switches Default-on latch-off MOSFET power switch

Posted on Feb 7, 2014

It should default on when first connected, but if a particular signal goes high for a small amount of time (say, 10 microseconds, ) then it should turn off, and stay off. It`s the `default on` that gives me trouble. The closest I can get is something like this: From null, `OFF` is floating. Supply 15V, and the capacitor starts out acting like a bit of wire, which pulls the M1 N-channel gate high, which makes the N-channel conduct

switches Default-on latch-off MOSFET power switch
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(I`m likely to use a BS170, not the IRF on the schematic. ) That pulls the gate of the P-channel M2 low, which makes it conduct. That, in turn provides voltage into R3/R2 divider that keeps the circuit latched open. Now, when OFF goes low-impedance to ground, it will turn off M1, and thus R1 will pull the M2 gate high, and M2 will turn off. Because C1 already has a charge, the system will now stay with M2 turned off even if the OFF signal goes high impedance again (which it will, because it`s a MCU output eventually powered by the SWITCHED voltage. ) While off, the leakage through C1 and the leakage through R1/M1 (and, I guess, leakage through M2 and the entire powered subsystem) will still draw a little bit on the battery, so if it`s a LiPo, I shouldn`t leave it in that state forever or it will die, but as a safeguard for a hobby robot, I think it might work. Is this correct, or am I getting something wrong Will the system oscillate when the "off" is triggered Will the system automatically turn on from "zero" as intended

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