Author Topic: world's simplest soft latching power switch circuit  (Read 2260 times)

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Offline rockstar

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world's simplest soft latching power switch circuit
« on: June 30, 2015, 12:15:52 pm »
The finished circuit as per blog262 with a single switch that latches and toggles: it works well at 12v on breadboard and when soldered up. BUT it has output when powered up. How can I modify circuit so that it is naturally off when powered up, then has output when button first pushed?? I've tried lots of things and it does what I want if there is a resistor between battery positive and 'power in' PLUS power up the gate to the mosfet prior to powering the rest of circuit. Trouble is I want 12v out so need to put 12v in, not add resistor. I am happy to power the gate first as will be using in my car so can have gate permanently powered and rest of circuit powered when ignition on. Maybe I need a different mosfet or different resistor values?
Used mosfet TP0604N3-G (-40V;-2A;740mW;T092;channel enhanced) 2x transistor 2n3904, resistors as suggested inc 10k on load parallel to out

Offline 4JimBeam

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  • Country: de
Re: world's simplest soft latching power switch circuit
« Reply #1 on: February 15, 2016, 11:09:09 am »
Hi All,

I have the same issue, that the output is high first time I power up the ciruit.  :bullshit:
So I have the following questions:
1.) Why is it so?
2.) What can I do to have it stay OFF when powerd time?
3.) Additional I would like to have a soft-start (inrush current limited), what do I need to change?

My own guesses:
to 1.) The P-Mos is inital at least a bit on, so that the bi-polar transitor at the gate of P-MOS pulls the gate faster down to GND than the 100k between gate-sorce can pull the gate up.  :-//

to 2.) A) Bypass 100k gate-sorce with a 100nF cap to have an pulse current to carge up the gate faster? B) add a cap at the base of that bipolar transitor that pulls the gate down, to slow this lathcing down?  :-/O

to 3.) add some extra gate-sorce cap to the Mosfet to make it slower on (and off)?

Thank's for your help and advice!  :-+


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