Power Switching Circuit/Resettable Fuse - Page 1
Forum section: electronics / projects

author: Bidski
author_rank: Newbie
timestamp: February 14, 2014, 08:38:34 am
content: Hi All,I am wondering whether this idea I have is possible to accomplish.I am designing a system that will be using a micro-controller to monitor and direct communications in a system as well as monitor and control power to the system. One of the features that I would like to implement is for the micro-controller to be able to shut-off the power to the entire system (including itself). Currently the system uses a toggle switch as its manual ON/OFF control and it would be best if this switch remained in place.My initial thought would be to use a sort of soft latching circuit that has an auto-on feature. Then the micro-controller can disable the power. My concern with this type of circuit would be what would happen once the power has been? The micro-controller is no longer actively disabling the soft latch, so would the system immediately turn back on again?Is there a way to make sure that the toggle switch must first be turned off before the system can power up again?An example of the sort of circuit I was looking at implementing is attached.Thanks in advance.BidskiEDIT: I guess I should also mention that I need to allow for around about 50A of current draw (allowing for more would be desirable to let things run a little cooler). The battery 11.1V LiPo.

author: tszaboo
author_rank: Super Contributor
timestamp: February 14, 2014, 04:44:10 pm
content: What you need is an RS flip-flop.http://en.wikipedia.org/wiki/Flip-flop_(electronics)CD4043 is probably the simplest in existence.

author: dannyf
author_rank: Super Contributor
timestamp: February 14, 2014, 04:59:07 pm
content: You have some conflicting requirements.It is quite possible and easy to implement a switch that would turn on power to the whole system, including the mcu.It is not possible to implement a switch input to the mcu that will turn on / off power to the whole system, including the mcu.It is possible to implement a switch to the circuit that will turn on / off power to the whole system, including the mcu: add a button (+serial resistors) from Q1's base to Vbat. When you push the bottom, it turn on Q1 and the rest of the circuit, including the mcu. Once powered up, the mcu can maintain Ven, forcing the whole system to be powered.Turning off the system with the same button is tricky.1) approach one: Assuming the button is wired from Vbat -> serial resistor -> button -> Q1's base, take a sense wire between the serial resistor and button to the mcu. The mcu would monitor that voltage. Once the button is pressed, that voltage will drop to 0.7v and that tells the mcu to turn off Ven.2) approach two: Use an opto coupler to achieve the same. This offers far better isolation.

author: Bidski
author_rank: Newbie
timestamp: February 14, 2014, 09:54:02 pm
content: Sorry, I guess I could have explained things a little better. I have attached another diagram showing a "big picture" view of the system. The "Master Power Switch" is a simple ON/OFF toggle switch.This is how I would like the system to operate.First assume the master switch has been in the OFF position for a long time. The master switch is then turned ON and the soft latching circuit also turns on, providing power to the system and the MCU.After some period of time with the system being powered up, the MCU determines that the power to the system should be cut-off, so it drives Ven to disable the soft latch.My concern here is that once the MCU has disabled the soft latch and power has been removed from the system the soft latch, with its auto-on feature, will immediately restore power to the system (since the master switch is still ON).Is there any way to ensure that, once the MCU has driven Ven to disable the soft latch, the system remains powered down until the master switch has been turned OFF and then back ON again?Best RegardsBidski

author: Kjelt
author_rank: Super Contributor
timestamp: February 14, 2014, 10:19:38 pm
content: Quote from: Bidski on February 14, 2014, 09:54:02 pmWhat NANDBlog said, you can use a flipflop that needs another upgoing flank to switch on again.Other possibilities is replace the toggle switch with a NO pushbutton switch and create a circuit that can be shutdown by the microcontroller.For instance can be done with Fets but also with relais where the pushbutton will power the relais coil , it will engage the relais and one of the secondary contacts of the relay will then continue to power the coil of the relay itself via another microcontroller operated FET or small relay. In that way the microcontroller can only shutdown the power and the pushbutton can only activate the power, but I am unsure if this is exactly what you want.