A constant-current diode (also called a current-regulating diode, CRD, or by the trade name "CurrentReg") is a two-terminal semiconductor device that behaves like a current source over a wide voltage range. Internally, it's a JFET with gate tied to source — the pinch-off current I_DSS becomes the regulated output. Once you exceed a small compliance voltage (typically 1–2 V), the current stays remarkably flat until you approach the breakdown voltage (usually 100 V).

Compared to a resistor pulling current from a rail, the CCD gives you three big wins: current is immune to supply variation, immune to load resistance, and requires zero design math beyond picking the part number. The Central Semiconductor 1N5283–1N5314 series, for example, offers ~30 discrete current values from 220 µA to 4.7 mA in a DO-35 glass package.

Key parameters to watch:

• Regulator current (I_P): the nominal current, typically ±10% tolerance
• Minimum compliance voltage (V_K): below this, you're on the knee and current drops
• Dynamic impedance (Z_D): usually 1–25 MΩ — this is what makes it act like a current source
• Temperature coefficient: roughly +0.3%/°C above 25°C, negative below — there's a "zero-tempco" current around 500 µA for some series

Real-world example: LED backlight for a panel meter. You want a 2 mA LED indicator that stays the same brightness whether powered from 5 V USB, 12 V wall wart, or 24 V industrial rail. Traditional approach: three different current-limiting resistors, or one resistor sized for worst-case (dim on 5 V, hot on 24 V). CCD approach: drop in a 1N5297 (2.0 mA nominal) in series with the LED. Brightness is now identical across the entire 3–100 V range, and you've eliminated an entire BOM line item for supply-voltage variants.

Rule of thumb for compliance headroom: Budget V_supply ≥ V_load + V_K + 1 V margin. So for a 3.3 V white LED (V_F ≈ 3.2 V) driven by a 2 mA CCD with V_K = 1.3 V, minimum supply is ~5.5 V. On 5 V USB you'll be sitting right at the knee — brightness will vary. Move to a 1 mA CCD with lower V_K, or accept the dimming.

Other classic uses: biasing the tail of a discrete differential pair (better CMRR than a tail resistor), pre-biasing bandgap references, providing a stable reference current to a log amp, and driving Zener chains for staircase voltage references.

Limitation: they're expensive (often $1–3 in singles) and current values are quantized. For tight custom values, a JFET + source resistor or a proper current mirror is better.