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Transistors      Transistor Substitution

 

De-soldering and replacing a transistor takes very little time.  More time is spent figuring out which one to replace and, sometimes, what to replace it with!

The original transistor might need special ordering, or be discontinued and unavailable.  Knowing how to substitute an alternate transistor type can speed up your repair, or make an otherwise impossible repair possible.

 

TRANSISTOR TYPES

 

Transistors are given registered type numbers by the Joint Electron Device Engineering Council (JEDEC).  The type number is often printed on the transistor, although product manufacturers sometimes have their own part numbers printed instead.  In that case, the product's Service Manual or Parts List might provide you with the transistor type.

If you don't have that particular type, a transistor cross-reference guide can sometimes help you to find a compatible, generic part.  The NTE Cross Reference is a good example - NTE branded replacements are widely available.  But you might already have a suitable substitute in your parts drawers.

We arrange our transistor stock, not by their type number, but by their specifications  This makes it easy to go right to the drawers holding good candidates for substitution.

You can find transistor specs in data sheets available online.  Use your browser to search for the transistor's JEDEC type, or use a search engine like AllDataSheet.com.

The first spec to consider is the Transistor Outline.  This is the only spec you can actually see.

 

TRANSISTOR OUTLINES

 

A Transistor Outline number, or TO, refers to a transistor's physical size, shape, and mounting style.  Some TO numbers are shown here:







 

The transistor outline doesn't usually tell you which of the three transistor leads connects to its Base terminal (B), its Emitter terminal (E), or its Collector terminal (C).  However, the transistor's data sheet will show you that.

If the lead arrangement of a small replacement transistor differs from that of the the original, you can sometimes bend the transistor leads across one another to route them into the correct holes in the circuit board.  Just use a bit of spaghetti tubing on the leads to keep them from touching each other.

 

BIPOLAR TRANSISTOR SPECS

 

Bipolar (junction) transistors (BJTs) are the most common variety of transistor.  The following electrical specs are important when choosing a substitute BJT.

 

TRANSISTOR POLARITY

 

A junction transistor is a sandwich of "N" and "P" type semiconductors.  Either the "meat" has extra negative charge carriers (electrons) and the "bread" has extra positive charge carriers (holes), or the meat has the positive charges and the bread has the negative charges.

A substitute transistor must have the same polarity, either "PNP" or "NPN", as the original.   If you install the wrong polarity, the sandwich won't work.

 

MAXIMUM VOLTAGES

 

If more than a maximum voltage rating is applied to a transistor, it can be permanently damaged.  At the maximum voltage, also called a breakdown voltage (BV), electrons begin to avalanche in the transistor.

During an avalanche, electrons in the P-N transition regions are accelerated to energies so high that they hit bound electrons with enough force to free them, creating additional charge carriers and greatly multiplying the transistor current.

There are three breakdown voltages:

  • VCB - the maximum voltage across the Collector-Base terminals
  • VCE - the maximum voltage across the Collector-Emitter terminals
  • VEB - the maximum voltage across the Emitter-Base terminals

In each of these ratings, the 3rd terminal is assumed to be electrically open (unconnected).  VCE, for example, may be written as VCEO, BVCEO, or most correctly as V(BR)CEO.

The VEB rating isn't usually a factor in choosing a substitute transistor.

VCB is always equal to or greater than VCE and you can use either of these maximum voltages to compare transistors.  Choose a substitute transistor with a breakdown voltage rating at least as high as the original.

 

MAXIMUM CURRENT

 

Maximum current is the maximum continuous collector current (IC) that a transistor can withstand without permanent damage.

Small, TO-92 or TO-98 transistors, depending on their fabrication, can handle between about 100 and 1000mA.  A TO-5 package might be rated as high as 5 amps; a TO-220, as high as 25A; and a TO-3, up to 500A.

Be sure to choose a substitute transistor with a maximum current rating at least as high as the original.

 

MAXIMUM POWER

 

Maximum Power, called PD, is the overall power a transistor can dissipate, through heat, without burning up.

Heat sinks and fans increase the ability of a transistor to dissipate heat.  A TO-5 transistor with a PD of 3 watts might be able to dissipate 8-10 watts with a heat sink.

Choose a substitute transistor with a maximum power rating at least as high as the original.

 

CURRENT GAIN

 

Current gain is only occasionally significant when choosing a substitute transistor.  Actual circuit gain depends on other components.  But if the original transistor has a high gain, try to match it.

Current gain falls off at higher frequencies, so a high-gain transistor can deliver a wider frequency response than a low-gain transistor.

One measure of gain, called hFE, is often used for comparing transistors.  The capital FE subscript refers to the Forward DC current transfer ratio in a common Emitter circuit.  In other words, IC / IB.

Data sheets often specify a minimum or typical value of hFE, or else a range of values that applies at a certain collector current (IC).

"Darlington" transistors are made up of two transistors in series and have gains in the thousands, instead of the tens or hundreds.  They also have double the input voltage drop since there are two semiconductor junctions in series.

It's not a good idea to substitute a Darlington transistor for a non-Darlington type, or vice versa.

 

FIELD-EFFECT TRANSISTOR SPECS

 

Field-effect transistors (FETs) come with the same transistor outlines as the BJTs  but their electrical specs are somewhat different:

 

FET TYPES

 

Field-effect transistors come in three basic types:

  • Type A - the Junction-gate FET (JFET)
  • Type B - the Insulated-gate FET (IGFET) in Depletion mode
  • Type C - the Insulated-gate FET (IGFET) in Enhancement mode

IGFETs are usually called MOSFETs (Metal-Oxide Semiconductor FETs) because, originally, all IGFETs used metal gates coated with oxide insulators.  Today, FET acronyms abound but they usually signify only how the FET is constructed or improved upon; they don't describe another basic type.

Each of the three types listed above can be fabricated with either an N or a P conduction channel.  So, altogether, there are six types of field-effect transistors.

Be sure to substitute a FET of the same type as the original.

 

MAXIMUM VOLTAGES

 

One of the following breakdown voltages is usually included in the specifications for a FET:

  • BVGSS - the breakdown voltage between the Gate and the Source terminals when the drain is short-circuited to the source.  (This rating is used primarily with JFETs.)

  • BVDSS - the breakdown voltage between the Drain and the Source terminals when the gate is short-circuited to the source.  (This rating is used primarily with Power MOSFETs.)

A breakdown voltage can also be written as V(BR)GSS or simply VGSS.

You can use either of the above voltages when comparing FETs.  Just make sure to compare apples with apples.  Pick a substitute FET with a rating at least as high as the original.

 

CURRENT

 

IDSS is the Drain to Source leakage current, often provided for small-signal FETs.  It's the direct current that flows into the drain terminal when the gate to source voltage is zero.

In a depletion type device, IDSS is an on-state current.  In an enhancement type device, it's an off-state current.  Minimum and maximum values are usually given.  Select a substitute with the same general range of values.

ID(cont), the Continuous Drain Current, is usually provided for power MOSFETs.  It's a maximum current rating so choose a substitute with a rating at least as high as the original.

 

MAXIMUM POWER

 

PD is the overall power the FET can dissipate through heat.  This is the same spec used for bipolar transistors.  Choose a substitute with a rating at least as high as the original.

 

"ON" RESISTANCE

 

rDS(on) is the DC resistance between the Drain and Source terminals when a specified gate to source voltage is applied to bias the FET to the on-state.

For a depletion-type FET, the gate-source bias voltage might be 0 V (i.e., a gate to source short).

rDS(on) could be important when replacing a power MOSFET.

 

SUMMARY

 

Finding a substitute replacement transistor isn't difficult if you know the specs of the original transistor and organize your stock by specs instead of type numbers.  You'll find that fewer devices need to be stocked, and turnaround time can be reduced.

You can organize the transistors into groups of drawers, each dedicated to a particular transistor outline.  Subdivide the TO groups by electrical ratings like VCEO, IC, and PD, in whatever order you choose.

Then, when you need to pick a substitute for an original transistor, you can quickly home in on all your potential candidates.

 





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