A current regulating diode (CRD)—also known as a constant current diode or current limiting diode—is a two-terminal semiconductor device that clamps current flow to a specific maximum value, regardless of the voltage applied across it (within its operating limits). Under the hood, a CRD is simply a Junction Field Effect Transistor (JFET) with its gate internally tied to its source, forcing it to operate permanently in the saturation (pinch-off) region. If you need to drive an LED from a noisy 28V aircraft bus or bias a sensitive sensor without building a complex active feedback loop, a CRD is the simplest, most robust solid-state solution available.
The Current Regulating Diode: Symbol, Pinout, and Core Concept
Unlike a standard rectifier or Zener diode, the schematic symbol for a CRD features a standard diode triangle but with a vertical bar intersecting the arrow, or an arrow pointing inward toward the cathode. This visually distinguishes it as a current-controlled device rather than a voltage-controlled one.
Physically, CRDs are packaged exactly like standard signal diodes. The most common through-hole package is the DO-35 or DO-7 glass axial lead. The pinout is straightforward:
- Anode (A): The positive terminal. Current enters here.
- Cathode (K): The negative terminal, marked by a black or colored band on the glass body. Current exits here.
Operation Regions and Biasing for the Job
To select and bias a CRD correctly, you must understand its three distinct voltage-current operating regions. If you bias the device in the wrong region, it will either act like a simple resistor or fail catastrophically.
| Operation Region | Voltage Condition | Device Behavior | Design Action |
|---|---|---|---|
| Ohmic (Linear) | 0V to Vk (Knee Voltage) | Acts like a fixed resistor. Current increases linearly with voltage. | Avoid. The device is not regulating yet. Ensure your supply voltage exceeds Vk. |
| Saturation (Constant Current) | Vk to Vmax | Current clamps at Ip (Pinch-off Current). Dynamic impedance is extremely high. | Target Region. Design your circuit so the voltage drop across the CRD stays strictly in this window. |
| Breakdown (Avalanche) | > Vmax | The internal JFET junction breaks down. Current spikes uncontrollably. | Fatal. Ensure Vsupply - Vload never exceeds the datasheet Vmax. |
How to bias it: Calculate the expected voltage drop across the CRD by subtracting your load voltage from your supply voltage. This drop must be higher than the knee voltage (Vk, typically 1.5V to 4V) to ensure regulation, but lower than the maximum regulating voltage (Vmax, typically 50V to 100V) to prevent avalanche breakdown.
Safe Default Part Numbers for the Bench
When stocking your lab or designing a new board, these are the most reliable, widely available CRD families. Prices reflect typical 2026 distributor rates for single units.
| Part Number | Nominal Current (Ip) | Max Regulating Voltage (Vmax) | Max Power (Pd) | Package / Price |
|---|---|---|---|---|
| 1N5283 | 0.22 mA | 100V | 300 mW | DO-35 / ~$0.85 |
| 1N5305 | 2.0 mA | 100V | 300 mW | DO-35 / ~$0.60 |
| 1N5314 | 4.7 mA | 100V | 300 mW | DO-35 / ~$0.75 |
| Semitec E-103 | 10.0 mA | 50V | 400 mW | DO-41 / ~$1.20 |
| Semitec E-202 | 20.0 mA | 50V | 500 mW | DO-41 / ~$1.50 |
Note: For higher currents (20mA to 300mA), modern designs often use Constant Current Regulators (CCRs) like the ON Semi NSI45020, which are functionally similar but built on a different silicon topology to handle higher wattages.
Application Circuit: 28V Avionics LED Indicator
Aircraft and heavy machinery often use 28V nominal DC buses that can spike to 36V during load dumps. Driving a standard 2.2V indicator LED directly with a resistor is a nightmare because the current (and brightness) will fluctuate wildly with bus voltage changes. A CRD solves this instantly.
Circuit Topology:
- Source: 28V DC Bus (protected by a 1A fast-blow fuse).
- Regulator: 1N5305 CRD (Anode to 28V, Cathode facing load). Clamps current to 2.0 mA.
- Sense/Protection Resistor: 10Ω, 1/4W carbon film resistor in series. (Optional, but allows you to measure the voltage drop across it with a multimeter to verify current on the bench: 2mA * 10Ω = 20mV).
- Load: High-efficiency red LED (Vf = 2.1V @ 2mA).
- Return: Circuit Ground.
Math Check: The voltage drop across the CRD is 28V - 2.1V - 0.02V = 25.88V. This is well above the ~2V knee voltage and well below the 100V Vmax. Power dissipated by the CRD is 25.88V * 0.002A = 51.7 mW, safely inside the 300 mW DO-35 package limit.
Bench Scenario: The 4-20mA Loop Thermal Derating Trap
Theory is clean; the workbench is not. Here is a real-world failure scenario involving a Semitec E-202 (20mA CRD) used to build a portable 4-20mA current loop simulator for testing industrial PLC analog inputs.
The Setup: I needed a precise 20mA source powered by a 24V bench supply. The load was a 250Ω precision sense resistor inside the PLC, dropping 5V. I wired the 24V supply to the Anode of the E-202, and the Cathode to the PLC input.
The Numbers: Voltage across the CRD was 24V - 5V = 19V. Power dissipation was 19V * 0.020A = 380 mW. The datasheet absolute maximum power rating for the DO-41 package is 500 mW. I assumed I had 120 mW of headroom.
The Outcome: After 10 minutes inside a sealed plastic project enclosure, the CRD was too hot to touch. Worse, the current measured at the PLC dropped from 20.0 mA down to 16.4 mA, throwing off the calibration entirely.
What Went Wrong: I ignored the thermal derating curve. The 500 mW rating assumes a 25°C ambient temperature with infinite heatsinking (long leads). Inside the sealed box, ambient hit 65°C. At 65°C, the derating curve slashed the maximum allowable power to roughly 250 mW. The JFET junction exceeded 140°C, causing the internal pinch-off voltage to shift and the current to droop. Furthermore, the negative temperature coefficient of the JFET caused thermal runaway.
The Fix: I added a 470Ω, 1W power resistor in series before the CRD. This dropped an additional 9.4V (at 20mA), shifting 188 mW of heat off the tiny glass CRD and into the beefy power resistor. The CRD dissipation dropped to 192 mW, the junction stayed cool, and the current locked dead at 20.1 mA regardless of enclosure temperature.
How to Test a CRD with a Multimeter
Because a CRD is just a JFET with shorted pins, you cannot test it exactly like a standard PN-junction diode. A standard diode will read ~0.6V forward and 'OL' reverse. A CRD behaves differently. Here is the definitive bench test procedure using a standard digital multimeter (DMM).
- Set your DMM to Diode Test Mode. Ensure the leads are in the correct COM and V/Ω ports.
- Forward Bias Test: Place the red probe on the Anode and the black probe on the Cathode. The DMM sources a small test current (usually 1mA to 2mA). The screen will display the JFET's pinch-off voltage (Vp), which typically reads between 1.2V and 3.5V depending on the specific part number. If it reads 0.00V, the device is internally shorted.
- Reverse Bias Test: Swap the probes (red on Cathode, black on Anode). The internal JFET gate-source diode is now reverse-biased. The DMM should read 'OL' (Open Loop) or a very high voltage. If it reads a low voltage or 0.00V, the junction is blown.
- The Live Current Test (Mandatory): A DMM diode test only checks the junction, not the regulating capability. To truly verify a CRD, wire it in series with a 100Ω resistor and a variable DC power supply. Sweep the supply from 5V to 30V while measuring the voltage across the 100Ω resistor. The voltage across the resistor must remain flat (indicating constant current) once you pass the knee voltage.
