EEVblog #262 - World's Simplest Soft Latching Power Switch Circuit - Page 1
Forum section: electronics / general

author: HopWorks
author_rank: Contributor
timestamp: December 10, 2012, 10:46:50 pm
content: I absolutely love your videos! Usually, analog+me=fire! but your detailed explanations of the theory has given me confidence, and I have less fires and explosions on my breadboards now. So not only have you helped my understanding of electricity, you have helped to save? the world as well! Hey! Bob's your uncle! Great job and thank you!It would be nice if you had parts links along with your video's though. I tried searching for an IRF9110 as used in your circuit but came up empty. I found a lot of P-Channel Mosfets, but without a full understanding of Vgs (+-20v???), I can't figure out a replacement.Anywho... NICE JOB!

author: IanB
author_rank: Super Contributor
timestamp: December 10, 2012, 11:15:09 pm
content: Quote from: HopWorks on December 10, 2012, 10:46:50 pmFires and explosions? Well, electronics for you is certainly more exciting than for most people!

author: HopWorks
author_rank: Contributor
timestamp: December 10, 2012, 11:40:49 pm
content: Quote from: IanB on December 10, 2012, 11:15:09 pmYou have no idea.

author: Fezder
author_rank: Regular Contributor
timestamp: March 27, 2013, 07:20:40 pm
content: Nice tutorial, easy looking schematics and well explained. BUT, i did of course try this out, 'been looking this kind solution/s for some time. mine version of that first one which has 2 push-buttons, doesn't latch. it turns on right away, and turns off only the time i hold the off button. i've checked wirings many times, i'm confused what's the problem. i dont know is it because of incorrect components but here's the list if anyone could lend a hand:...and sorry if this is NOT the place for asking help :S.not even SCR equilavent circuit worked, tired that too, same symptoms.ps: its not the end of the world, but i found couple other latching circuits which i succeeded to get workingsupply voltage is 12vtransistors: bc547 / bc557resistors: both 1k, tried 10k and 100kload was LED with 1k resistor

author: con-f-use
author_rank: Unknown
timestamp: July 07, 2013, 11:15:29 am
content: I can confirm that the first schematic does in fact NOT work with a BC527/537 combination.Here is what I used for a small ?C-project. It's not pretty but very simple and does the job. The 2N7000 is only rated for 200mA so others might need something beefier. Also it switches ground, which generally is a bad idea, but should be fine for 5V. I don't care about the cap slowly leaking and eventually turning the whole thing off by itself as it never runs more than a day anyway and the circuit gave me not problems.Additionally because I'm tired of looking, for your convenience here the links to a few spin-offs:https://www.eevblog.com/forum/beginners/world%27s-simplest-soft-latching-power-switch-circuit-%28modification%29/msg121811/#msg121811https://www.eevblog.com/forum/projects/two-mosfet-soft-latching-startup-switch/msg167730/#msg167730https://www.eevblog.com/forum/projects/characteristics-of-a-soft-latch-power-switch/msg152164/#msg152164https://www.eevblog.com/forum/projects/yet-another-self-latching-mosfet-switch/msg171647/#msg171647https://www.eevblog.com/forum/beginners/world%27s-simplest-soft-latching-power-switch-circuit-%28modification-2%29/msg121872/#msg121872https://www.eevblog.com/forum/projects/an-improvement-on-the-soft-latching-power-switch/msg171112/#msg171112https://www.eevblog.com/forum/projects/mosfet-power-switch-with-auto-off/15/

author: opty
author_rank: Regular Contributor
timestamp: December 19, 2013, 10:58:37 pm
content: In the first circuit PNP's leakage current will immediately pull NPN up.You could add a resistor (parallel to the off switch) to prevent it.

author: jimjam
author_rank: Regular Contributor
timestamp: June 14, 2014, 05:51:31 pm
content: Quote from: opty on December 19, 2013, 10:58:37 pmI just discovered this while breadboarding itWhy is there a leakage current?

author: SeanB
author_rank: Super Contributor
timestamp: June 14, 2014, 05:55:40 pm
content: Because real transistors always have some leakage current, so there is leakage. Typically you will add a resistor between base and emitter of both transistors to ensure that they will stay turned off even with a low amount of leakage. In this particular circuit 100k or so will work, as the leakage currents are in the femtoamp region. If you are driving big power devices then you will be much lower in value.Necromancing the thread..........