Using an external pass transistor with a voltage regulator like the 78XX series allows you to safely boost its current output beyond its internal limit. Using an external pass transistor with a 78XX regulator reduces the heating of the regulator itself, but does not eliminate overall heat generation—it simply shifts the thermal load to the external transistor. Here is how.
Integrated circuit (IC) voltage regulators, such as the popular 78XX series, such as LM7805 voltage regulator, are reliable and easy to use, but they come with a limitation: they can only supply a limited output current—typically up to 1.3 A under standard conditions. When the load current goes beyond this limit, the regulator can overheat, triggering thermal shutdown due to excessive internal power dissipation.
🛠️ When You Need More Current: Use an External Pass Transistor
In applications where your load demands more current than the regulator can handle, a clever workaround is to use an external pass transistor (denoted Qext). This configuration allows the excess current to bypass the internal regulator safely, preventing overload.
The circuit diagram below shows how a standard three-terminal voltage regulator can be enhanced with an external pass transistor to handle higher load currents.
⚙️ How It Works:
A current-sensing resistor (Rext) is placed between the voltage regulator output and the transistor’s base-emitter junction. This resistor determines the current level at which the external transistor starts conducting. The basic behavior is as follows:
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When the load current is below the threshold (Imax):
The voltage across Rext is less than 0.7 V, so Qext remains off. The regulator supplies all the current, as shown in the figure below.
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When the load current exceeds Imax:
The voltage across Rext rises to 0.7 V or more, turning Qext on. The external transistor then conducts the excess current, relieving the internal regulator, as shown in the figure below.
🧮 Designing Rext:
To choose the right resistor value that sets the internal regulator’s current limit (Imax), use the formula:
For example, if you want the regulator to handle only 1 A internally:
If your total load requires 3 A, then:
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The internal regulator supplies 1 A.
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Qext supplies the remaining 2 A.
🌡️ Don’t Forget Thermal Considerations:
Since the external pass transistor is now conducting significant current, it must be:
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A power transistor (e.g., TIP2955, 2N3055)
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Properly heat-sinked to handle thermal dissipation
The power dissipated in the pass transistor is approximately:
This ensures the transistor can safely operate under full load conditions without overheating.
✅ Summary Tips:
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Use an external pass transistor to extend the current capability of a standard 78XX regulator.
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Select Rext using the simple 0.7 V/Imax rule.
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Ensure the transistor is rated for the current and power, with a proper heat sink.
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Monitor for thermal shutdown or overload issues in high-current designs.
