A current diode—formally known as a Current Regulating Diode (CRD) or constant current diode—is a two-terminal semiconductor device that limits current to a specific maximum value, regardless of voltage fluctuations across it. Unlike a standard Zener diode that clamps voltage, a CRD clamps current. Internally, it is essentially an N-channel JFET with its gate terminal permanently shorted to its source terminal. This simple architecture makes it one of the most reliable, low-noise, and space-efficient ways to bias transistors, drive LEDs, or protect sensitive analog stages without the overhead of a multi-pin constant current IC.
Symbol, Pinout, and Internal Structure
Before dropping a CRD into a circuit, you need to understand how it is represented on a schematic and how to orient it physically on the bench.
Schematic Symbol: The standard IEC/IEEE symbol for a current diode resembles a regular diode (a triangle pointing toward a vertical bar) but with an additional vertical line intersecting the triangle, or an inward-pointing arrow on the cathode bar. This denotes its current-limiting behavior. In some older schematics, you will simply see an N-channel JFET symbol with the gate pin drawn directly into the source pin.
Physical Pinout: Most through-hole CRDs come in a standard DO-35 glass package, looking identical to a 1N4148 signal diode.
- Anode (+): Corresponds to the internal JFET's Drain terminal. This is the unmarked end of the package.
- Cathode (-): Corresponds to the internal JFET's Source terminal. This is marked by the black or colored band on the glass body.
Operation Regions and Spec-Sheet Parameters
To use a CRD effectively, you must operate it in the correct region. A common mistake among hobbyists is assuming the device regulates current all the way down to 0V. It does not. It requires a minimum "headroom" voltage to pinch off the internal JFET channel.
| Region | Voltage Across CRD (V_D) | Current Behavior | Circuit State |
|---|---|---|---|
| Ohmic (Linear) | 0V to V_p (e.g., 0V - 1.5V) | Acts as a fixed resistor. Current rises linearly with voltage. | Under-biased. Regulation fails. |
| Saturation (Active) | V_p to V_BO (e.g., 1.5V - 50V) | Constant current (I_p). Channel is pinched off. | Target operating zone. |
| Breakdown | > V_BO (e.g., > 50V) | Avalanche breakdown. Current spikes uncontrollably. | Destructive. Device will burn out. |
Note: V_p is the Pinch-Off Voltage (sometimes listed as V_k or knee voltage on Semitec datasheets). V_BO is the Breakdown Voltage. Always ensure your supply voltage minus your load voltage drop is greater than V_p but less than V_BO.
How to Select and Bias a Current Diode (Application Circuit)
Let us design a constant-current driver for a standard high-brightness white LED using a 12V DC supply. We want exactly 15mA of current to ensure long LED life and stable brightness, even if the 12V rail sags to 11V or spikes to 14V (like in an automotive environment).
Component Selection:
- Power Supply: 12V DC nominal.
- Load: Standard 5mm White LED (Forward Voltage, V_f = 3.2V at 15mA).
- CRD: Semitec S-153 (Nominal Current I_p = 15mA, Pinch-off Voltage V_p max = 2.0V, Max Power P_d = 300mW).
Step-by-Step Circuit Verification:
- Calculate Voltage Drop Across the CRD: V_CRD = V_supply - V_LED = 12V - 3.2V = 8.8V.
- Verify Headroom: Is 8.8V greater than V_p (2.0V)? Yes. The device will fully pinch off and regulate at 15mA. (Even if the supply sags to 6V, V_CRD becomes 2.8V, which is still > 2.0V, so regulation holds).
- Calculate Power Dissipation: P = V_CRD × I_p = 8.8V × 0.015A = 132mW.
- Verify Thermal Limits: The S-153 is rated for 300mW at 25°C ambient. 132mW is well within the safe operating area. The DO-35 package will get warm, but not hot enough to require a heatsink or derating.
Wiring: Connect the 12V positive rail to the Anode of the S-153. Connect the Cathode of the S-153 to the Anode of the LED. Connect the Cathode of the LED to Ground. No series resistor is needed; the CRD replaces it entirely. For a deeper look at the underlying JFET physics that makes this possible, review this guide on JFET constant current sources.
Failure Modes and Multimeter Testing
Current diodes are robust, but they do fail if abused. The two primary failure modes are thermal destruction (exceeding the 300mW-500mW package limit, causing the internal wire bond to melt open) and avalanche puncture (exceeding V_BO, causing the JFET channel to short out permanently).
You can quickly verify a CRD on the bench using a standard digital multimeter (DMM). Because a CRD is just a JFET, it will not read like a standard silicon diode (which shows ~0.6V).
Testing Steps:
- Set your DMM to Diode Test Mode. (This mode typically outputs 2V to 3V at the probes).
- Forward Bias Test: Place the Red probe on the Anode (unmarked end) and the Black probe on the Cathode (banded end).
- Expected Reading: The DMM will display a voltage drop between 1.0V and 2.5V. This is the internal pinch-off voltage (V_p). If your DMM's open-circuit voltage is lower than the CRD's V_p, it may instead display a low resistance value (e.g., 100Ω to 500Ω) if switched to Ohms mode.
- Reverse Bias Test: Swap the probes (Black on Anode, Red on Cathode).
- Expected Reading: OL (Overload) or infinite resistance. The internal PN junction is reverse-biased and blocks the DMM's test voltage.
- Diagnose Failures: If you read 0.00V or a dead short in both directions, the device has suffered avalanche puncture and is shorted. If you read OL in both directions, the internal channel has melted open from thermal overload. Throw it in the bin.
Safe Default Part Numbers for the Workbench
If you are stocking your lab, do not buy random unbranded CRDs from bulk marketplaces; the pinch-off voltages are often untested and wildly out of spec. Stick to established manufacturers like Semitec, Central Semiconductor, or Vishay. You can source these from major distributors like Digi-Key's CRD category.
| Part Number | Manufacturer | Nominal Current (I_p) | Max Pinch-Off (V_p) | Breakdown (V_BO) | Approx. Price (USD) |
|---|---|---|---|---|---|
| S-103 | Semitec | 10 mA | 2.0V | 50V | $0.45 |
| S-153 | Semitec | 15 mA | 2.0V | 50V | $0.45 |
| 1N5283 | Central Semi | 0.22 mA | 1.5V | 100V | $0.80 |
| 1N5305 | Central Semi | 1.2 mA | 1.5V | 100V | $0.80 |
| 1N5314 | Central Semi | 4.7 mA | 1.5V | 100V | $0.85 |
Bench Tip: The Semitec S-series is generally preferred for LED driving (10mA-20mA range) due to lower cost and tighter thermal stability. The Central Semiconductor 1N52xx/1N53xx series is better for low-current transistor biasing and sensor excitation where you need sub-milliamp precision and higher breakdown voltages.
Frequently Asked Questions
Can I use a current diode instead of a constant current IC?
Yes, but it depends on your power and precision requirements. A current diode is vastly superior for low-power applications (under 300mW) because it requires zero external components, generates no switching noise, and has infinite power supply rejection ratio (PSRR) at low frequencies. However, if you need to drive a 1A LED string or require 0.1% current accuracy over a wide temperature range, a CRD will fail. CRDs have a temperature coefficient of roughly -0.05%/°C to -0.1%/°C, meaning the current will drift as the part heats up. For high-current or high-precision needs, use a dedicated constant current IC or an LM317 configured as a current source.
Why is my current diode getting too hot to touch?
A glass DO-35 package has a thermal resistance of about 250°C/W to ambient air. If your CRD is dissipating 200mW, the junction temperature will rise by 50°C above room temperature. The glass body will easily reach 75°C (167°F), which is too hot to hold your finger on, but perfectly safe for the silicon inside (max junction temp is usually 175°C). If you want it to run cooler, you must either lower the supply voltage (reducing the voltage drop across the CRD) or place a standard power resistor in series with the CRD to share the heat dissipation. The resistor will drop the excess voltage, and the CRD will only drop its V_p, keeping it cool.
How do I increase the current limit of a single current diode?
You cannot change the internal physical limit of a single CRD, but you can wire multiple CRDs in parallel to sum their currents. For example, placing two Semitec S-153 (15mA) diodes in parallel will yield a constant current of 30mA. When doing this, ensure they are thermally coupled (e.g., zip-tied together or glued to a small copper pad). Because CRDs have a negative temperature coefficient, if one gets hotter than the other, its current will drop, forcing the cooler one to take more current, which then heats it up. This natural thermal balancing prevents runaway, but keeping them at the same ambient temperature ensures the most accurate total current.
