Diodes Tutorial

Diodes are polarised, which means that they must be inserted into the PCB the correct way round.
This is because an electric current will only flow through them in one direction (like air will only flow one way through a tyre valve).

Diodes have two connections, an anode and a cathode.
The cathode is always identified by a dot, ring or some other mark.



The pcb is often marked with a + sign for the cathode end.

Diodes come in all shapes and sizes.
They are often marked with a type number.
Detailed characteristics of a diode can be found by looking up the type number in a data book.

If you know how to measure resistance with a meter then test some diodes. A good one has low resistance in one direction and high in the other.

There are specialised types of diode available such as the zener and light emitting diode (LED).

Electric current

Current is the flow of charge. The unit of current is the ampere, A.
"One ampere is the current in a conductor when a charge of one coulomb passes through a cross section of the conductor each second."
Note that this is consistent with the formal SI definition of the ampere, namely,
"The ampere is the constant current which, when flowing in each of two parallel conductors of infinite length and 1m apart in a vacuum, produces between them a force of 2 x 10-7 N per metre of length".
Current is measured using an instrument called an ammeter.
Ammeters are connected in series with the part of the circuit through which one wishes to measure the current, I, and they have negligible resistance.
Potential difference:
Current flows in a circuit as a result of a difference in potential between two points in the circuit.
The potential difference between two points in a conductor is the work per unit charge done by the charge in moving from a point of higher potential to a point of lower potential.
The unit of potential difference is called the volt, V. One volt of potential difference exists between two points if one joule of work is done by each coulomb of charge in moving between them. Potential difference is measured by an instrument called a voltmeter.
Voltmeters are connected in parallel to the component across which one wishes to measure the potential difference. They have a resistance which is several orders of magnitude higher than the resistance of the component.
The current which flows through a voltmeter is negligible.
Resistance:
Georg Ohm discovered that for any conductor, the ratio of the potential difference across the conductor and the current flowing through it, is constant. This constant is called the resistance of the conductor, R. We can write:

The unit of resistance is the OHM, .
"A conductor has a resistance of one ohm if on application of a potential difference of one volt, a current of one ampere flows through it."
Resistance is a property of a particular conductor and depends on:
1. The material of which the conductor is made.
2. The length, L, of the conductor (R L).
3. The cross-sectional area, A, of the conductor (R  A-1)
4. The temperature of the conductor. (Resistance increases non-linearly with temperature).

Electric power:
Remember that power is defined as the rate at which work is done:

By substituting from W = VIt, we obtain the formula for the power dissipated in an electric circuit, as follows:

This formula gives the power which is dissipated when a current I moves through a conductor across which there is a potential difference V.
From Ohm's law we may also write

The unit of power is the WATT, W which is equivalent to one joule per second, J.s-1.

SCR Tester

This handy tester will provide a visual "on" or "off" switching and latching indication. When finished, you can test all those posible 'duds' in your junkbox and dump some of those in the garbage. If the scr is latching and can hold the latch it is most likely okay.

Look at the circuit diagram, it shows a 3-amp, 50-volt SCR (under test) and a test circuit. Points "Gate" and "Kathode" are temporary connections, so that they can easily be opened. I used toggle switches for each, but use whatever you feel comfortable with, a simple jumper wire would do the trick. I use this gadget in my shop and so have it mounted in a small case. This circuit can even be bread-boarded for your purpose.

When "Kathode" is closed, the lamp doesn't light. When "Gate" is also closed, the lamp lights to its full intensity. The lamp remains lit even if "Gate" is opened again. But when "Kathode" is opened, even momentarily, the lamp does not close again when "Kathode" is closed. That illustrates the "ON" and "OFF" operation of the SCR.

I tested the following SCR types: C106D1 (400V/4A)     
                              T106Y1 (30V/4A)
                              C106F (50V/4A)        
                              C106F1 (50V/4A)
                              C106B (200V/4A)      
                              CSM2B2 (100V/4A)
                              T106D1 (400V/4A)     
                              NTE5402 (100V/0.8A)
                              TIC106M (600V/8A)    
                              NTE5457 (400V/4A)
                              TIC126M (600V/25A)   
                              CR6AM-8 (400V/10A)
                              MCR106-3 (100V/4A)   
                              NTE5455 (200V/4A)
         
In all cases the tester was accurate in telling 'good'
from the 'bad'.

DC circuit equations and laws

Ohm's and Joule's Laws

NOTE: the symbol "V" is sometimes used to represent voltage instead of "E". In some cases, an author or circuit designer may choose to exclusively use "V" for voltage, never using the symbol "E." Other times the two symbols are used interchangeably, or "E" is used to represent voltage from a power source while "V" is used to represent voltage across a load (voltage "drop").

Kirchhoff's Laws

"The algebraic sum of all voltages in a loop must equal zero."

Kirchhoff's Voltage Law (KVL)

"The algebraic sum of all currents entering and exiting a node must equal zero."

Kirchhoff's Current Law (KCL)

From electronicsteacher

How to calculate transformer rating

How to calculate transformer rating?

The basic formula is P=V*I and between input output of the transformer we have Power input = Power output

For example if we want a 220W output at 220V then we need 1A at the output. Then at the input we must have at least 18.3V at 12V because: 12V*18.3 = 220v*1

So you have to wind the step up transformer 12v to 220v but input winding must be capable to bear 20A.