TRANSISTORS

SECTION TEN                                               

                                        TRANSISTORS

Transistors are solid-state devices and although they operate completely differently to a diode, they appear as two back-to-back diodes when tested.

There are basically 2 types of transistor NPN and PNP.

A transistor is sometimes referred to as BJT (Bi-polar Junction Transistor) to distinguish it from other types of transistor such as Field Effect transistor, Programmable Unijunction Transistor and others.

In the following diagram, two diodes are connected together and although the construction of a transistor is more complex, we see the transistor as two diodes when testing it.

 

A TRANSISTOR APPEARS AS TWO DIODES WHEN TESTING IT

All transistors have three leads. Base (b), Collector (c), and Emitter (e).

For an NPN transistor, the arrow on the emitter points away from the base.

It is fortunate that the arrow on both symbols points in the direction of the flow of current (Conventional Current) and this makes it easy to describe testing methods using our simplified set of instructions. The symbols have been drawn exactly as they appear on a circuit diagram.

All transistors are the same but we talk about digital and analogue transistors. There is no difference between the two.

The difference is the circuit. And the only other slight difference between transistors is the fact that some have inbuilt diodes and resistors to simplify the rest of the circuit.

All transistors work the same way. The only difference is the amount of amplification they provide, the current and voltage they can withstand and the speed at which they work. For simple testing purposes, they are all the same.

NPN transistors are the most common and for an NPN transistor, the following applies. (the opposite applies for PNP)

To test a transistor, there is one thing you have to know:

When the base voltage is higher than the emitter, current flows though the

collector-emitter leads.

As the voltage is increased on the base, nothing happens until the voltage reaches

0.55v. At this point a very small current flows through the collector-emitter leads. As the voltage is increased, the current-flow increases. At about 0.75v, the current-flow is a MAXIMUM. (can be as high as 0.9v). That's how it works. A transistor also needs current to flow into the base to perform this amplifying function and this is the one feature that separates an ordinary transistor from a FET.

If the voltage on the base is 0v, then instantly goes to 0.75v, the transistor initially passes NO current, then FULL current. The transistor is said to be working in its two states: OFF then ON (sometimes called: "cut-off" and "saturation"). These are called digital states and the transistor is said to be a

DIGITAL TR ANSISTOR or a SWITCHING TRANSISTOR, working in

DIGITAL MODE.

If the base is delivered 0.5v, then slowly rises to 0.75v and slowly to 0.65v, then 0.7v, then 0.56v etc, the transistor is said to be working in ANALOGUE MODE and the transistor is an ANALOGUE TRANSISTOR.

Since a transistor is capable of amplifying a signal, it is said to be an active device.

Components such as resistors, capacitors, inductors and diodes are not able to amplify and are therefore known as passive components.

In the following tests, use your finger to provide the TURN ON voltage for the base (this is 0.55v to 0.7v) and as you press harder, more current flows into the base and thus more current flows through the collector-emitter terminals. As more current flows, the needle of the multimeter moves UP-SCALE.

TESTING A TRANSISTOR ON A DIGITAL METER

Testing a transistor 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 the resistance settings and will not produce an accurate reading.

The "DIODE" setting must be used for diodes and transistors. It should also be called a "TRANSISTOR" setting. TESTING AN unknown TRANSISTOR

The first thing you may want to do is test an unknown transistor for COLLECTOR, BASE AND EMITTER. You also want to perform a test to find out if it is NPN or PNP.

That's what this test will provide. You need a cheap multimeter called an ANALOGUE METER - a multimeter with a scale and pointer (needle).

It will measure resistance values (normally used to test resistors) - (you can also test other components) and Voltage and Current. We use the resistance settings. It may have ranges such as "x10"  "x100"   "x1k"   "x10" Look at the resistance scale on the meter. It will be the top scale.

The scale starts at zero on the right and the high values are on the left. This is opposite to all the other scales.

When the two probes are touched together, the needle swings FULL SCALE and reads "ZERO." Adjust the pot on the side of the meter to make the pointer read exactly zero.

How to read:  "x10"  "x100"   "x1k"   "x10"

Up-scale from the zero mark is "1" When the needle swings to this position on the "x10" setting, the value is 10 ohms.

When the needle swings to "1" on the "x100" setting, the value is 100 ohms.

When the needle swings to "1" on the "x1k" setting, the value is 1,000 ohms = 1k.

When the needle swings to "1" on the "x10k" setting, the value is 10,000 ohms = 10k.

Use this to work out all the other values on the scale.

Resistance values get very close-together (and very inaccurate) at the high end of the scale. [This is just a point to note and does not affect testing a transistor.]

Step 1   - FINDING THE BASE and determining NPN or PNP

Get an unknown transistor and test it with a multimeter set to "x10"

Try the 6 combinations and when you have the black probe on a pin and the red probe touches the other pins and the meter swings nearly full scale, you have an NPN transistor. The black probe is BASE

If the red probe touches a pin and the black probe produces a swing on the other two pins, you have a PNP transistor. The red probe is BASE

If the needle swings FULL SCALE or if it swings for more than 2 readings, the transistor is FAULTY.

Step 2   - FINDING THE COLLECTOR and EMITTER

Set the meter to "x10k."

For an NPN transistor, place the leads on the transistor and when you press hard on the two leads shown in the diagram below, the needle will swing almost full scale.

For a PNP transistor, set the meter to "x10k " place the leads on the transistor and when you press hard on the two leads shown in the diagram below, the needle will swing almost full scale.

SIMPLEST

TRANSISTOR TESTER

The simplest transistor tester uses a 9v battery, 1k resistor and a LED (any colour).

Keep trying a transistor in all different combinations until you get one of the circuits below. When you push on the two leads, the LED will get brighter.

The transistor will be NPN or PNP and the leads will be identified

The leads of some transistors will need to be bent so the pins are in the same positions as shown in the diagrams. This helps you see how the transistor is being turned on.

This works with NPN, PNP transistors and Darlington transistors.

HEATSINKING

Heat generated by current flowing between the collector and emitter leads of a transistor causes its temperature to rise. This heat must be conducted away from the transistor otherwise the rise may be high enough to damage the P-N junctions inside the device. Power transistors produce a lot of heat, and are therefore usually mounted on a piece of aluminum with fins, called a HEATSINK.

This draws heat away, allowing it to handle more current. Low-power signal transistors do not normally require heat sinking. Some transistors have a metal body or fin to connect to a larger heatsink. If the transistor is connected to a heatsink with a mica sheet (mica washer), it can be damaged or cracked and create a short-circuit. (See Testing Mica Washers).  Or a small piece of metal may be puncturing the mica.

Sometimes white compound called Heatsink Compound is used to conduct heat through the mica. This is very important as mica is a very poor conductor of heat and the compound is needed to provide maximum thermal conduction.

TRANSISTOR FAILURE

Transistor can fail in a number of ways. They have forward and reverse voltage ratings and once these are exceeded, the transistor will ZENER or conduct and may fail. In some cases a high voltage will "puncture" the transistor and it will fail instantly. In fact it will fail much faster via a voltage-spike than a current overload.

It may fail with a "short" between any leads, with a collector-emitter short being the most common. However failures will also create shorts between all three leads.

A shorted transistor will allow a large current to flow, and cause other components to heat up.

Transistors can also develop an open circuit between base and collector, base and emitter or collector and emitter.

The first step in identifying a faulty transistor is to check for signs of overheating. It may appear to be burnt, melted or exploded. When the equipment is switched off, you can touch the transistor to see if it feels unusually hot. The amount of heat you feel should be proportional to the size of the transistor's heat sink. If the transistor has no heat sink, yet is very hot, you can suspect a problem.

DO NOT TOUCH A TRANSISTOR IF IT IS PART OF A CIRCUIT THAT CARRIES 240VAC.

Always switch off the equipment before touching any component s.

TRANSISTOR REPLACEMENT

If you can't get an exact replacement, refer to a transistor substitution guide to identify a near equivalent.

The important parameters are:

- Voltage

- Current

- Wattage

- Maximum frequency of operation

The replacement part should have parameters equal to or higher than the original.

Points to remember:

- Polarity of the transistor i.e. PNP or NPN.

- At least the same voltage, current and wattage rating.

- Low frequency or high frequency type.

- Check the pinout of the replacement part

- Use a desoldering pump to remove the transistor to prevent damage to the printed circuit board.

- Fit the heat sink.

- Check the mica washer and use heat-sink compound

- Tighten the nut/bolt - not too tight or too loose.

- Horizontal output transistors with an integrated diode should be replaced with the same type.

DIGITAL TRANSISTORS

There is no such thing as a DIGITAL TRANSISTOR, however some transistors are available with built-in resistors between base and emitter (to save space on the board) and these transistors are often used in digital circuits. The transistor will amplify analogue signals but when the signal is 0v then immediately goes to a voltage above 0.7v, the transistor is in a DIGITAL CIRCUIT and the transistor is called a DIGITAL TRANSISTOR.   It is tested like an ordinary transistor but the low value resistor between base and emitter will produce a low reading in both directions.

DARLINGTON TRANSISTORS

A DARLINGTON TRANSISTOR is two transistors in a single package with three leads.

They are internally connected in cascade so the gain of the pair is very high. This allows a very small input signal to produce a large signal at the output. They have three leads (Base, Collector and Emitter and can be PNP or NPN) and are equivalent to the leads of a standard individual transistor, but with a very high gain. The second advantage of a Darlington Transistor is its high input impedance. It puts very little load on the previous circuit.

Some Darlington transistors have a built-in diode and/or built-in resistor and this will produce a low reading in both directions between the base and emitter leads.

Darlington transistors are tested the same as an ordinary transistor and a multimeter will produce about the same deflection, even though you will be measuring across two junctions, (and a base-emitter resistor is present).

HORIZONTAL OUTPUT TRANSISTORS, SWITCH-MODE

TRANSISTORS, FLYBACK TRANSISTORS, POWER TRANSISTORS,

VERTICAL TRANSISTORS . . . .

These are all names given to a transistor when it is used in a particular circuit. ALL these transistors are the same for testing purposes.

We are not testing for gain, maximum voltage, speed of operation or any special feature. We are just testing to see if the transistor is completely faulty and SHORTED.

A transistor can have lots of other faults and the circuit using the transistor is the best piece of TEST EQUIPMENT as it is detecting the fault.

TESTING MOSFETs and FETs

MOSFETs and JFETs are all part of the FET family. MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor.

FETs operate exactly the same as a "normal" transistor except they have different names for the input and output leads and the voltage between the  gate and the source has to between 2v to 5v for the device to turn on fully. A FET requires almost NO CURRENT into the Gate for it to turn on and when it does, the voltage between drain and source is very low (only a few mV). This allows them to pass very high currents without getting hot. There is a point where they start to turn on and the input voltage must rise higher than this so the FET turns on FULLY and does not get hot.

Field Effect Transistors are difficult to test with a multimeter, but "fortunately" when a power

MOSFET blows, it is completely damaged. All the leads will show a short circuit. 99% of bad MOSFETs will have GS, GD and DS shorted.

The following symbols show some of the different types of MOSFETs:

Most MOSFET transistors cannot be tested with a multimeter. This due to the fact that the Gate needs 2v - 5v to turn on the device and this voltage is not present on the probes of either meter set to any of the ohms ranges.

You need to build the following Test Circuit:

Touching the Gate will increase the voltage on the Gate and the MOSFET will turn on and illuminate the LED. Removing your finger will turn the LED off.