EEVblog 1746 - Schottky vs PN Diodes & Measurement Traps

The video discusses an interesting question posed on X by Blind Via regarding a simple circuit with a diode. The circuit involves a 5V USB input powering circuitry, with a diode used for series input protection to prevent damage from reverse biased input voltage. The diode is supposed to allow current flow in one direction only, from the power supply into the circuit, and prevent it from flowing back to the input. The question arises because, when measuring the USB port with the diode in series and reverse biased, the multimeter still reads 5 volts instead of 0 volts. Initially, the speaker demonstrates the setup with a 5V power supply and a surface-mount diode (STP 2SL60A) in series. With the diode reverse biased, the multimeter reads 5 volts. Even when the diode's polarity is reversed, it still reads 5 volts, leading to confusion. Trying a different diode, an SS24, yields the same result. However, when a classic 1N4148 signal diode is used and reverse biased, the multimeter correctly reads 0 volts. Forward biased, it allows the voltage through. The same behavior is observed with a 1N41 power diode. This leads to the core question: why do some diodes work as expected (reading 0V when reverse biased) while others don't (reading 5V)? The explanation lies in the type of diode. The diodes that failed to block the voltage (STP 2SL60A and SS24) are Schottky diodes, while the ones that worked (1N4148 and 1N41) are regular PN junction diodes. Schottky diodes, named after Walter Schottky, are different from PN junction diodes. A PN junction diode is a semiconductor PN junction, allowing current in one direction. A Schottky diode, however, is a metal-semiconductor junction, featuring an extra metal layer. This construction gives Schottky diodes a significant advantage: a much lower forward voltage drop compared to PN junction diodes (e.g., 0.142V for Schottky versus 0.5V for 1N41 and 1N4148). This low voltage drop makes Schottky diodes ideal for applications where efficiency is critical, such as high-frequency switch-mode power supplies. The original schematic in the X post actually used a Schottky diode symbol, which has distinct "square tails" instead of the straight line of a regular diode symbol. This indicates it's a Schottky barrier diode. Despite their advantages, Schottky diodes have a major downside: higher reverse leakage current. When tested with a diode tester on a multimeter, Schottky diodes appear to function like regular diodes, blocking current in one direction. However, when the multimeter is switched to measure current in microamps, a reverse-biased Schottky diode at 5 volts shows a non-zero reverse current (e.g., 2.4 microamps). This leakage current is the reason for the unexpected 5-volt reading. Multimeters, both digital and analog, have an input impedance, typically around 10 megaohms for digital multimeters. This means the multimeter acts like a 10-megaohm resistor. The small reverse leakage current (e.g., 2.2 microamps) flowing through this 10-megaohm input impedance creates a voltage drop. According to Ohm's law (V = I * R), 2.2 microamps * 10 megaohms would ideally result in 22 volts. However, since the power supply is only 5 volts, the multimeter reads a maximum of 5 volts. This leakage current is significantly higher in Schottky diodes—about three orders of magnitude worse than a typical PN junction diode. The data sheets confirm this. A Schottky diode like the one used in the example can have reverse current in the milliamp range (e.g., 1mA at 40V, increasing to 10mA at 100°C). In contrast, a 1N4148 PN junction diode has reverse current in the nanoamp range (e.g., 25nA at 20V). The leakage current in Schottky diodes also increases significantly with temperature and voltage. The question highlights a classic measurement trap: the multimeter's input impedance. The 10-megaohm input impedance of the multimeter, combined with the higher reverse leakage current of the Schottky diode, allows a measurable voltage to appear across the multimeter, even when the diode is supposedly blocking current. For a PN junction diode, with its much lower leakage current (nanoamps), the voltage drop across the 10-megaohm input impedance would be negligible (e.g., 0.1 volts), effectively reading as zero. Schottky diodes also have other characteristics, such as generally lower maximum reverse voltage ratings compared to PN junction diodes (e.g., 100-200V versus 500-1000V). Despite these drawbacks, Schottky diodes are excellent for applications requiring low forward voltage drop, fast switching, and high-frequency operation. In the context of reverse polarity protection, a Schottky diode is superior to a PN junction diode for clamping reverse voltages. Its lower forward voltage drop means it clamps at a lower voltage (e.g., 2V), protecting other sensitive components like transistors and ICs from exceeding their typical silicon threshold (around 0.6V). A PN junction diode, clamping at 0.6-0.7V, might allow higher voltages (e.g., 6-7V) to appear across the protected circuit, potentially damaging components. The video concludes by demonstrating how an analog multimeter with a lower input impedance (e.g., 120k ohms on the 12V range) can correctly read closer to 0 volts when reverse-biased, effectively "swamping" the leakage current. This illustrates that while high input impedance is generally desirable for voltage measurements, in this specific case, a lower input impedance helps reveal the true blocking behavior. The takeaway is that understanding diode types and multimeter characteristics is crucial for accurate circuit analysis and troubleshooting.