Quote of the Day
Concentration is my motto. First honesty, then industry, then concentration.
— Andrew Carnegie
I recently was asked if it was possible to design a circuit that will isolate a battery from a circuit until a momentary switch is closed (Figure 1). Once the momentary switch is closed, the battery is connected to the rest of the circuit and it cannot be disconnected by further mechanical switch closures. This was just a proof-of-feasibility exercise and NOT a final implementation. However, it was a good example of how to use LTSpice to verify the first design concept and is worthwhile documenting here.
The circuit is a simple MOSFET/JFET latch combo. It has to meet a few requirements:
- Very low DC load on the battery when in isolation mode.
- Capable of latching with a charged battery (~3.5 V)
- Stay latched down to ~2.0 V.
Figure 2 shows the basic circuit. The switch S2 represents the momentary switch. MOSFETs M1 and M2 form the latch.
Figure 3 shows my LTSpice simulation.
The simulation shows that the circuit works for this simple test case. Much more checking is needed. This is enough for now – I have shown that a circuit like this is possible. For those who are interested, my LTSpice schematic is available here.
Many thanks to Kurt Raichle for lending his superb analog expertise to this exercise.