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What Are the Causes of Blown NPN Transistors

Updated on August 5, 2011

Issues of blown transistors


Transistors function as current regulators by using a small current introduced at the base to control a larger current flow between the collection and emitter. By varying the base current flow, or base voltage, the total current flow or voltage difference between the collector and emitter can be varied. The variation is known as “amplification factor.”

NPN transistors and their counterparts, PNP transistors, are types of bipolar-junction transmitters (BJT) that consist of three terminals – the base, collector, and emitter terminals. The difference between the PNP and NPN transistor is the direction in which current flows between the collector and emitter. In the case of an NPN transistor, it can be characterized as two diodes with a common anode, constructed of semi-conductor materials of two types, P and N. In operation, a small induced base current regulates a large emitter-to-collector current flow.

Transistors were developed in the late 1940s, and quickly replaced the bigger, slower, and more fragile vacuum tubes that were used prior to that time. Although transistors are incredibly rugged by comparison, they can still be damaged or destroyed (blown) if circuitry goes awry. Listed below are a few of the ways to blow an NPN transistor.

Excessive Voltage

Because transistors are sized based on a circuit’s operating voltage, any spike or large increase in the base system voltage will cause an NPN transistor to blow. This is because the transistor is designed to be a voltage storage container with a limited capacity, and will always be damaged when more voltage is pushed into the container than it was designed to hold or pass.

Thermal Load

Transistors must be kept cool to keep their semiconductor material from overheating and breaking down. If the material is allowed to begin breaking down due to heat, it will cause unnecessary resistance inside the transistor which will add to the heat load, and the likelihood of damage to the semiconductor material in the transistor becomes inevitable. Components and circuit boards must be kept cool at all times through the use of heat sinks and cooling fans.

Partial Operation

A bipolar-junction transmitter operates at its best when the collector-emitter flow is either turned completely on or completely off. Operation anywhere in between these two extremes will increase resistance in the component, and increased resistance is converted to heat in an electrical circuit. Left to operate at partial capacity for long periods of time will cause the transistor to overheat and fail, even when it is being properly cooled under normal operating conditions.

Excess Current Flow

Similar to the way a transistor responds to excessive voltage is the way it responds to excessive current flow. Overcurrent can occur as the result of a voltage spike, breakdown in the base current semiconductor material, or just by pushing more current through the system than it was designed. Short over-current bursts can be handled because they are dissipated as heat, but a long-duration over-current event will function with the mechanism similar to those above and cause the transistor to overheat and fail. In the same vein as overcurrent, current flow in the wrong direction will also blow NPN transistors. This can happen if a portion of the circuit’s polarity is reversed, or additional components are installed in the system which force current to flow opposite its expected path.


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