Diodes

A diode is a device that conducts the current in just one direction: the direction of the arrow in the diode symbol, which looks like this:


The most important characteristics of a diode are: maximum forward current, forward voltage, maximum power dissipation and reverse voltage.
The forward current is the current flow in the direction of the arrow of the diode symbol. This current causes a voltage across the diode: the forward voltage drop. The power dissipation of a diode is the forward current multiplied by the forward voltage drop.
The reverse voltage is the voltage across a diode when it is reverse biased.
If you want to know how a diode works internally, you'll have to take a peek inside.

An AC Voltage Rectifier
Since diodes conduct current in only one direction, they can be used as an AC Voltage rectifier. Take a look at the picture below.





A triangular AC voltage is connected to the input terminals of the rectifier. The output voltage will be measured across resistor R1. When the upper input terminal is positive, there will be a current flow through the diode and the resistor. This current causes a voltage across R1. Assume the peak voltage is (plus and
minus) 9V, and the forward voltage of the diode is 0.7V. The peak current will then be (9V - 0.7V) / 1k = 8.3mA. The maximum power dissipation of the diode will be 9V • 8.3mA = 74.7mW. When the voltage at the upper input terminal becomes negative, the diode is reverse biased blocking the current flow. Since the diode has a very large resistance, all the voltage will be across the diode. This should not exceed the maximum reverse voltage.

So if you want to perform this experiment, you'll need a diode with the following requirements: the maximum forward current must be 8.3mA or higher; the maximum power dissipation must be 74.7mW or higher; and the maximum reverse voltage must be 9V or higher. Any small signal diode will meet these requirements. The resistor can be a regular 0.25W resistor since the maximum power dissipation is (8.3mA) 2 • 1k = 69mW. The circuit above is called a half wave rectifier, since the ouput contains only the positive half of the input. The circuit below shows a full wave rectifier.



This circuit works as follows. When the input signal is positive, the currents flows from the upper terminal, via diode D1, resistor R1, and diode D3 to the lower terminal. When the input signal is negative, the currents flows from the lower terminal, via diode D2, resistor R1, and diode D4 to the upper terminal. Notice that the current always flows through two diodes: either D1 and D3, or D2 and D4. This means that the output voltage will always be about 1.4 volts (two 'forward voltage drops') less than the input voltage. The circuit D1...D4 is called a bridge rectifier. When you look at a bridge rectifier, you'll probably see something imprinted like 'B80C5000/3300'. The number after the 'B' indicates the maximum (reverse) voltage, in this case 80V. The number after the 'C' indicates the maximum peak/continuous (forward) current in mA. In this case the maximum peak current is 5A and the maximum continuous current is 3.3A.
Smaller bridge rectifiers only indicate the maximum voltage and current, e.g. 'B40C800'.

LEDs
The abbreviation LED stands for Light Emitting Diode. LEDs consume less power than light bulbs, and have a much longer life time: about 100000 hours. A regular LED needs a current flow of 10...20mA, and has a forward voltage drop of 1.5 to 2 volts, depending on the color.
With the circuit below, you can test and experiment with LEDs.


Question: What will be a good value for R1? Assume that the voltage across LED D1 is 2 volts, and we want a current flow of 15mA.

Electronics Course - Diodes
Answer: The voltage across R1 will be 9V - 2V = 7V. The current flow through R1 will also be 15mA. So R1 should have a value of 7V / 15mA = 467ohms. From the E12 series, 470ohms is a good choice.

Zener Diodes
A zener diode in conducting state acts like a normal diode. It's the reverse voltage that distinguishes a zener diode from a regular diode. Take a look at the picture below.





In this picture you see a reverse connected zener diode. The 'value' of a zener diode is given in volts; this is the reverse voltage. But a zener diode doesn't blow when the voltage tends to get higher. A zener diode stabilizes the voltage at the reverse voltage. So the voltage across the zener diode in the picture above will always be 4.7 volts, even when the battery voltage increases. Again, we need to calculate the value of R1. Unfortunately, it's difficult to say what's the ideal current flow through a zener diode. (Yep, altough the diode is reverse biased there is a current flow!) In most cases 5mA is fine. Since the voltage across R1 will be 9V - 4.7V = 4.3V, a good value of R1 is 4.3V/5mA = 860ohms.

A zener diode manufacturer publishes the maximum power dissipation of a zener diode. 0.4 or 0.5W is a very common value for a small zener diode. Using this characteristic, we can calculate the minimum value of R1: Assume we use a 0.4W zener in the design above. Since the voltage across the zener is 4.7V, the maximum current flow is 0.4W/4.7V = 85mA. The voltage across R1 will be 9V - 4.7V = 4.3V. So the minimum value for R1 is 4.3V/85mA = 51ohms. So a good value of R1 ranges from 51 to 860ohms. 820ohms is a good choice. Note however that the calculations above only count for a zener without a load. Take a look at the picture below.





In this design zener D1 has a 50ohms load (R2). Again, we'll calculate a proper value for R1. Since the voltage across the load R2 is always 4.7V, the current flow through R2 will always be 4.7V/50 = 94mA. The current flow through D1 should be between 5 and 85mA. So the current flow through R1 ranges from 99 to 179mA. The voltage across R1 is always 4.3V, so the resistance should be between 24 and 43ohms.

Electronics Course - Diodes
39ohms may be a good choice. In that case, the power dissipation is 4.32 / 39 = 0.47W. So you'd better take a 1W resistor! But what should we do if the 50ohms load can be detached, e.g. because it's an external load? With the load connected, the *maximum* value of R1 is 43ohms, but without the load the *minimum* value is 51ohms!

The answer is simple: use a higher wattage zener diode, e.g. 1.3W. In that case, the maximum current flow through D1 is 1.3W/4.7V = 276mA. This means, without the load connected, a minimum value of R1 of 4.3V/276mA = 16ohms. Now we have an overlapping range of values for R1 from which you may choose one. Again, a 39ohms / 1W resistor is a good choice.

Testing diodes using a multimeter

Most digital multimeters look like this:





1 = Display
2 = Function switch
3 = Transistor socket (optional)
4...6 = Test lead jacks






If you want to test a diode, set the function switch to "diode test".
Next, connect the test leads. Mulimeters usually come with two test leads: a black one and a red one.
- Connect the black test lead to the COM jack and the red lead to the V/ jack.
- Connect the other ends of the test leads across the diode.
- Connect the black wire to the cathode and red wire to the anode. The display should now read about 0.6V (600mV). If you swap the test leads, the display will indicate an overflow. Note that in-circuit testing may lead to wrong results, since other components may be parallel-connected to the diode. Also make sure that the equipment-under-test has been turned off!

Notes:



Power Supply Unit
Power Failure (Kerosakan Disebabkan Kilat)
- Check Fius
- Check Capasitor Seramik
- Check Diod
- IC/Chip – Power IC

Contoh kerosakan diod
Probe +/- pada diod +/- , multimeter naik, probe -/+ pada diod -/+ multimeter naik

Contoh Diod Baik
Probe +/- pada diod +/- multimeter naik , probe -/+ pada diod -/+ multimeter tidak naik.

Fungsi diod ialah menukar arus dari AC ke DC

September 9 2009 - Activity

September 8 2009 - Activity

September 6 2009 - Activity

Power Supply Fuse

Some power supplies come with their own integrated fuse. The fuse is designed to protect the circuits in the power supply from damage should an over-current situation arise. You can read more about fuses on this PC Fundamentals page about basic electrical components. If there is a problem with the electrical system (surge, lightning strike) or internal fault within the power supply, the fuse will blow. It can then be replaced and if it did its job properly, the supply should operate normally.

Fuse Symbol

Unfortunately, many PC power supplies don't have fuses at all. I suppose this is a cost-savings measure but it seems pretty short-sighted to me. Even many power supplies that do have fuses hide them from the user within the power supply case. It's not a good idea to open up the power supply unless you are sure you know what you are doing, so I don't recommend opening the supply to search for a fuse (especially since too many units no longer have them). It's a good idea though to search the back of your system to see if there is a user-replaceable power supply fuse.

Fius Keranda

Fius Pisau

Switches

Switch Contacts - pole, throw etc.
Standard Switches - SPST, SPDT, DPST, DPDT.
Special Switches - multiway, key, tilt, reed etc.

Selecting a Switch

There are three important features to consider when selecting a switch:

• Contacts (e.g. single pole, double throw)
• Ratings (maximum voltage and current)
• Method of Operation (toggle, slide, key etc.)

Switch Contacts

Several terms are used to describe switch contacts:

• Pole - number of switch contact sets.
• Throw - number of conducting positions, single or double.
• Way - number of conducting positions, three or more.
• Momentary - switch returns to its normal position when released.
• Open - off position, contacts not conducting.
• Closed - on position, contacts conducting, there may be several on positions.

For example: the simplest on-off switch has one set of contacts (single pole) and one switching position which conducts (single throw). The switch mechanism has two positions: open (off) and closed (on), but it is called 'single throw' because only one position conducts.

Switch Contact Ratings

Switch contacts are rated with a maximum voltage and current, and there may be different ratings for AC and DC. The AC values are higher because the current falls to zero many times each second and an arc is less likely to form across the switch contacts.

For low voltage electronics projects the voltage rating will not matter, but you may need to check the current rating. The maximum current is less for inductive loads (coils and motors) because they cause more sparking at the contacts when switched off.

Capacitors

Function

Capacitors store electric charge. They are used with resistors in timing circuits because it takes time for a capacitor to fill with charge. They are used to smooth varying DC supplies by acting as a reservoir of charge. They are also used in filter circuits because capacitors easily pass AC (changing) signals but they block DC (constant) signals.

Capacitance

This is a measure of a capacitor's ability to store charge. A large capacitance means that more charge can be stored. Capacitance is measured in farads, symbol F. However 1F is very large, so prefixes are used to show the smaller values.

Three prefixes (multipliers) are used, µ (micro), n (nano) and p (pico):

• µ means 10^-6 (millionth), so 1000000µF = 1F
• n means 10^-9 (thousand-millionth), so 1000nF = 1µF
• p means 10^-12 (million-millionth), so 1000pF = 1nF

Capacitor values can be very difficult to find because there are many types of capacitor with different labelling systems!

There are many types of capacitor but they can be split into two groups, polarised and unpolarised. Each group has its own circuit symbol.

Polarised capacitors (large values, 1µF +)

Electrolytic Capacitors

Electrolytic capacitors are polarised and they must be connected the correct way round, at least one of their leads will be marked + or -. They are not damaged by heat when soldering.

There are two designs of electrolytic capacitors; axial where the leads are attached to each end (220µF in picture) and radial where both leads are at the same end (10µF in picture). Radial capacitors tend to be a little smaller and they stand upright on the circuit board.

It is easy to find the value of electrolytic capacitors because they are clearly printed with their capacitance and voltage rating. The voltage rating can be quite low (6V for example) and it should always be checked when selecting an electrolytic capacitor. If the project parts list does not specify a voltage, choose a capacitor with a rating which is greater than the project's power supply voltage. 25V is a sensible minimum for most battery circuits.

Tantalum Bead Capacitors

Tantalum bead capacitors are polarised and have low voltage ratings like electrolytic capacitors. They are expensive but very small, so they are used where a large capacitance is needed in a small size. Modern tantalum bead capacitors are printed with their capacitance, voltage and polarity in full. However older ones use a colour-code system which has two stripes (for the two digits) and a spot of colour for the number of zeros to give the value in µF. The standard colour code is used, but for the spot, grey is used to mean × 0.01 and white means × 0.1 so that values of less than 10µF can be shown. A third colour stripe near the leads shows the voltage (yellow 6.3V, black 10V, green 16V, blue 20V, grey 25V, white 30V, pink 35V). The positive (+) lead is to the right when the spot is facing you: 'when the spot is in sight, the positive is to the right'.

For example: blue, grey, black spot means 68µF
For example: blue, grey, white spot means 6.8µF
For example: blue, grey, grey spot means 0.68µF

Unpolarised capacitors (small values, up to 1µF)

Small value capacitors are unpolarised and may be connected either way round. They are not damaged by heat when soldering, except for one unusual type (polystyrene). They have high voltage ratings of at least 50V, usually 250V or so. It can be difficult to find the values of these small capacitors because there are many types of them and several different labelling systems!

Many small value capacitors have their value printed but without a multiplier, so you need to use experience to work out what the multiplier should be!

For example 0.1 means 0.1µF = 100nF.

Sometimes the multiplier is used in place of the decimal point:
For example: 4n7 means 4.7nF.

Capacitor Number Code

A number code is often used on small capacitors where printing is difficult:

• the 1st number is the 1st digit,
• the 2nd number is the 2nd digit,
• the 3rd number is the number of zeros to give the capacitance in pF.
• Ignore any letters - they just indicate tolerance and voltage rating.

For example: 102 means 1000pF = 1nF (not 102pF!)

For example: 472J means 4700pF = 4.7nF (J means 5% tolerance).

Polystyrene Capacitors

This type is rarely used now. Their value (in pF) is normally printed without units. Polystyrene capacitors can be damaged by heat when soldering (it melts the polystyrene!) so you should use a heat sink (such as a crocodile clip). Clip the heat sink to the lead between the capacitor and the joint.

Real capacitor values (the E3 and E6 series)

You may have noticed that capacitors are not available with every possible value, for example 22µF and 47µF are readily available, but 25µF and 50µF are not!

Why is this? Imagine that you decided to make capacitors every 10µF giving 10, 20, 30, 40, 50 and so on. That seems fine, but what happens when you reach 1000? It would be pointless to make 1000, 1010, 1020, 1030 and so on because for these values 10 is a very small difference, too small to be noticeable in most circuits and capacitors cannot be made with that accuracy.

To produce a sensible range of capacitor values you need to increase the size of the 'step' as the value increases. The standard capacitor values are based on this idea and they form a series which follows the same pattern for every multiple of ten.

The E3 series (3 values for each multiple of ten)
10, 22, 47, ... then it continues 100, 220, 470, 1000, 2200, 4700, 10000 etc.
Notice how the step size increases as the value increases (values roughly double each time).

The E6 series (6 values for each multiple of ten)
10, 15, 22, 33, 47, 68, ... then it continues 100, 150, 220, 330, 470, 680, 1000 etc.
Notice how this is the E3 series with an extra value in the gaps.

The E3 series is the one most frequently used for capacitors because many types cannot be made with very accurate values.

Variable capacitors

Variable capacitors are mostly used in radio tuning circuits and they are sometimes called 'tuning capacitors'. They have very small capacitance values, typically between 100pF and 500pF (100pF = 0.0001µF). The type illustrated usually has trimmers built in (for making small adjustments - see below) as well as the main variable capacitor.

Many variable capacitors have very short spindles which are not suitable for the standard knobs used for variable resistors and rotary switches. It would be wise to check that a suitable knob is available before ordering a variable capacitor.

Variable capacitors are not normally used in timing circuits because their capacitance is too small to be practical and the range of values available is very limited. Instead timing circuits use a fixed capacitor and a variable resistor if it is necessary to vary the time period.

Trimmer capacitors

Trimmer capacitors (trimmers) are miniature variable capacitors. They are designed to be mounted directly onto the circuit board and adjusted only when the circuit is built.

A small screwdriver or similar tool is required to adjust trimmers. The process of adjusting them requires patience because the presence of your hand and the tool will slightly change the capacitance of the circuit in the region of the trimmer!

Trimmer capacitors are only available with very small capacitances, normally less than 100pF. It is impossible to reduce their capacitance to zero, so they are usually specified by their minimum and maximum values, for example 2-10pF.

Trimmers are the capacitor equivalent of presets which are miniature variable resistors.

AC and DC

AC means Alternating Current and DC means Direct Current. AC and DC are also used when referring to voltages and electrical signals which are not currents! For example: a 12V AC power supply has an alternating voltage (which will make an alternating current flow). An electrical signal is a voltage or current which conveys information, usually it means a voltage. The term can be used for any voltage or current in a circuit.

Alternating Current (AC)

Alternating Current (AC) flows one way, then the other way, continually reversing direction.

An AC voltage is continually changing between positive (+) and negative (-).

The rate of changing direction is called the frequency of the AC and it is measured in hertz (Hz) which is the number of forwards-backwards cycles per second.

Mains electricity in the UK has a frequency of 50Hz.

See below for more details of signal properties.

An AC supply is suitable for powering some devices such as lamps and heaters but almost all electronic circuits require a steady DC supply (see below).

Direct Current (DC)

Direct Current (DC) always flows in the same direction, but it may increase and decrease.

A DC voltage is always positive (or always negative), but it may increase and decrease.

Electronic circuits normally require a steady DC supply which is constant at one value or a smooth DC supply which has a small variation called ripple.

Cells, batteries and regulated power supplies provide steady DC which is ideal for electronic circuits.

Power supplies contain a transformer which converts the ma

ins AC supply to a safe low voltage AC. Then the AC is converted to DC by a bridge rectifier but the output is varying DC which is unsuitable for electronic circuits.

Some power supplies include a capacitor to provide smooth DC which is suitable for less-sensitive electronic circuits, including most of the projects on this website.

Lamps, heaters and motors will work with any DC supply.

Please see the Power Supplies page for further information.

Power supplies are also covered by the Electronics in Meccano website