The thyristor or silicon-controlled rectifier (SCR) provides a convenient latching mechanism for switching power to a load. However, once the thyristor is triggered into conduction, it supplies no means of controlling the current flow, which is limited only by the external components.
On the other hand, the circuit in the a portion of the figure allows a low-level trigger pulse to switch a fixed current through a load. As such, it behaves like a latching current sink. In the figure, TR1 and TR3 are connected in a "pseudo-SCR" configuration. When the input is low, all transistors are normally off. Applying a pulse, VP, of sufficient amplitude causes TR3 to switch on. This will switch on TR1, which supplies additional base drive for TR3. Now the circuit is latched, and TR1 and TR3 remain in the "on" state when the input returns to a low level.
Unlike a conventional SCR, however, the additional transistor, TR2, servos TR3's base and limits TR3's emitter current to a level determined by current-sense resistor R5 and TR2's base-emitter voltage, VBE2. Provided that TR3 has reasonably large hFE, we may assume that:
IC3 @ IE3 = VBE2/R5
If TR1 also has large hFE (forward current transfer ratio), then practically all of IC3 will flow through light-emitting diode LED1. The LED serves a dual purpose. Beyond illuminating to indicate that the circuit is latched, it supplies a constant reference voltage at TR1's base. Consequently, the voltage across R3 is a VBE drop less than LED1's forward voltage drop (VF) and remains constant in spite of large changes in the supply voltage (VS).
Therefore, TR1 forms part of the SCR structure and behaves as a constant-current source. Assuming that TR1 has large hFE , its collector current may be approximated by:
IC1 @ IE1 = (VF -VBE1)/R3
R3's value should be selected to ensure that IC1 can satisfy TR3's base current requirements and to make up the current shunted by TR2 and R1 (or R2 later on). The total current, ISINK, flowing through the load is given by:
ISINK @ IE1 + IC3 = \[(VF - VBE1)/R3\] + (VBE2/R5).
Because all of these factors are reasonably constant at a given temperature, the sink current also stays the same even though VS may vary considerably. In tests on a prototype circuit with R3 = 2.2 k Ù, R5 = 62 Ù, and using an HLMP-1000 (3-mm red) for LED1, the sink current was 10.01 mA at VS = 5 V. This increased to 10.05 mA at VS = 35 V, equivalent to a 0.4% change in ISINK for a 600% increase in VS.
As long as the input returns to zero, R2 isn't required. However, if the input could be left floating, R2 may be necessary to minimize the noise effects at TR3's base. R4 provides a similar function at TR1's base and could also be required to minimize the photovoltage generated by LED1, which might otherwise cause the circuit to latch in the presence of bright light. Such an effect can occur with certain LEDs that generate a significant photovoltage in bright-enough light conditions when lightly loaded.
Tests on the prototype using an HLMP-1503 (3-mm green) for LED1 demonstrated that the circuit could function as a latching light detector if the LED was subjected to strong light. But inserting a 100-kÙ shunt resistor (R4) across the LED eliminated this effect completely. Shielding the LED from any light source will naturally have a similar result.
As most of the sink current flows through LED1, the circuit is restricted to currents below 50 mA or so. But substituting the LED with some other voltage reference, such as a low-voltage zener or two series-connected diodes, can increase the current level. Then, ISINK is limited only by the current rating of the diode(s) and that of TR3.
Once latched, the circuit may be reset by either cycling the supply voltage or shorting TR3's base to 0 V. When using a device like the HLMP-1000 for LED1—resulting in VF @ 1.6 V—the circuit itself (minus the load) will function down to about 3 V. This allows plenty of voltage across the external load with sufficient supply voltage. Maximum voltage permitted across the circuit depends mainly on the VCE(MAX) ratings of TR1 and TR3, plus the power rating of TR3 that conducts the bulk of the sink current. R1's value will depend largely on the magnitude of the trigger pulse: A value of 10 kÙ was found to be adequate for pulses of a few volts, although larger values may be acceptable.
The circuit can be used to latch a constant current to loads such as LEDs or relay coils. It also functions well as a "glitch catcher": The prototype circuit responded to low-amplitude (3-V) input pulses as narrow as 100 ns. In view of the circuit's ability to latch with very narrow input pulses, it may be necessary to connect C1 at the input to provide a degree of noise immunity.
The b portion of the figure shows a variation on the circuit. Here, an optocoupler has replaced the LED. In addition to providing a stable voltage reference for TR1's base, the optocoupler furnishes an isolated signal to indicate that the current sink has been latched.