1. When current flows
2. Voltages and currents are either DC or AC. AC Voltage or Current varies with
time, DC Voltage or Current stays constant with time. AC Voltage, Current, and
Power are usually measured in RMS (root mean square), as it varies with time.
3. The source of current is a potential difference (p.d.) given by batteries
(DC) or Power Supplies (DC/AC). A potential difference across two connected points
causes current to flow through it. As an analogy, picture water flowing through
a pipe.
4. Units: Voltage-Volts(v). Current-Amperes(A). Resistances-Ohms.
5. Two Resistors R1 and R2 in series: total R = R1 + R2. Two Resistors R1 and R2
in parallel: total R = R1 R2 / (R1 + R2)
6. Resistances are given by resistors. When you buy resistors, ask for certain
resistances only, because they come in standard values. Common Resistor values:
10 11 12 13 15 16 18 20 22 24 27 30 33 36 43 47 51 56 62 68 75 82 91. (got it?
- the above numbers can be used to obtain standard values. That means don't ask
for a resistor if 19 kiloohms. the only avialiable are of 16 and then 18. The
above numbers is applicable for ohms, kiloohms ('000), and megaohms ('000,000).
That means you can find resistances of 1.0 1.1 .12 1.3 1.5 ... 10 11 12 13 15 ...
100 110 120 130 150 160 ... 1000 (1K) 1100 (1K1) 1200 (1K2) 1300 (1K3) ... 10K 11K
12K 13K 15K ...
7. You should combine resistors in series to add, and in parallel to reduce the resistance
to obtain oddball values like 62 parallel 62 series 10 = 41.
8. The resistance can be measured by an ohmmeter, or a multimeter. It can also
be obtained by looking at the color codes. Just remember: BBROY of Great Britian
had a Very Good Wife - meaning Black(1) Brown(2) Red(3) Orange(4) Green (5) Blue(6)
Violet(7) Gray(8) White(9). The first line color gives the first digit, the
second line gives the second digit, and the third line gives the number of zeros
to add. Thus a 10K resistor is Brown Black Orange, and a 1K resistor is Brown
Black Yellow while a 47ohm is Yellow Violet Black.
8. Besides resistances, you should be aware of power handling capacity of resistors
Power = Voltage * Current. Resistors come in ratings of 1/4 watt, 1/2 watt 1 watt and more
Power Ratings mean the power dissipated by the resistor (voltage across it times
the current thorough it) should not exceed the rated value, otherwise... just try
and you will find out why!
9. Capacitors store charges. They are measured in Farads. One farad in a big value.
Usually caps. are measured in uF (10-6 pronounced microfarads) or pF (10-12 picofarads).
10. An inductor is an energy storage device. It can be as simple as a single loop of wire or consist of many turns of wire wound around a special core. Energy is stored in the form of a magnetic field in or around the inductor. Whenever current flows through a wire, it creates a magnetic field around the wire. By placing multiple turns of wire around a loop, we concentrate the magnetic field into a smaller space where it can be more useful. When you apply a voltage across an inductor, a current starts to flow. It does not instantly rise to some level, but rather increases gradually over time. The relationship of voltage to current vs. time gives rise to a property called inductance. The higher the inductance, the longer it takes for a given voltage to produce a given current. Whenever there is a moving or changing magnetic field in the presence of an inductor, that change attempts to generate a current in the inductor. An externally applied current produces an increasing magnetic field which in turn produces a current opposing that applied externally, hence the inability to create an instantaneous current change in an inductor. This property makes inductors useful as filters in power supplies.
Many types of cores are commonly used in inductors. The simplest core is basically nothing, or air. Any core consisting of non-magnetic material behaves essentially the same as air. Most commonly used inductors, however, use some type of magnetic material in the core. This tends to concentrate the inductor’s magnetic field inside the core and increases the effective inductance. While a magnetic core can provide greater inductance in a given volume, there are also drawbacks. A magnetic core can contain only a limited magnetic field. As you increase current in a magnetic core inductor, the magnetic field increases. At some point, further increasing the current no longer produces an increase in the magnetic field. At this point, the core is said to be saturated, a condition that generally is undesirable.
The relationship between the inductance with a given core and the number of turns on it is called its AL value. The unit of inductance is the henry. The formula for inductance is: L = n2(squared) x AL where L is the inductance in henries and n is the number of turns. Most commercially available cores have published AL values. Inductor cores with higher AL values tend to saturate more readily than cores with lower AL values.
11. A transformer is an energy transfer device. It has an input side (primary) and an output side (secondary). Electrical energy applied to the primary is converted to a magnetic field which in turn, induces a current in the secondary which carries energy to the load connected to the secondary. The energy applied to the primary must be in the form of a changing voltage which creates a constantly changing current in the primary, since only a changing magnetic field will produce a current in the secondary.
A transformer consists of at least two sets of windings wound on a single magnetic core. There are two main purposes for using transformers. The first is to convert the energy on the primary side to a different voltage level on the secondary side. This is accomplished by using differing turns counts on primary and secondary windings. The voltage ratio is the same as the turns ratio. The second purpose is to isolate the energy source from the destination, either for personal safety, or to allow a voltage offset between the source and load.
Transformers are generally divided into two main types. Power transformers are used to convert voltages and provide operating power for electrical devices, while signal transformers are used to transfer some type of useful information from one form or location to another.
- Contact Mr. Indra Gopal Shrestha. 340-3F Super Market. New Road, Kathmandu. Phone: 4229161, 4228135, 4242240. Email: shivaelx@hotmail.com
Ason, Kamalachhi. Phone: 4229391
Contact Mr. Ganesh Shrestha. Ason. Bangemuda. Phone: 4228069. Email: ganeshshrestha2002@yahoo.com
You can also try other various electronic
shops at Bangemuda. To order components, you can try Mahavir Shree International. Old Baneshwor. Phone: 4461848. They are the dealers of Farnell.
Best o luck! |