When you apply a positive voltage to the cathode and a negative voltage to the anode of a semiconductor junction, you create a reverse biased diode. In this state, the diode acts as an open switch, blocking current flow and allowing only a microscopic leakage current to pass—until the voltage exceeds its Peak Inverse Voltage (PIV) rating, triggering an avalanche breakdown. Understanding this blocking behavior is critical for designing power supplies, protecting inductive loads, and preventing reverse polarity damage.
This guide moves past abstract physics to give you the exact part numbers, operating thresholds, and testing procedures you need on the bench. We will cover how to select the right diode for reverse-bias applications, map out its operating regions, build a practical flyback protection circuit, and test for failure modes using a standard digital multimeter.
The Anatomy and Symbol of a Reverse Biased Diode
Before wiring a circuit, you must correctly identify the diode's terminals. The standard schematic symbol for a diode consists of a triangle pointing toward a vertical line.
- Anode (Triangle Side): The P-type semiconductor material. In forward bias, conventional current flows into this terminal.
- Cathode (Vertical Bar): The N-type semiconductor material. On a physical through-hole component like a 1N4007, the cathode is marked by a painted band or stripe on the body.
To reverse bias the diode, you must connect the positive terminal of your voltage source to the cathode and the negative terminal to the anode. This polarity pulls the majority charge carriers (holes in the P-side, electrons in the N-side) away from the PN junction. The result is a widened depletion region that acts as an insulator, effectively blocking current flow.
Selecting the Right Part: Default Diodes and PIV Ratings
When selecting a diode to operate in a reverse biased state (such as blocking reverse polarity or clamping inductive spikes), the most critical specification is the Peak Inverse Voltage (PIV), also known as Peak Reverse Voltage (PRV). If your circuit's reverse voltage exceeds the PIV, the diode will avalanche and likely short-circuit.
Below is a data-dense reference table of the most common bench-stock diodes, their reverse-bias limits, and their ideal use cases.
| Part Number | Type | Peak Inverse Voltage (PIV) | Max Forward Current (If) | Reverse Leakage (Ir @ Max PIV) | Safe Default Use Case |
|---|---|---|---|---|---|
| 1N4148 | Small Signal | 100 V | 300 mA | 25 nA (at 20V) | High-speed logic, low-voltage flyback |
| 1N4007 | Standard Rectifier | 1000 V | 1.0 A | 5.0 µA (at 1000V) | Mains AC rectification, high-voltage blocking |
| UF4007 | Ultrafast Rectifier | 1000 V | 1.0 A | 10 µA (at 1000V) | SMPS flyback, high-frequency switching |
| 1N5819 | Schottky | 40 V | 1.0 A | 1.0 mA (at 40V) | Low-voltage reverse polarity protection |
| 1N4733A | Zener | 5.1 V (Vz) | 1.0 W (Pd) | N/A (Designed to break down) | Voltage clamping, reference generation |
Operating Regions: From Leakage to Avalanche Breakdown
A diode does not simply turn 'off' when reverse biased. It transitions through distinct operating regions depending on the applied voltage and the junction temperature. Understanding these regions is vital for predicting how a reverse biased diode will behave under fault conditions.
| Operating Region | Applied Voltage (V) | Current (I) | Physical State & Circuit Behavior |
|---|---|---|---|
| Forward Bias | V > 0.7V (Si) | mA to Amps | Depletion region collapses; diode conducts freely. |
| Reverse Bias (Leakage) | 0V to < PIV | nA to µA | Depletion region widens. Only minority carriers drift across the junction. Acts as an open circuit. |
| Avalanche / Zener Breakdown | V ≥ PIV (or Vz) | Rapidly increasing | Electric field tears electrons from covalent bonds. Current multiplies. Non-destructive only if current is limited by external resistance. |
| Thermal Runaway (Destruction) | V > PIV + High I | Max until failure | Junction heats up, generating more electron-hole pairs, increasing current, generating more heat. Results in a melted, shorted silicon die. |
According to All About Circuits, the reverse leakage current (Ir) roughly doubles for every 10°C increase in junction temperature. A 1N4007 that leaks 1 µA at 25°C might leak 128 µA at 95°C. In high-impedance analog circuits, this temperature-dependent leakage can introduce significant measurement errors.
Practical Application: Inductive Flyback Protection
The most common application relying on a diode's reverse biased state is the flyback diode (or freewheeling diode) across an inductive load like a relay coil or DC motor. When the power to an inductor is abruptly cut, the collapsing magnetic field induces a massive reverse voltage spike ($V = L \frac{di}{dt}$). Without a diode, this spike will arc across switch contacts or destroy the driving transistor.
12V Relay Flyback Circuit
Here is a complete, bench-tested circuit for driving a 12V automotive-style relay using an NPN transistor, with a reverse biased diode protecting the transistor.
- Power Supply: 12V DC
- Load: 12V Relay Coil (approx. 120Ω, drawing 100mA)
- Switching Element: 2N2222 NPN Transistor
- Base Resistor: 1kΩ (limits base current to ~11mA, ensuring transistor saturation)
- Flyback Diode: 1N4148 (Chosen over the 1N4007 because its 4ns reverse recovery time is fast enough to clamp the spike before the 2N2222's collector-emitter junction breaks down).
Wiring and Operation:
- Connect the relay coil between the +12V rail and the collector of the 2N2222.
- Connect the 1kΩ base resistor between your microcontroller GPIO (5V logic) and the base of the 2N2222. Ground the emitter.
- Place the 1N4148 diode in parallel with the relay coil. Crucially, connect the cathode (band) to the +12V rail and the anode to the transistor collector.
Why this works: When the transistor is ON, the collector sits at roughly 0.2V. The diode's cathode is at +12V and its anode is at +0.2V. The diode is reverse biased by 11.8V. Because 11.8V is well below the 1N4148's 100V PIV, it blocks current and does not interfere with the relay. When the transistor turns OFF, the inductor's voltage spikes positive at the collector. The anode voltage instantly exceeds the cathode voltage, forward-biasing the diode. The diode conducts, clamping the spike to roughly 12.7V (12V rail + 0.7V forward drop) and safely dissipating the stored inductive energy as heat in the coil.
Failure Modes and Multimeter Diode Testing
Diodes rarely fail gracefully. When subjected to voltages exceeding their PIV or currents exceeding their thermal limits, they typically fail in one of three ways:
- Short Circuit (Most Common): An avalanche breakdown event with insufficient current limiting melts the silicon junction, fusing the anode and cathode together. The diode will conduct in both directions.
- Open Circuit: A massive forward current surge melts the internal bond wire connecting the silicon die to the external lead. The diode blocks current in both directions.
- High Leakage (Degraded): Prolonged operation near the maximum junction temperature damages the crystal lattice. The diode still blocks, but reverse leakage increases from nanoamps to milliamps, causing circuit malfunction.
How to Test a Reverse Biased Diode with a DMM
You can quickly verify the health of a diode using the Diode Test mode on a standard digital multimeter (DMM). As noted in Fluke's official testing guide, this mode applies a small test voltage (usually 2V to 3V) and measures the voltage drop.
- Isolate the Component: Remove the diode from the circuit, or ensure at least one leg is lifted. Testing in-circuit often yields false readings due to parallel parallel paths.
- Set the DMM: Turn the dial to the Diode Test symbol (a triangle with a line).
- Forward Bias Test: Place the Red probe on the Anode and the Black probe on the Cathode. A healthy silicon diode will read between 0.500V and 0.750V. (Schottky diodes will read 0.150V to 0.300V).
- Reverse Bias Test: Swap the probes. Place the Red probe on the Cathode and the Black probe on the Anode. A healthy diode will display 'OL' (Overload) or a '1' on the far left of the display, indicating infinite resistance.
- Interpret Failures: If the reverse bias test reads '0.00' or a low voltage, the diode is shorted. If the forward bias test reads 'OL', the diode is open.
For further reading on semiconductor characteristics and safe derating practices, refer to the Nexperia discrete diodes documentation. Always select a diode with a PIV rating at least 20% to 50% higher than the maximum expected reverse voltage in your circuit to account for transient spikes and temperature derating.
