Electronica 101

Ohm's law

When we apply a direct current (DC) voltage source “V” (which could be a battery, or a power supply using mains power) to a resistor “R”, it results in a current “I” circulating on the circuit.The relationship between these magnitudes is described by Ohm's Law: | |
This equation shows that with a constant resistor (whose value is measured in ohms, symbol Ω), as voltage increases,current (measured in amperes) also increases proportionally.

For example, if V=12 Volts, and R=24 Ω (ohms) , then I=0.5 Amperes.

The same formula can be expressed like this:
                            

Thus as the current over a resistor grows, voltage on the resistor rises too.

Power

Power is a measure of how much energy a component or a system consumes over a given period oftime. The general expression for electrical power is:
                            

Using Ohm's law, we obtain two expressions for power over a resistor:

                                        and            

Resistor Identification

Common commercially available resistors are usually identified by color bands.  The color code is explained below.
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    1st band: first digit    2nd band: second digit

    3rd band: number of zeros to append

    4th band is the tolerance,

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5% (gold)
or
10%(silver)  |

For example, in the case of the resistor pictured above:

    1st band: red (2)

    2nd band: orange (3)

    3rd band: yellow (add 4 zeros)

    4th band: silver (10%)

So, this is a 230 000 ohm (or 230 Kohm) resistor, with + -10% toleranceThe next gadget calculates the resistor value given the color codes, and the other way round. You can use it here or insert it into your iGoogle page.

Resistor Measurement

You can measure a resistor simply by setting a tester to resistance measurement, and contacting terminals with the probes. It's the same in both ways, there is no polarity.

If the resistor is mounted on a board, thecircuit must not be energized, and at least one terminal should be disconnected for measuring.

Resistors in Series

If we connect two resistors in series, the equivalent resistance is the sum of both resistors' values:

The same current flows through both resistors, and is calculated as follows:

   where     

And the voltage drops on each resistance are:

                           

Resistors in Parallel

If we connect two resistors in parallel, the equivalent conductance is the is the sum of both resistors' conductances. The conductance is the inverse of the resistance.

                                       

  Both resistors are under the same tension, and total current is derived in two branches:

                            

    

Alternate CurrentIt is a sinusoidal electric signal that alternates between positive and negative values. There is a positive hemicycle and a negative one.
The term "alternate current" (AC) is used for both current and voltage signals.

Vrms, Frequency and Period:

When somebody refers to voltage, such as "110", he is speaking about the rms value (root mean square) of the AC voltage signal. The rms valueis an equivalent DC voltage which applicated over a resistor dissipates the same power than the AC voltage that we are describing. It is not the peak value but a kind of average.

USA:             VRMS=110 Volt                                            

Some other countries:                               VRMS= 220 Volt
There is a relationship between rms and peak values:

                  so, in USA:            

Ohm's law in AC
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Of course it is the same Ohm's law that for DC current.The only thing to take in account is that we must use rms values for both V and I. |

Power Transformer |
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It is an array of coils with at least two pairs of terminals corresponding to primary and secondary windings. When we connect an AC source at the primary winding, we...