A typical SCR (silicon-controlled rectifier) requires a trigger current, which causes the SCR structure to latch on. Once the device latches, the current through the SCR is solely a function of external component values. The SCR has no inherent ability to limit the current flow once it latches on. Current continues to flow, as long as the current exceeds a minimal value known as the holding current. The circuit in Figure 1 is similar to an SCR, because it also requires a trigger current to latch into its on state. However, once latched, the circuit conducts a constant current. The constant current continues to flow, as long as the external circuitry can provide it, and the minimum compliance voltage of the SCR circuit is satisfied. When these conditions are no longer valid, the circuit latches off. The circuit in Figure 1 provides a constant-current pulse to drive an LED with current sourced from a capacitor. You trigger the circuit with a narrow, negative-going pulse. The pulse, coupled through R1 and D2, turns Q3 on. Q3 provides base drive to Q1. As Q1 turns on, current begins to flow through the LED and current-sense resistor R2.
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| Figure 1. | Resembling an SCR, this circuit provides a constant current of controlled pulse width and amplitude to a load. |
When 0.6 V develops across R2, the current-limiting transistor, Q2, begins to turn on and shunt base current from Q1, through diode D1. Q2 thus maintains the current through R2 at a constant level (~0.6 V/R2) by controlling the base current to Q1. At the same time, because the collector voltage of Q2 must be one diode drop lower than the base voltage of Q1 while in constant-current mode, Q2 also draws current through R3. Q2 thus maintains Q3 in the on state (providing base current to Q1), even after the trigger pulse disappears. The circuit maintains the constant-current mode, with Q1 drawing a constant current through the LED, the storage capacitor C1, and R2 until Q1 can no longer sustain the constant current. This situation occurs when the voltage across C1 drops low enough to be unable to maintain 0.6 V across R2. Then, Q2 begins to turn off, which allows Q3 to turn off, thereby depriving Q1 of base current. Q1 turns off, which results in a constant-current (flat-topped) pulse through the LED with sharply rising and falling edges. By choosing the proper values of R2 and C1, you can easily control pulse width and amplitude.
An apt application for this circuit is constant-current battery charging. Once you trigger the circuit, it provides constant current to charge a battery. When the battery charges to a point where the charging current falls below the constant-current level, the circuit latches off. Note that the circuit does not provide a continuous trickle charge, which could overcharge some batteries.
