytic capacitors have a numeric code which is explained on a separate sheet.

Most non-electrolytic types will work at the voltages used in electronic circuits but for power applications, particularly when used with the mains, the voltage rating becomes important. A 1000 V capacitor is needed at mains voltages. Suppressor capacitors or capacitors used to start mains motors need to be carefully selected.

Measurement of Capacitance

The farad is the unit of capacitance but it is far too large for practical purposes.

Practical electrolytic capacitors are rated in microfarads.

In the power supply section expect to find capacitors with values up to 10,000 μF These are physically large but smaller ones with values in the tens or hundreds of microfarads are found at various other places on the board, with values up to thousands available. The largest non-electrolytic types are only a few μF and most are measured in picofarads

1 pf = 1/1,000,000,000,000F

or nanofarad

1nF = 1000pF = 1/1,000,000,000F

What are they used for?

Electrolytic capacitors are used to smooth the rectified mains supply for use in electronic equipment. Smaller ones will be used to allow AC signals to pass but to block DC signals.

The diagram opposite shows an electrolytic capacitor that may be found in a power supply.

To help identify it correctly fill in the details inside the box.

Non-electrolytic types have a variety of applications. Ceramic types can be effective at removing ' spikes' from supplies or motors, polyester types may be used as suppressors and polystyrene is used at radio frequencies. When there are many integrated circuits there is likely to be a number of very small capacitors used for decoupling purposes - this prevents unwanted triggering of the electronic switching circuits.

Colour Coding of Capacitors

The first two digits are written down (colours are the same as the resistor colour code). The third band is the multiplier. Multiply the first two digits by the multiplier value in the table below.

 

Colour

First digit

Second digit

Multiplier
(value in pf)

Tolerance

Voltage

 

Black

-

0

1

 

 

 

Brown

1

1

10

 

 

 

Red

2

2

100

 

 

 

Orange

3

3

-

 

 

 

Yellow

4

4

1

 

 

 

Green

5

5

2

 

 

 

Blue

6

6

3

 

 

 

Violet

7

7

4

 

 

 

Grey

8

8

5

 

 

 

White

9

9

6

 

 

 

No colour

-

-

7

 

 

 

Silver

-

-

8

 

 

 

Gold

-

-

9

 

 

Numerical Coding of Capacitors

Newer capacitors have a numerical code. This may be difficult to read without a magnifying glass but gives the capacitance by the following method.

  1. Find the three-digit number.

  2. Write down the first two digits as in the order on the body of the capacitor.

  3. Note the third digit. This is the multiplier and gives the number of zero's to put at the end of the value.

  4. The number is the value in picofarads.

Practical example.

  1. The first two digits are 4 and 7 so write down 47.

  2. The digit 3 gives the number of 0's which is 3. Add these to the end of 47 to give 47 000.

  3. The capacitance is 47 000 pF (picofarads). Since 1000 pF is equal to 1 nF (nanofarad) it could also be written as 47 nF.

Note: Other information gives the working voltage and dielectric material of the capacitor.

Find the value of each of the capacitors in the box opposite. Express the left capacitor in picofarads, the middle one in nanofarads and the capacitor on the right in microfarads.

Types of Resistor

The most familiar type of resistor is the one with coloured bands. They will be seen on older and some new PCBs.

Sometimes, several identical resistors are needed. It is convenient to pack them in a common package, such as the SIL and DIL packages shown.

Surface mount technology has introduced tiny resistors without leads. Modem boards are using these to increase reliability and reduce costs.

Preset variable resistors or potentiometers are found on PCB'S where they are used to set up the circuit correctly. They may set the temperature at which a component will switch on or off or determine a time interval. Multi-turn potentiometers give higher precision.

If a value is intended to be altered to set new operating conditions an ordinary potentiometer will be used. This has a spindle which is intended to have a knob fitted. They can cause trouble if they are altered regularly over many years.

A number of transducers or sensors change their resist~ when the operating conditions change. A common transducer is the thermistor. In hot water systems, they are mounted in a metal package with a screw and will be connected to the main electronics controller board by leads with push-on connectors. They are robust.

Other thermistors come in smaller packages which have a lacquered coating. They can be identified by a colour code or by a number printed on the component. These are sometimes found on a PCB where they may monitor the temperature around the board and could switch the system off if it became too hot.

The letters NTC are often connected with thermistors. The letters stand for ' negative temperature coefficient' which simply means that the resistance goes down as the temperature goes up. Positive temperature coefficient thermistors are also available. The thermistor has a rating which gives the resistance at a particular temperature (usually 25*C and replacement types must have the same value as the original.

A varistor, or voltage-dependent resistor, is used to suppress mains-borne transients or spikes by suddenly reducing its resistance and absorbing the high voltage spike. These are found as part of the power supply on the mains side of the transformer (suppression capacitors and chokes may be nearby). They protect devices which are sensitive to high voltages or which may cause spurious triggering. They have a similar appearance to a thermistor.

Identifying Resistors

In electronic circuits, there are many resistors. Most are physically small and are identified by coloured bands. Newer boards contain very small devices that are surface mounted and have their value printed on them. These are not easy to distinguish from small capacitors. Resistors which have a higher power rating may have their resistance value printed on them.

The Resistor Color Code

The colour coding may have 4 or 5 bands. One system is shown below (band 5 may not be present). A slightly different coding has a third band that works in a similar way to bands 1 and 2. Band 4 then becomes the multiplier and the remaining band the tolerance.

 

Colour

Band 1

Band 2

Band 3

Band 4

Band 5

 

Black

0

0

x1

 

200 ppm

 

Brown

1

1

x10

1%

100 ppm

 

Red

2

2

x100

2%

50 ppm

 

Orange

3

3

x1000

 

15 ppm

 

Yellow

4

4

x10,000

 

25 ppm

 

Green

5

5

x100,000

0.5%

 

 

Blue

6

6

x1,000,000

0.25%

10 ppm

 

Grey

7

7

 

0.1%

5 ppm

 

White

8

8

 

 

1 ppm

 

Gold

9

9

 

5%

 

 

Silver

 

 

 

10%

 

Resistive Sensor

Resistors in electronic circuits are expected to keep the same resistance. If this resistance changes the working conditions of the circuit, particularly the voltages in it, will be affected. Sometimes the change in resistance of a component can be put to good use.

resistors in a circuit can divide up the voltage that is connected across them. If one resistance changes the voltages at the junctions will be affected. Electronic circuits, called comparators can compare voltages and cause an output to change state. This is the basis of an electronic switch - it will perform the same function as a mechanical switch but there are no moving parts.

Sensors are available that respond to light, temperature, position, pressure and many other physical properties. The use of electronics should improve reliability and increase the level of control available. A further advantage is the ability to make decisions based on the information from a wide variety of sensors. Digital logic circuits are widely used to make this possible.

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