Reverse Polarity Protection for Chargers
⚠️ The Problem: Accidental Reverse Polarity in Chargers
Connecting power in reverse is a common mistake, especially when building or modifying DIY chargers. This mistake can instantly damage your charger circuit, batteries, or other connected components. Fortunately, reverse polarity protection is easy to implement and saves your device from costly damage.
🔧 The Solution: Use a Diode or P-Channel MOSFET
The simplest solution is to use a Schottky diode in series with the input to block reverse current. For more efficient protection, use a P-channel MOSFET configured with its source to the input and drain to the load—this gives low voltage drop and fast switching.
💡 Practical Example: DIY Li-Ion Charger
Imagine making a 5V USB Li-Ion charger. If someone plugs in the power supply backwards, the diode or MOSFET blocks the current, preventing instant burnout of the charging IC or battery. A 1N5819 Schottky diode can do the job for low currents; for higher loads, a P-channel MOSFET like IRF9540 is better.
🧮 Sample Calculation: Power Loss in Diode
For a 1A load using a 0.3V drop Schottky diode:
P = V × I = 0.3V × 1A = 0.3W
Always consider thermal dissipation if power loss exceeds 0.5W.
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