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CHOOSING A REPLACEMENT TRANSISTOR

Clearly the ideal replacement for any transistor is a new one of exactly the same type. But letÂ’s face it: there have been around 200,000 different transistor types produced so far over the years,

Syeda Tasneem Rumy
Syeda Tasneem Rumy
Jul 12, 2010
4 Comments | 3638 Views | 1 Hits


Transistor Photo

When you’re servicing a piece of faulty electronic equipment, the problem can easily involve a transistor that has shorted, gone open circuit or developed low gain and/or high leakage. Tracking down the dud may not be too hard, but finding a suitable replacement can pose problems — especially for newcomers.

Clearly the ideal replacement for any transistor is a new one of exactly the same type. But letÂ’s face it: there have been around 200,000 different transistor types produced so far over the years, many of them almost impossible to get nowadays.

 

So what do you do if the exact type is impossible to obtain?

As it happens, many circuits are really quite tolerant of transistor parameters, and because many transistors are very similar in terms of their main parameters, you can often substitute any of a number of transistors and still achieve normal circuit operation or very close to it. But you do need to know what youÂ’re doing, in selecting a substitute device.

The first step is to identify the faulty transistor. Preferably, look it up in one of the many excellent Transistor Data Books or Comparison Tables that are available (Electus stocks some very good examples of these — see the current Catalogue).

 

Basic info & parameters

Now make a note of its basic information: whether itÂ’s germanium (Ge) or silicon (Si); NPN or PNP; high power or low power; designed for linear amplification or switching; and low noise or not. Also note its main performance parameters: voltage breakdown rating (usually Vcemax or BVceo); current gain (b or hFE) at the likely collector current; and upper frequency limit (fT or Fmax).

What if you can’t even find it in the data books? Perhaps you can find out at least some basic information about it from the circuit schematic for the equipment — if you have access to it. For example this will tell you if it’s a PNP or an NPN; and if the base-emitter voltage is shown, this will tell you if it’s germanium (0.25V) or silicon (0.6V). Of course if it’s reasonably new equipment, you can probably assume it’s silicon anyway.

Transistor Photo

YouÂ’ll probably also be able to get a good idea of whether itÂ’s being used for switching or amplification, the supply voltage, the approximate collector current/power level (from the supply voltage divided by load impedance/resistance) and perhaps the operating frequency. If itÂ’s being used in the first stage of a preamp or high-gain amplifier, you can reasonably assume that itÂ’s a low-power, low noise type. If you donÂ’t even have a schematic, it would be a good idea to trace out the circuit in the immediate vicinity of the transistor in question. This probably wonÂ’t take long, and should let you work out at least the basics: whether itÂ’s an NPN (collector connected to the positive rail) or a PNP (collector to the negative rail), and what kind of resistance/impedance is limiting the collector-emitter current.

If the transistor seems to be being used for switching, you’ll hopefully also be able to get an idea of the base drive current available to switch it on. This will let you make a rough calculation of the minimum current gain needed, simply by dividing the approximate collector current when it’s ‘on’ (assuming it will then have very little voltage drop — say 200mV), by the estimated base drive current.

Transistor Photo

The dud transistorÂ’s own package, and whether or not itÂ’s fitted with a heatsink, will give you a pretty good idea of its power rating. For example if itÂ’s in a metal T0-18 or plastic TO-92 package, the odds are it has a power rating below 500mW. If itÂ’s in a TO-126 or TO-220 package with a small heatsink, itÂ’s probably a mediumpower device; and if itÂ’s in an SOT-93 or TO-3 package bolted to a large heatsink, itÂ’s clearly a high-power device.

How about transistors in RF or IF amplifier stages, or in oscillator circuits? Here youÂ’ll probably have a very good idea of the frequency it is working at, and if so, you can use this to estimate its Fmax or fT. A safe guess in most cases is to multiply the working frequency by a factor of 10.


Picking a substitute

At this stage youÂ’ve hopefully found or worked out the basic information and parameters for the dud transistor. Now itÂ’s a matter of looking through the data for the new transistors you can obtain, and choosing one that seems closest to it. This is basically just a process of progressive elimination:


1. First off, is it NPN or PNP? This narrows the field by 50% straight away, and the same applies for Si or Ge. Needless to say your substitute generally has to have the same polarity and semiconductor material, so this narrows your selection down quite a bit already.


2. Now, is it high or low power? You should try to match this reasonably carefully, or at the very least go for a device in the same or a very similar type of package to the original.


3. How about the voltage rating? YouÂ’ll need to find a device with either the same or a higher voltage rating, to make sure it doesnÂ’t break down in operation.


4. Now we come to maximum current rating, and here you have to be a bit more careful — especially with silicon transistors. The current gain of these devices is roughly proportional to collector current level, but reaches a peak and then drops again at higher current levels. Where this peak occurs depends on the transistor’s construction, but it can have an important effect on how the transistor behaves in your circuit. That’s why a transistor with much higher current rating may not work properly in your circuit, because its gain might actually be quite low at low working current levels; conversely a transistor with too low a current rating might be driven past its peak, where it will again have low gain.


This can be very important in switching circuits, because if the working gain is low, there might not be enough base current drive to switch the transistor properly on, and it could be damaged due to abnormally high power dissipation. Switching transistors spend most of the time either turned off or ‘hard on’ (saturated) — both conditions where there’s a relatively low power dissipation. So it’s important to match the maximum current rating fairly closely, to ensure that the transistor is going to be switched on and off properly.


5. You should now have narrowed the field down quite a lot, and be more or less at the fine-tuning level. HereÂ’s where you need to look at things like matching the original transistorÂ’s current gain hFE and its maximum frequency fT. In many cases it wonÂ’t matter much if you go for a device which is even better than the original in both these respects, as most circuits are designed to cope with a reasonably wide range in these parameters anyway.


Summary


Hopefully youÂ’ll find this rough guide to choosing a replacement transistor useful. The basic steps weÂ’ve covered should generally allow you to find a reasonable substitute for most transistors in typical circuit applications, assuming the exact type simply isnÂ’t available.

Note, however, that there will be specific cases where youÂ’ll need to be particularly cautious in choosing a replacement transistor, either because the device concerned is very special or the circuit itself is more critical of device parameters. For example horizontal output transistors in TV sets and monitors can be especially tricky, as they frequently have very fast charge-storage and turn-off characteristics, and may also have an inbuilt protective or booster diode. Be very careful about choosing a substitute for these, and also for transistors in DC-DC converters, motor control circuits, regulated power supplies and so on.

Watch out also for Darlington or ‘compound’ transistors, which will generally need to be replaced with a similar type. And of course for SCRs and triacs, which may physically look like a transistor — but definitely aren’t!

Finally, be very careful with RF power transistors, especially those designed for use at VHF and UHF. Here a replacement will generally need to be in exactly the same specialised RF package, and a very close match in terms of inter-lead and internal capacitances, etc.


Author's note: (Copyright © Electus Distribution, 2001) Note that Electus Distribution cannot accept responsibility for the outcome of any transistor substitution made on the basis of the above advice.
Keywords: Electus Distribution Reference Data Sheet, transistors, germanium , silicon, PNP, NPN, current/power level, amplifier , RF, IF , substitute


Comments on this article


Tina, S , ConveyLive.com, Photos, Picture, Album

Tina S : How did you attach photos in article? I asked several times here. Why didn't you clearly explain?! There is no option for general users to attach photo. It's so unfair.

Jul 13, 2010 at 05:17 AM, Tuesday  

Syeda, Tasneem Rumy , ConveyLive.com, Photos, Picture, Album

Syeda Tasneem Rumy : There will be options to attach photos for general users in next version. I had already told you that to add photos to articles, I use html codes. It will not be easy for normal users to add such codes and therefore it is not a recommended method. You will have to wait till the next version of conveylive comes which is hopefully very soon or that you may wish to discuss this issue with the main admin of conveylive: aazhbd to give you a solution. And what is fair and what is not, is the almighty Allah to decide. May Allah show you the right path.

Jul 14, 2010 at 09:52 AM, Wednesday  

Tina, S , ConveyLive.com, Photos, Picture, Album

Tina S : I didn't understand last 2 lines of your comment. I was talking about attaching photo in article nothing else. I am not coming here to decide anything. And for your kind information Almighty Allah has shown me the right and correct path. Anyway...may Allah show you the right way to talk over people and choose friends..

Jul 14, 2010 at 10:00 AM, Wednesday  

Tina, S , ConveyLive.com, Photos, Picture, Album

Tina S : And don't mind, its not necessary to bring Allah in all topics. I know you are religious but it doesn't mean that you always show it. "Judge not, that ye be not judged."

Jul 14, 2010 at 12:23 PM, Wednesday  

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