Tag Archives: PNP

Old Kit Review – Silicon Chip Transistor Beta Tester


After exploring a quiet , dusty electronics store in the depths of suburbia the other week, I came across this kit from Altronics (K2534) which is the subject of this review. The Transistor Beta tester is the second revision of a tester designed by John Clarke for the March 1991 issue of Silicon Chip magazine, and promises to offer a simple way of measuring the gain of almost any NPN or PNP bipolar transistor. But first some public answers to recent feedback…

John – Why do you publish these “Old Kit Reviews”?

They’re more of  a selfish article, like many electronics enthusiasts I’ve enjoyed kits for decades – and finding kits from days gone by is a treat. From various feedback some of you are enjoying them, so I’ll continue with them for fun and some nostalgia. If you’re not interested, just ignore the posts starting with “Old”!

Where’s the schematic?

After publishing a few kit reviews, people have been asking me for the schematics. For kits that are based on magazine articles from Silicon Chip and the like, the details are Copyright and I can’t legitimately give you a copy. You need to contact the magazine or kit supplier. The surviving electronics magazines often run “on the smell of an oily rag” so in order to support them I promote the idea of paying for copies which are obtainable from the magazine. Plus Australia is a small country, where people in this industry know each other through first or second connections – so I don’t want to annoy the wrong people. However Google is an awesome tool,  and if you want to make your own beta tester there are many example circuits to be found – so have fun.

Back to the review – what is “beta”?

Apart from a letter of the Greek alphabet and a totally-underrated form of VCR format, beta is a term used to define the amount of gain of a transistor. From the guide:

Silicon Chip transistor beta tester what is beta


Here’s our kit from 1991, rescued from the darkness of the store:

Silicon Chip transistor beta tester packaging

Which contained the nice box, plus all the required components except for an IC socket, and a few screws and mounting nuts that should have been included. The instructions looked to be a photocopy of a photocopy, harking back to the 1980s…

Silicon Chip transistor beta tester contents

Silicon Chip transistor beta tester components

Looks like an off-brand 555 has been used (or substituted), however a bit of research indicated that it is most likely from LG Semiconductor:

Silicon Chip transistor beta tester off brand 555

The PCB was made to the usual standard at the time, just drilled:

Silicon Chip transistor beta tester PCB rear

Silicon Chip transistor beta tester PCB front

The front panel was well done, and kindly pre-drilled by a previous customer. The kit came with a 3mm LED however this mystery person had drilled the hole out for a 5mm:

Silicon Chip transistor beta tester front panel

… but hadn’t cut the oblong for the slide switch wide enough. But the biggest problem was that the PCB was just a smidge too wide for the included enclosure:

Silicon Chip transistor beta tester PCB not fitting

Nevertheless it was time to get started, and the resistors were measured, lined up and fitted:

Silicon Chip transistor beta tester resistors lined up

Then the rest of the components fitted as normal, however they need to stay below the horizontal level of the slide switch bezel:

Silicon Chip transistor beta tester assembly 1

… which was somewhat successful. Then to fit the potentiometer, battery snap …

Silicon Chip transistor beta tester assembly 2

and the test leads:

Silicon Chip transistor beta tester assembly 3

 And we’re finished:

Silicon Chip transistor beta tester finished

How it works

Operation is quite simple, just wire up the test leads to the transistor’s base, collector and emitter – set the PNP/NPN switch and press test. Then you turn the knob until the LED just turns on – at which point the scale indicates the gain.

“Modern-day” replacements

Digital technology has taken over with this regard, and a device such as the one below can not only give the gain, but also the component details, identify legs, and much more:

Silicon Chip transistor beta tester modern tester gain

I’ll be sticking with this one for the time being. Jaycar have discontinued the analyser shown above, but Altronics have the “Peak” unit which looks even more useful.


Well… that was fun. A lot of promise, however with a few details not taken care of the kit was just a bit off. Considering this was around twenty years old and possibly shop-soiled I can’t complain. For the record the good people at Altronics have a great line of kits. Full-sized images and a lot more information about the kit are available on flickr.

And while you’re here – are you interested in Arduino? Check out my new book “Arduino Workshop” from No Starch Press.


In the meanwhile have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column? And join our friendly Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

Education – the Bipolar Transistor – part two

Hello readers

Today we continue with the second in a series of articles about the bipolar transistor. The first section is here. In this article we look at using the bipolar transistor as an amplifier. That is, change a very small alternating-current signal and make it larger, increasing the amplitude of the signal. Although originally it would seem to be rather simple, perhaps it is not. There are many, many ways to construct a transistor amplifier circuit, but I hope this introduction helps your basic understanding of the process.

When we used the transistor as a switch in part one, we were concerned about the amount of current that flowed between the base and the emitter – that it did not exceed the maximum rating for the particular transistor. When a transistor is allowing the most amount possible of current to flow, it is saturated – the point where the transistor cannot handle any more current. However, to use a transistor as an amplifier we need to bias the transistor so that it is passing current, but not saturated. The procedure of setting the output DC level is known as biasing. The procedure for biasing is outside the scope of this article.

When selecting transistors one needs to take note of the hFE (DC current gain), as variations in this will require a complete recalculation of the values for the bias resistors. Even a common model such as the BC548 is available with hFE ranges between 110~520.


one transistor amplifier

Consider the example schematic above. The transistor is not saturated, due to the bias being set by the two 10k ohm resistors, which drops the voltage over the base by around half. In this case with our 6V supply this drops to around 3V. When power is applied, the transistor is biased and allows a small amount of current to flow, but it still has a lot more current-handling capacity. In testing this example, without an input the base current Ib is 0.32 milliamps, and the collector current Ic is 19.9 milliamps . These amounts of current are known as the quiescent current values.

The purpose of the 0.1 uF capacitor is to block DC current and only allow AC current to flow. When the AC current passes through the 0.1 uF capacitor, it is combined with the DC quiescent current running through the base and rides the stronger current out of the emitter. At which point the 100 uF capacitor before the speaker stops the DC current and only allows the AC signal through to the speaker, but amplified. The level of amplification is dependent upon the gain of the transistor, and the amount of base current. Let’s have a look at the behaviour of the current as it passes through the example circuit above:

At the end stage of the video clip we increased the input signal greatly. Did you notice the clipping at the output? This occurs when the voltage is too great for the transistor, and therefore it cannot pass the complete signal through to the emitter. In an audio signal situation, this will cause distortion. That is another reason to check the specification of the transistor against your requirements.

Moving along. You can also connect more than one transistor together to increase the amplification, for example:


two-transistor amplifier

The left half of the circuit above should be familiar. The 10uF capacitor at the bottom is to stop the DC current being passed through to the base of the BC548 transistor. The second transistor, the BC558 is a PNP transistor, and amplifies the signal at the collector of the BC548. Finally, the 1uF capacitor blocks the DC current from reaching the output. However in using two or more transistors in such a method, you need to ensure the emitter current rating of the second transistor is much higher, as the gain of two transistors is the product of the individual transistors’ gain.

As stated at the beginning, this is only an introduction. There are literally hundreds of thousands of pages of material written about the use of transistors, so don’t stop here – experiment and do your own research and learning!

As always, thank you for reading and I look forward to your comments and so on. Furthermore, don’t be shy in pointing out errors or places that could use improvement. Please subscribe using one of the methods at the top-right of this web page to receive updates on new posts. Or join our new Google Group.

Education – the Bipolar Transistor

Hello readers

Today we continue with the series of articles on basic electronics with this introductory article about the bipolar transistor, and using it as a switch.

What is a transistor?  It is a semiconductor with three leads, with which a small voltage or current applied to one lead can control a much larger current flowing through the other two leads. A transistor can be used as either a switch or an amplifier. Furthermore, there are two main types of transistor – the bipolar and the field-effect transistor. This article will examine and refer to bipolar transistors as transistors. Let’s go!

A transistor consists of three layers of silicon, P- and N-type in fact. Do you recall the diode article? A transistor is basically two diodes connected together in a Y-formation, in one of two ways as shown below:


Also notice the circuit symbols for NPN and PNP-type transistors. Transistors can be found in many shapes and sizes, the size usually being directly proportional to the amount of current the particular transistor is designed to handle. Thankfully the physical shape or package design has been standardised, and each casing type has a designation. Let’s look at some of the more common ones now:

TO-92 casing. When the flat-side is facing you, the pin numbering is 1-2-3. This casing style is for transistors that usually handle up to 100 mA. Unfortunately there are three varieties with regards to which pin is the base, collector and emitter – so always check your data sheet if in doubt.

TO-220 casing. When the metal tab is at the rear (above), the pin numbering is 1-2-3. The metal tab acts as a heatsink, and the hole enables one to bolt it to a larger heatsink, metal chassis, etc. This casing style is for transistors that usually handle up to around 8 amps.

TO-3 casing. These are all metal in order to dissipate heat – as they can handle up to around 75 amps of current. The entire metal case and ends are pin 2; pins 1 and 3 are the usual leads. Check your data sheet for pins 1 and 3! There are many other types of casing, but the three above are usually the most common.

How do transistors work?

For current to flow from the base of a transistor to the emitter, it needs to be forward-biased by at least 0.6 volts. In other words, there must be a potential difference between the base and emitter by 0.6V. If the base is connected to ground, the transistor will not let current pass from the collector to the emitter:


The transistors in the circuits above are NPN transistors. The current that flows from the base to the emitter is known as base current or Ib. The current that flows from the collector to the emitter is known as the collector current, or Ic. An interesting property of the transistor is this: the ratio of Ic to Ib is constant, and Ic is always larger than Ib. The ratio of Ic/Ib is known as the gain of the transistor. When reading a data sheet, gain is usually defined as hFE. This formula also proves that if there is no base current, there will be no collector current – you can’t divide by zero.

Using the transistor as a switch

To use a transistor as a switch, we need to know several things to be successful. For example:

To use the transistor to turn on the “load” we need to:

  • know the current drawn by the load. This is also the transistor’s Ic (collector current). Or the load’s resistance, as Ohm’s law can be used to calculate the current
  • know the transistor gain (hFE)
  • calculate Ib (base current)
  • use the above data to find a value for that lonely resistor

Let’s do that now with a contemporary problem… we have an Arduino that needs to turn a relay on and off at certain times. However you can only source up to 20 mA from a digital output on the Arduino, so we want to use it instead to switch a transistor which can control the relay coil. The problem is, what value resistor to use to control the base current?


First of all, let’s note what we do know. The relay (data sheet) coil requires 60 mA of current to activate, it is a 5 volt relay, and the coil has a resistance of 83 ohms. The transistor (data sheet) is a BC548 NPN transistor, very cheap and easy to find. It can handle a collector/load current of 100 mA, and the hFE (gain) is 110.  That diode is there to loop back pulse current when the relay is switched off. The supply voltage is 5 volts, and the digital output from the arduino is also 5 volts when active. There is also one more thing to take note of – the base-emitter junction is a diode, and therefore has a voltage drop of 0.7 volts. When you are switching large voltages, this is not an issue – however as we are working with a small voltage, the drop needs to be taken into account.

So, let’s calculate Ib, the base current. If hFE = Ic/Ib then 110 = 0.06 A/Ib; which translates to Ib = 0.06/110 = 0.0005 A. Which is basically nothing, so we’ll round it up to 1 milliamp.

Next, the resistor value. Using Ohm’s law (voltage = current x resistance):

Voltage = (5-0.7) = 4.3 volts (we need to take into account the voltage drop over the base-emitter junction of the transistor)

Current = 0.005 A (Will use a slightly higher current just to be on the safe side)

So, resistance = 4.3/0.005 = 860 ohms. For such a tiny current and small voltage, a 1/4-watt resistor is fine. (power = volts x current; = 4.3 * 0.005 = 0.0215 < 0.25)

If we didn’t have an 860 ohm resistor, a little higher is OK. I have used a 1k ohm resistor and it has worked nicely.

And there you have it – a transistor used as a switch. As stated at the beginning, this is only an introduction. There are literally hundreds of thousands of pages of material written about the use of transistors, so don’t stop here – experiment and do your own research and learning! In the next few weeks we will look at using transistors as amplifiers.

As always, thank you for reading and I look forward to your comments and so on. Furthermore, don’t be shy in pointing out errors or places that could use improvement. Please subscribe using one of the methods at the top-right of this web page to receive updates on new posts. Or join our new Google Group.

Some information for this post is from: historical info from Wikipedia; various technical information and inspiration from books by Forrest Mims III;  TO-3 package photo from Farnell Australia.