components
~ Transistors ~
BC327 / 337 - Epitaxial Silicon Transistor, Low power applications (PNP/NPN, 1-B 2-C 3-E)
others
~ Voltage Regulators ~
LM7805 - 5V regulator
LM7812 - +12V regulator
transistor latch
trans latch
The two transistors normally are off. To fire the latch, you drive either the base of the PNP low or the base of the NPN high until one of the transistors turns on. This creates collector current flow and turns the other transistor on, which further turns the original transistor on. The circuit latches in a regenerative way. Current is only limited by the source impedance and the transistor characteristics, allowing this circuit to rapidly discharge a capacitor. An interesting characteristic of this circuit is that you can establish the holding current of the SCR by choosing the resistor values. For the latch to remain on after being fired, there has to be sufficient voltage (~0.7 V) across the two base-emitter junctions to keep them on. This means that the circuit latches, if it is supplied with a current of at least Vbe / R1 + Vbe / R2. If the latch is connected across a capacitor that is fed with a small current, the latch discharges the capacitor. Once the current in the circuit falls below the holding current, it turns off.
