Diodes and rectifiers

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Some basics

A diode is a device that will allow current flow in one direction. It is the electrical equivalent of a non-return valve in a water system.
A certain voltage is required to allow current to flow. This is true of all diodes but is important when considering using a silicon or germanium diode. But diodes will only tolerate a certain voltage before they “breakdown” and allow current to flow the “wrong way”. This property, however, is useful and a device called a zener diode makes use this to good effect.
Diodes can be semiconductor or vacuum “thermionic” valves. Although valves are more of historical interest technicians may encounter the basic “Teltron” model.
Rectification is the term used to convert alternating current (a.c.) into direct current (d.c.). NOTE a voltage that varies but does not go negative is a direct current.

a: anode, c: cathode


An easy way to remember the orientation of the diode is that the silver bar on a plastic case corresponds to the vertical stripe on the symbol. Thus the end that the current flows to (the cathode) has the silver bar.

Illustration of rectification

The simplest method of showing a LED acting as a rectifier is to use a single LED connected in series to a resistor across a power supply. This will show that the LED will only light when connected in one direction.
It is a short step then to suggest that instead of manually reversing the applied voltage, an a.c. source can be used.
This can be shown simply using two LEDs and protective resistors.

current path illustration


This simple to construct demo will illustrate that a.c. current will only flow through one LED at a time. Note that the wave form produced across the resistors are only half waves.
Half wave rectification shows a "half wave" when the diode conducts but the second half of the cycle is zero as no current flows.
To make the the apparatus more visual two different coloured LEDs should be used
An LED requires about 2V before it will light. It also has a maximum current of about 40mA that it can carry (you can see why these are not used for power applications!). So using a 6V signal generator and working at an "LED safe" 10mA. The protective resistor must drop 4V. Using V=IR we can see that the value of R is V/I =4/0.01 or 400 Ohm. The prefered value that is closest to this (but higher) is 470 Ohm.
Preferred values (external link)

Full wave rectification

waveforms

The wave forms that may be traced out are given in the diagram opposite.
The normal a.c wave form is shown and, on the same time base, the rectified outputs are shown.
Because of the nature of the diodes and their "turn on" voltages the diagrams are not what would actually be found. There would be a step taken out of each cycle where the diodes are not conducting. The diagram is often referenced as having this form so is shown as an "ideal" trace.

The circuit for this can be built using the circuit diagram lower right:

led demo of Full wave rectification


Note LEDs will not allow much power to be passed so this is for visual explanation only. The "load" resistor R should be of such a size that the diodes are within their working loads. Calculation:
For a 6V supply from a Signal generator a 220 Ohm resistor should suffice.
Calculation of load resistance:
As above an LED needs to drop about 2V to light as 2 are used only 2V is left to be dropped across the resistor. Using the LED safe 10mA we have R=V/I =2/0.010 =200 Ohm or 220 for the nearest preffered value.


Explanation of circuit

Consider the right hand side of the AC supply as momentarily being more positive than the left.
Current flows along the blue path (it is blocked by the action of the diodes from taking the other routes).
As the current falls to zero the diode stops conducting.
The LHS then becomes more positive and the current follows the green path.
The current flows in the same direction no matter which way the ac voltage is applied
Thus the wave form is a half wave form when conducting right to left and half wave form conducting left to right.
Please refer to the waveform diagrams above to make sure that this is understood.

If this is done slowly the turn on voltage of the LED can be commented on to illustrate the off-set of Si or Ge diodes.

Bridge rectifiers

These are packages that are power examples of the 4 LED circuit above.
They noramlly come in two forms a square (footprint) plastic case with a pin at each corner or a rectanglar package with four pins in line.
The body has the markings (2x)~ for ac inputs and + and - for the rectified output. Different makes may have different layouts so be careful if replacing faulty bridges. Bridge rectifiers taking several amps are not uncommon, although high current situations usually have distinct diodes.
Bridge rectifiers can be un-soldered from a circuit and tested as if you were testing diodes.

How does a diode work?

The theory behind how the diodes work can involve some serious maths and physics, however a general idea should be more than adequate for a school technician.
A diode is made from a semi-conducting material such as silicon or germanium. This in itself would not be of much use until some impurities are introduced into the semiconductor. The impurity is introduced so as to either allow an excess of electrons or “holes”. Where the semiconductor has an excess of electrons it is called n-type (n for negative) similarly where holes predominate, a p-type (p-positive) semiconductor exists.
A diode is formed at a junction of the n- and p-type materials. Here the electrons and holes move across the junction to capture each other through diffusion. This leaves a “depletion region” where few charge carriers exist. The migration of these charges leaves a negative charge in the p-type and a positive charge in the n-type material. This causes a “potential barrier” to exists to prevent further charge movement. The size of this can be modified by the application of an external potential difference.
If a potential difference is applied to a Silicon p-n junction, so that the p-type semiconductor is connected to the positive (and the n-type is negative). The potential barrier decreases, but no current flows -until about 0.6V is applied and the barrier is overcome. If the connections are reversed the potential barrier becomes greater and so no current flows.
In Germanium this forward bias limit is less at about half that of Silicon (0.3 V compared to 0.6V). Germanium based devices are more expensive and are less common. They are also more likely to be damaged when soldering.

Testing a diode

This section concerns testing diodes out of circuits with a multimeter. The diode can be shown if it works and wheather it is a silicon or germanium diode.
Most multimeters have a diode check facility. This is shown with diode symbol. The diode check acts differently to the continuity test in that it shows the voltage dropped over the diode.

  1. Place the positive probe of the multimeter in the socket associated with the diode. The black probe should go to common.
  2. Test the multimeter by shorting the probes (should give approximately 0.00 on the display)
  3. Put the positive probe to the Anode and the negative to the cathode. Note the arrow indicates the way current flows.
  4. note the reading (0.7 to 0.9 v implies a silicon diode, 0.4 ish implies germanium, 0 ish implies short, OL or 1 on left of display implies blown diode)
  5. Reverse the diode
  6. Display should show OL or 1 on left.


Internal Links

External Links

--D.B.Ferguson 20:42, 6 April 2008 (BST)
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