CAPACITORS

 SECTION SIX

                                          CAPACITORS

Capacitors are one of the most difficult things to test. That's because they don't give a reading on a multimeter and their value can range from 1p to 100,000u.

A faulty capacitor may be "open" when measured with a multimeter, and a good capacitor will also be "open." You need a piece of test equipment called a CAPACITANCE METER to measure the value of a capacitor.

HOW A CAPACITOR WORKS

There are two ways to describe how a capacitor works. Both are correct and you have to combine them to get a full picture. A capacitor has INFINITE resistance between one lead and the other. This means no current flows through a capacitor. But it works in another way.

Suppose you have a strong magnet on one side of a door and a piece of metal on the other. By sliding the magnet up and down the door, the metal rises and falls. The metal can be connected to a pump and you can pump water by sliding the magnet up and down.

A capacitor works in exactly the same way.

If you raise a voltage on one lead of a capacitor, the other lead will rise to the same voltage. This needs more explaining - we are keeping the discussion simple. It works just like the ma gnetic field of the magnet through a door. The next concept is this:

Capacitors are equivalent to a tiny rechargeable battery

They store energy when the supply-voltage is present and release it when the supply drops.

These two concepts can be used in many ways and that's why capacitors perform tasks such as filtering, time-delays, passing a signal from one stage to another and create many different effects in a circuit.

CAPACITOR VALUES

The basic unit of capacitance is the FARAD. (C) This is the value used in all equations, but it is a very large value. A one FARAD capacitor would be the size of a car if made with plates and paper. Most electronic circuits use capacitors with smaller values such as 1p to 1,000u. 1p is about equal to two parallel wires 2cm long. 1p is one picofarad.

The easiest way to understand capacitor values is to start with a value of 1u. This is one microfarad and is one-millionth of a Farad. A 1 microfarad capacitor is about 1cm long and the diagram shows a 1u electrolytic.

 

Smaller capacitors are ceramic and they look like the following. This is a 100n ceramic

 

To read the value on a capacitor you need to know a few facts. The basic value of capacitance is the FARAD. 1 microfarad is one millionth of 1 farad.

1 microfarad is divided into smaller parts called nanofarad.

1,000 nanofarad = 1 microfarad

Nanofarad is divided into small parts called picofarad

1,000 picofarad = 1 nanofarad.

Recapping:

1p = 1 picofarad.   1,000p = 1n ( 1 nanofarad)

1,000n = 1u (1 microfarad)

1,000u = 1millifarad

1,000,000u = 1 FAR AD.

Examples:

All ceramic capacitors are marked in "p" (puff")

A ceramic with 22 is 22p = 22 picofarad

A ceramic with 47 is 47p = 47 picofarad

A ceramic with 470 is 470p = 470 picofarad

A ceramic with 471 is 470p = 470 picofarad

A ceramic with 102 is 1,000p = 1n

A ceramic with 223 is 22,000p = 22n

A ceramic with 104 is 100,000p = 100n  = 0.1u

TYPES OF CAPACITOR

For testing purposes, there are two types of capacitor.

Capacitors from 1p to 100n are non-polar and can be inserted into a circuit around either way.

Capacitors from 1u to 100,000u are electrolytics and are polarised. They must be fitted so the positive lead goes to the supply voltage and the negative lead goes to ground (or earth).

There are many different sizes, shapes and types of capacitor. They are all the same.

They consist of two plates with an insulating material between. The two plates can be stacked in layers or rolled together.

The important factor is the insulating material. It must be very thin to keep things small. This gives the capacitor its VOLTAGE RATING.

If a capacitor sees a voltage higher than its rating, the voltage will "jump through" the insulating material or around it. If this happens, a carbon deposit is left behind and the capacitor becomes "leaky" or very low resistance, as carbon is conductive.

CERAMIC CAPACITORS

Nearly all small capacitors are ceramic capacitors as this material is cheap and the capacitor can be made in very thin layers to produce a high capacitance for the size of the component. This is especially true for surface-mount capacitors.

All capacitors are marked with a value and the basic unit is: "p" for "puff" However NO surface mount capacitors are marked and they are very difficult to test

 

VALUE: VALUE

WRITTEN ON THE   COMPONENT:

0.1p    0p1,             0.22p  0p22,         0.47p  0p47,          1.0p   1p0,              2.2p    2p2,

4.7p   4p7,             5.6p   5p6,             8.2p   8p2,            10p   10 or 10p,      22p   22 or 22p,

47p  47 or 47p,       56p  56 or 56p,     100p  100 on 101,  220p  220 or 221,   470p  470 or 471,

560p  560 or 561,   820p  820 or 821,  1,000p (1n)  102,   2200p (2n2)  222,   4700p (4n7)  472,

8200p (8n2)  822,  10n    103,             22n    223,             47n     473,            100n   104,

220n  224,             470n  474,             1u      105,

POLYESTER, POLYCARBONATE, POLYSTYRENE, MYLAR, METALIZED POLYESTER,

("POLY"), MICA and other types of CAPACITOR

There are many types of capacitor and they are chosen for their reliability, stability, temperate-range and cost.

For testing and repair work, they are all the same. Simply replace with exactly the same type and value

 

Capacitor Colour Code Table

Colour    Digit   Digit     Multiplier      Tolerance     Tolerance         Temperature  Coefficient

                A           B             D              (T) > 10pf     (T) < 10pf                        (TC)

Black      0            0             x1                 ± 20%             ± 2.0pF

Brown     1            1            x10               ± 1%                ± 0.1pF                        -33x10- 6

Red         2            2            x100             ± 2%                ± 0.25Pf                       -75x10- 6

Orange    3            3            x1,000          ± 3%                                                    -150x10^-6

Yellow    4            4            x10,000        ± 4%                                                    -220x10- 6

Green      5            5           x100,000       ± 5%                ± 0.5pF                       -330x10- 6

Blue        6           6            x1,000,000                                                                -470x10^-6

Violet      7           7                                                                                     -750x10- 6

Grey        8          8             x0.01            +80% ,-20%

White      9          9             x0.1               ± 10%              ± 1.0pF

Gold                                x0.1               ± 5%

Silver                              x0.01              ± 10%

 

ELECTROLYTIC and TANTALUM CAPACITORS

Electrolytics and Tantalums are the same for testing purposes but their performance is slightly different in some circuits. A tantalum is smaller for the same rating as an electrolytic and has a better ability at delivering a current. They are available up to about 1,000u, at about 50v but their cost is much higher than an electrolytic.

Electrolytics are available in 1u, 2u2 3u3 4u7 10u, 22u, 47u, 100u, 220u, 330u, 470u, 1,000u, 2,200u, 3,300u, 4,700u, 10,000u and higher. The "voltage" or "working voltage" can be: 3.3v, 10v, 16v, 25v, 63v, 100v, 200v and higher. There is also another important factor that is rarely covered in text books. It is RIPPLE FACTOR. This is the amount of current that can enter and leave an electrolytic. This current heats up the electrolytic and that is why some electrolytics are much larger than others, even though the capacitance and voltage-ratings are the same.

If you replace an electrolytic with a "miniature" version, it will heat up and have a very short life. This is especially important in power supplies where current (ene rgy) is constantly entering and exiting the electrolytic as its main purpose is to provide a smooth output from a set of diodes that delivers "pulsing DC." (see"Power Diodes")

 

NON-POLAR CAPACITORS (ELECTROLYTICS)

Electrolytics are also available in non-polar values. It sometimes has the letters "NP" on the component. Sometimes the leads are not identified.

This is an electrolytic that does not have a positive and negative lead but two leads and either lead can be connected to the positive or negative of the circuit.

These electrolytics are usually connected to the output of an amplifier (such as in a filter near the speaker) where the signal is rising and falling.

A non-polar electrolytic can be created from two ordinary electrolytics by connecting the negative leads together and the two positive leads become the new leads. For example: two 100u  63v electrolytics will produce a 47u 63v non-polar electrolytic. In the circuit below, the non-polar capacitor is replaced with two electrolytics.

 

PARALLEL and SERIES CAPACITORS

Capacitors can be connected in PARALLEL and/or SERIES for a number of reasons

1. If you do not ha ve the exact value, two or more connected in parallel or series can produce the value you need.

2. Capacitors connected in series will produce one with a higher voltage rating.

3. Capacitors connected in parallel will produce a larger-value capaci tance.

Here are examples of two equal capacitors connected in series or parallel and the result s they produce:

VOLTAGE RATING OF CAPACITOR

Capacitors have a voltage rating, stated as WV for working voltage, or WVDC. This specifies the ma ximum voltage that can be applied across the capacitor without

puncturi ng the dielectric. Voltage ratings for "poly," mica and ceramic capacitors are typically 50v to 500 VDC. Ceramic capacitors with ratings of 1kv to 5kv are also available. Electrolytic capacitors are commonly available in 6v, 10v 16v, 25v, 50v, 100v, 150v, and 450v ratings.

CAUTION

If a capacitor has a voltage rating of 63v, do not put it in a 100v circuit as the insulation (called the dielectric) will be punctured and the capacitor will "short-circuit." It's ok to replace a 0.22uF 50WV capacit or with 0.22uF 250WVDC.

SAFETY

A capacitor can store a charge for a period of time after the equipment is turned off. High voltage electrolytic caps can pose a safety hazard. These capacit ors are in power supplies and some have a resistor across them, called a bleed resistor, to discharge the cap after power is switched off.

If a bleed resistor is not present the cap can retain a charge after the equipment is unplugged.

How to discharge a capacitor

Do not use a screw driver to short between the terminals as this will damage the capacitor internally and the screwdriver.

Use a 1k 3watt or 5watt resistor on jumper leads and keep them connected for a few seconds to fully discharge the electro. Test it with a voltmeter to make sure all the energy has been removed. Before testing any capacitors, especially electrolytics, you should look to see if any are damaged, ov erheated or leaking. Swelling at the top of an electrolytic indicates heating and pressure inside the case and will result in drying out of the electrolyte. Any hot or warm electrolytic indicates leakage and ceramic capacitors with portions missing indicates something has gone wrong.

TESTING A CAPACITOR

There are two t hings you can test with a multimeter:

1. A short-circuit within the capacitor

2. Capacitor values above 1u

You can test capacitors in-circuit for short-circuits. Use the x1 ohms range.

To test a capacitor for leakage, you need to remove it or at least one lead must be removed. Use the x10k range on an analogue or digital multimeter.

For values above 1u you can determine if the capacitor is charging by using an analogue meter. The needle will initially move across the scale to indicate the cap is charging, then go to "no deflection." Any permanent deflection of the needle will indicate leakage.

You can reverse the probes to see if the needle moves in the opposite direction. This indicates it has been charged. Values below 1u will not respond to charging and the needle will not deflect.

This does not work with a digital meter as the resistance range does not output any current and the electrolytic does not charge.

Rather than spending money on a capacitance meter, it is cheaper to replace any suspect capacitor or electrolytic.

Capacitors can produce very unusual faults and no piece of test equipment is going to detect the problem.

In most cases, it is a simple matter to solder another capacitor across the suspect component and view or listen to the result.

This saves all the worry of removing t he component and testing it with equipment that cannot possibly give you an accurate reading when the full voltage and current is not present.

It is complete madness to even think of testing critical components such as capacitors, with TEST EQUIPMENT. You are fooling yourself. If the Test Equipment says the component is ok, you will look somewhere else and waste a lot of time.

FINDING THE VALUE OF A CAPACITOR

If you want to find the value of a surface-mount capacitor or one where the markings have been removed, you will need a CAPACITANCE METER. Here is a simple circuit that can be added to your meter to read capacitor values from 10p to 10u

 

ADD-ON CAPACITANCE METER

REPLACING A CAPACITOR

Always replace a capacitor with the exact same type.

A capacitor may be slightly important in a circuit or it might be extremely critical.

A manufacturer may have taken years to select the right type of capacitor due to previous failures.

A capacitor just doesn't have a "value of capacitance." It may also has an effect called "tightening of the rails." In other words, a capacitor has the ability to react quickly and either absorb or deliver energy to prevent spikes or fluctuations on the rail.

This is due to the way it is constructed. Some capacitors are simply plates of metal film while others are wound in a coil. Some capacitors are large while others are small.

They all react differently when t he voltage fluctuates.

Not only this, but some capacitors are very stable and all these features go into the decision for the type of capacitor to use.

You can completely destroy the operation of a circuit by selecting the wrong type of capacitor.

No capacitor is perfect and when it gets charged or discharged, it appears to have a small value of resistance in series with the value of capacitance. This is known as

"ESR" and stands for EQUIVALENT SERIES RESISTANCE. This effectively makes the capacitor slightly slower to charge and discharge.

We cannot go into the theory on selecting a capacitor as it would be larger than this eBook so the only solution is to replace a capacitor with an identical type.

However if you get more than one repair with identical faults, you should ask other technicians if the original capacitor comes from a faulty batch.

The author has fixed TV's and fax machines where the capacitors have been inferior and alternate types have solved the problem.

Some capacitors are suitable for high frequencies, others for low frequencies.