CMPP6028R - 40V, 150mA Programmable UJT, SOT-23 | Central Semiconductor

Relaxation Oscillators

The CMPP6028R is purpose-built for relaxation oscillators: its externally programmable peak current (IP), valley current (IV), and intrinsic standoff ratio (eta) let a designer set frequency and hysteresis with just a timing capacitor and two gate-divider resistors. The device charges the capacitor through a resistor; when the anode crosses the programmed gate threshold, the PUT fires and discharges the capacitor, repeating cyclically. Because IP can be programmed low, very long time constants and low supply currents are achievable, which fixed-UJT designs cannot match. Ensure the charging current through the timing resistor stays above valley current (IV) at minimum supply, or oscillation will latch off; per the datasheet this programmability is the core design intent of the CMPP6027R/CMPP6028R family.

Precision Timers and Time-Delay Circuits

In long-duration timer circuits the CMPP6028R excels because its low programmable peak current allows large timing resistors and capacitors, stretching delay periods into seconds or minutes without electrolytic-quality capacitors. The PUT fires when the integrating node reaches the gate-set threshold, driving a load or an SCR. The standoff ratio (eta), programmed by the gate divider, defines the trigger point as a fraction of supply voltage, so timing accuracy follows divider precision - use 1% resistors for predictable thresholds. Compared with a 555 timer, the CMPP6028R draws far less standing current and tolerates higher supply voltages up to its 40 V rating, making it attractive for battery-backed and industrial delay-on/delay-off functions.

Overvoltage Detection and Crowbar Triggering

The CMPP6028R functions as a sensitive voltage-level detector: a resistor divider feeds the anode while the gate divider sets the trip threshold. When a monitored rail exceeds the programmed point, the PUT fires and can directly trigger an SCR crowbar, shutting down the faulted supply. Its 40 V rating covers most intermediate DC buses, and the SOT-23 package fits densely on protection PCBs. Programmable IP lets the designer set immunity against noise-induced false triggers while remaining sensitive enough to catch genuine overvoltage events quickly. Place the device upstream of the SCR gate with a small series resistor to limit gate current, and verify that sustained PUT conduction within the crowbar window stays within the 167 mW package dissipation limit.

SCR and TRIAC Gate Triggering

As a pulse generator for thyristor gates, the CMPP6028R provides a sharp, low-impedance discharge pulse from a timing capacitor when it fires, delivering the di/dt that SCRs and TRIACs need for reliable latch-up. The programmable standoff ratio allows the trigger phase angle to be set against a synchronized ramp, supporting simple phase-control circuits for heaters, lamps, and small motors. Its 150 mA pulse capability is adequate for sensitive-gate SCRs; larger devices may need a pulse transformer or transistor buffer after the PUT. Because it is obsolete, new designs should verify long-term supply or adopt the CMPP6027R sibling. Keep gate-drive pulse width and energy within the target thyristor datasheet requirements for the full operating temperature range.

Lamp Flashers and Indicator Circuits

The CMPP6028R makes efficient low-parts-count lamp or LED flashers: a capacitor charges toward the supply through a resistor, and when the PUT fires the stored charge dumps through the lamp or LED, producing a bright flash from a low average current. Because IP and IV are programmable, flash rate and duty cycle can be tuned over a wide range while operating from supplies up to 40 V, well above typical IC timer limits. Average battery drain is dominated by the gate divider and charging resistor, so select divider values that balance threshold stability against quiescent draw. The SOT-23 surface-mount package supports compact indicator PCBs in industrial panels, battery-powered devices, and warning-beacon applications.

Sawtooth and Ramp Generators

In analog function generation, the CMPP6028R resets a ramp capacitor each cycle: a constant-current source or high-value resistor charges the capacitor linearly, and the PUT discharges it rapidly at the programmed threshold, producing a sawtooth waveform for sweep, PWM-ramp, or ADC-driving applications. The programmable standoff ratio sets ramp amplitude precisely as a fraction of the supply, and low IP permits very slow, low-current ramps. Rise-time linearity depends on the charging network rather than the PUT itself, so a current source improves fidelity over a simple resistor. Recovery time after discharge is short in the SOT-23 device, supporting repetition rates from sub-hertz to tens of kilohertz depending on capacitor size and discharge-path impedance.