DIODE

SECTION SEVEN

                                                DIODE

TESTING DIODES

Diodes can have 4 different faults.

1. Open circuit in both directions.

2. Low resistance in both directions.

3. Leaky.

4. Breakdown under load.

TESTING A DIODE ON AN ANALOGUE METER

Testing a diode with an

Analogue Multimeter can be done on any of the resistance ranges. [The high resistance range is best - it sometimes has a high voltage battery for this range but this does not affect our testing]

There are two things you must remember.

1.   When the diode is measured in one direction, the needle will not move at all. The technical term for this is the diode is reverse biased. It will not allow any current to flow. Thus the needle will not move.

When the diode is connected around the other way, the needle will swing to the right (move up scale) to about 80% of the scale. This position represents the voltage drop across the junction of the diode and is NOT a resistance value. If you change the resistance range, the needle will move to a slightly different position due to the resistances inside the meter. The technical term for this is the diode is forward biased. This indicates the diode is not faulty.

The needle will swing to a slightly different position for a "normal diode" compared to a Schottky diode. This is due to the different junction voltage drops.

However we are only testing the diode at very low voltage and it may break-down when fitted to a circuit due to a higher voltage being present or due to a high current flowing.

2. The leads of an Analogue Multimeter have the positive of the battery connected to the black probe and the readings of a "good diode" are shown in the following two diagrams:

The diode is REVERSE BIASED in the diagram above and diodes not conduct

The diode is FORWARD BIASED in the diagram above and it conducts

TESTING A DIODE ON A DIGITAL METER

Testing a diode with a Digital Meter must be done on the "DIODE" setting as a digital meter does not deliver a current through the probes on some of t he resistance settings and will not produce an accurate reading.

The best thing to do with a "suspect" diode is to replace it. This is because a diode has

a number of chara cteristics that cannot be test ed with simple equipment. Some diodes have a fast recovery for use in high frequency circuits. They conduct very quickly and turn off very quickly so the waveform is processed accurately and efficiently.

If the diode is replaced with an ordinary diode, it will heat up as does not have the high-speed characteristic.

Other diodes have a low drop across them and if an ordinary is used, it will heat up.

Most diodes fail by going: SHORT-CIRCUIT.  This can be detected by a low resistance (x1 or x10 Ohms range) in both directions.

A diode can also go OPEN CIRCUIT. To locate this fault, pl ace an identical diode across the diode being tested.

A leaky diode can be detected by a low reading in one direction and a slight reading the other direction.

However this type of fault can only be detected when the circuit is working. The output of the circuit will be low and sometimes the diode heats up (more than normal).

A diode can go open under full load conditions and perform intermittently.

Diodes come in pairs in surface-mount packages and 4 diodes can be found in a bridge.

They are also available in pairs that look like a 3-leaded transistor.

The line on the end of the body of a diode  indicates the cathode and you cannot say "this is the positive lead." The correct way to describe the leads is to say the "cathode lead." The other lead is the anode. The cathode is defined as the electrode (or lead) through which an electric current flows out of a device.

The following diagrams show different types of diodes.

POWER DIODES

To understand how a power diode works, we need to describe a few things. This has

NEVER been described before, so read carefully.

The 240v AC (called the "mains") consists of two wires, one is called the ACTIVE and the other is NEUTRAL. Suppose you touch both wires. You will get a shock. The neutral is connected to an earth wire (or rod driven into the ground or connected to a water pipe) at the point where the electricity enters the premises and you do not get a shock from the NEUTRAL.

But the voltage on the active is rising to +345v then goes to -345v at the rate of 50 times per second (for a complete cycle).

345v is the peak voltage of 240v. You never get a 240v shock. (It is a 345v shock.)

In other words, if you touch the two wires at a particular instant, you would get a

POSITIVE 345v shock and at another instant you would get a negative 345v shock.

This is shown in the diagram below.

We now transfer this concept to the output of a transformer. The diagram shows an AC waveform on the output of the secondary.

This voltage is rising 15v higher than the bottom lead then it is 15v LOWER than the bottom lead. The bottom lead is called "zero volts." You have to say one lead or wire is not "rising and falling" as you need a "reference" or starting-point" or "zero point" for voltage measurements.

The diode only conducts when the voltage is "above zero" (actually when it is 0.7v above zero) and does not conduct (at all) when the voltage goes below zero.

This is shown on the output of the Power Diode. Only the positive peaks or the positive parts of the waveform appear on the output and this is called "pulsing DC."  This is called "half-wave" and is not used in a power supply. We have used it to describe how the diode works. The electrolytics charge during the peaks and deliver energy when the diode is not delivering current. This is how the output becomes a steady DC voltage.

Power supplies use FULL WAVE rectification and the other half of the AC waveform is delivered to the output (and fills in the "gaps") and appears as shown in "A."

DAMPER DIODES

A damper diode is a diode that detects a high voltage and SQUELCHES IT (reduces it - removes it). The signal that it squelches is a voltage that is in the opposite direction to the "supply voltage" and is produced by the collapsing of a magnetic field. Whenever a magnetic filled collapses, it produces a voltage in the winding that is opposite to the supply voltage and can be much higher. This is the principle of a flyback circuit or EHT circuit. The high voltage comes from the transformer.

The diode is placed so that the signal passes through it and less than 0.5v appears across it.

A damper diode can be placed across the coil of a relay, incorporated into a transistor or FET or placed across a winding of a flyback transformer to protect the driving transistor or FET.

It can also be called a "Reverse-Voltage Protection Diode," "Spike Suppression Diode," or "Voltage Clamp Diode." The main characteristic of a Damper Diode is HIGH SPEED so it can detect the spike and absorb the energy.

It does not have to be a high-voltage diode as the high voltage in the circuit i s being absorbed by the diode.

SILICON, GERMANIUM AND SCHOTTKY DIODES

When testing a diode with an analogue meter, you will get a low reading in one direction and a high (or NO READING) in the other direction. When reading in the LOW direction, the needle will swing nearly full scale and the reading is not a resistance-value but a reflection of the characteristic voltage drop across the junction of the diode. As we mentioned before, a resistance reading is really a voltage reading and the meter is measuring the voltage of the battery minus the voltage-drop across the diode.

Since Silicon, Germanium and Schottky Diodes have sli ghtly different characteristic voltage drops across the junction, you will get a slightly different reading on the scale. This does not represent one diode being better than the other or capable of handling a higher current or any other feature.

The quickest, easiest and cheapest way to find, fix and solve a problem caused by a faulty diode is to replace it.

There is no piece of test equipment capable of testing a diode fully, and the circuit you are working on is actually the best piece of test equipment as it is identifying the fault UNDER LOAD.

Only very simple tests can be done with a multimeter and it is best to check a diode with an ANALOGUE MULTIMETER as it outputs a higher current though the diode and produces a more-reliable result.

A Digital meter can produce false readings as it does not apply enough current to activate the junction.

Fortunately almost every digit al multimeter has a diode test mode.

Using this, a silicon diode should read a voltage drop between 0.5v to 0.8v  in the forward direction

and open in the reverse direction. For a germanium diode, the reading will be lower,

around 0.2v - 0.4v in the forward direction. A bad diode will read zero volts in both

directions.

LIGHT EMITTING DIODES (LEDs)

Light Emitting Diodes (LEDs) are diodes that produce light when current flows from anode to cathode. The LED does not emit light when it is revered-biased. It is used as a low current indicator in many types of consumer and industrial equipment, such as monitors, TV’s, printers, hi-fi systems, machinery and control panels.

The light produced by a LED can be visible, such as red, green, yellow or white. It can also be invisible and these LEDs are called Infrared LEDs. They are used in remote controls and to see if they are working, you need to point a digital camera at the LED and view the picture on the camera screen.

An LED needs about 2v - 3.6v a cross its leads to make it emit light, but this volt age must be exact for the type and colour of the LED. The simplest way to deliver the exact voltage is to have a supply that is higher than needed and include a voltage-dropping resistor. The v alue of the resistor must be selected so the current is between 2mA and 25mA. The cathode of the LED is identified by a flat on the side of the LED. The life expectancy of a LED is about 100,000 hours. LEDs rarely fail but they are very sensitive to heat and they must be soldered and de-soldered quickly. They are one of the most heat-sensitive components.

Light emitting diodes cannot be tested with most multimeters because the characteristic voltage across them is higher than the voltage of the battery in the meter.

However a simple tester can be made by joining 3 cells together with a 220R resistor

and 2 alligator clips:

Connect the clips to a LED and it will illuminate in only one direction.

The colour of the LED will determine the voltage across it. You can measure this voltage if you want to match two or more LEDs for identical operation.

Red LEDs are generally 1.7v to 1.9v. - depending on the quality such as "high-bright"

Green LEDs are 1.9v to 2.3v.

Orange LEDs are about 2.3v and

White LEDs and IR LEDs are about 3.3v to 3.6v.

The illumination produced by a LED is determined by the quality of the crystal. It is the crystal that produces the colour and y ou need to replace a LED with the same quality to achieve the same illumination.

Never connect a LED across a battery (such as 6v or 9v), as it will be instantly damaged. You must hav e a resistor in series with the LED to limit the current.

ZENER DIODES

All diodes are Zener diodes. For instance a 1N4148 is a 120v zener diode as this is its reverse breakdown voltage.

And a zener diode can be used as an ordinary diode in a circuit with a voltage that is below the zener value.

For instance, 20v zener diodes can be used in a 12v power supply as the voltage never reaches 20v, and the zener characteristic is never reached.

Most diodes have a reverse breakdown voltage a bove 100v, while most zeners are below 70v. A 24v zener can be created by using two 12v zeners in series and a normal diode has a characteristic voltage of 0.7v. This can be used to increase the voltage of a zener diode by 0.7v. See the diagram above. It uses 3 ordinary diodes to increase the output voltage of a 3-terminal regulator by 2.1v.

To tests a zener diode you need a power supply about 10v higher than the zener of the

diode. Connect t he zener across the supply with a 1k to 4k7 resistor and measure the voltage across the diode. If it measures less than 1v, reverse the zener.

If the reading is high or low in both directions, the zener is damaged.

Here is a zener diode tester. The circuit will test up to 56v zeners

ZENER DIODE TESTER

TRANSFORMERLESS POWER SUPPLY

Here's a circuit that uses zener diodes in a power supply to show how they work.  This clever design uses 4 diodes in a bridge to produce a fixed voltage power supply capable of supplying 35mA.

If we put 2 zener diodes in a bridge with two ordinary power diodes, the bridge will break-down at the voltage of the zener. This is what we have done. If we use 18v zeners, the output will be 17v4

SUPPLY USING ZENER DIODES

When the incoming voltage is positive at the top, the left zener provides 18v limit (and the other zener produces a drop of 0.6v).  This allows the right zener to pass current just like a normal diode.  The output is 17v4. The same with the other half-cycle.

You cannot use this type of bridge in a normal power supply as the zener diode will "short" when the input voltage reaches the zener value. The concept only works in the circuit above.