Yet Another self-latching mosfet switch - Page 1
Forum section: electronics / projects

author: onlooker
author_rank: Frequent Contributor
timestamp: December 15, 2012, 04:03:48 am
content: This is a continuation of my previous posts about this topic that were scattered in different threads. For reference purpose, they are in1).https://www.eevblog.com/forum/blog/eev-blog-262/60/,2).https://www.eevblog.com/forum/chat/ut61e-auto-power-off-modification/,and3).https://www.eevblog.com/forum/projects/an-improvement-on-the-soft-latching-power-switch/This time the new self latching switch has a soft-power-on feature. And in reverse of the manual button behavior, here, long press of the manual button does the power-on, and short press does the power-off.Also I have just noticed that the circuit in my last post had the soft-power-off feature built-in.More maybe coming.Update: Pictures, LTspice files, and some comments, plus a attempt for soft start.I have to run for now, I will put words after I come back.I added a third circuit here which is a further attempt along the line that the OP of link 3) tried for soft start. My comments in link 3) argued that the OP's cap can't do soft start. Now, I should also say it is in the right direction. I tried to add one more resistor and manged to have the soft start time reach about 200us. I wanted to make it to 100ms range, but have not been very successful.Update: Ok, I am back. I like to 1st make a general comment on the subject.What we call self-latching basically means bistable and the switches we see (e.g. in this forum) are, in essence, not much different than the regular "toggle type flip-flops" (or the simple old bistable multivibrators). At the core of a basic flip-flop (or bistable) is a pair of not gates connected into a serial loop.In terms of using mosfet, the attached picture flip-flop.jpg shows the circuit. What we all try to add are just different ways of doing the toggle ctrl.The second point is about using mechanical switch: I agree with the arguments in Dave's video. And more than that, in my particular case, it is not even a solution worth to consider. I started the whole thing when wanting to do a manual power on/off ctrl for my UT61E. A mechanical switch would mean to deface the meter and leave the APO signal not handled (the meter's own APO will only cut down the power-off current to about 170uA and it is too big).The 3rd point is about whether it is a bug or feature when the circuit starts in the on state: In the case of my UT61E mod, it can be a wanted feature. The meter before mod turns on when the rotary changes from "OFF" position to "V" position. To retain this behavior, "start in the on state" by the circuit is needed. Just imagine a friend borrowed your meter and he would be surprised to find out the rotary does not turn on the power.To summarize about the circuits here:1). all the circuits here do not oscillate.2). a 3rd mosfet can be used to handle non resistive loads.3). some can do soft start/off to various degrees.4). some starts in power-off state.

author: BravoV
author_rank: Super Contributor
timestamp: December 15, 2012, 04:26:45 am
content: Mind attach the ltspice file ? thanks

author: onlooker
author_rank: Frequent Contributor
timestamp: December 16, 2012, 12:14:18 am
content: Updated.