World's Simplest Soft Latching Power Switch Circuit (modification) - Page 1
Forum section: electronics / beginners

author: demat95
author_rank: Newbie
timestamp: June 19, 2012, 06:02:02 am
content: Hi guys, my name is Mateo and I am from Argentina... I am kinda embarrassed to ask this question, specially for being in my 4th year of electronic engineering grade career lol.Here is my question, I want to make dave´s soft latching switch circuit, but use an N-channel mosfet instead of a P-channel. I can´t get it to work on my protobard. I am using an IRF510 mosfet. Any hints? Where should I connect the Gate to? I connected the load between VCC and Drain, and Source straight to Ground..Thanks!

author: T4P
author_rank: Super Contributor
timestamp: June 19, 2012, 06:37:52 am
content: Quote from: demat95 on June 19, 2012, 06:02:02 amYou MUST use a P-channel, in any other case it's alright to ask questions here, nobody's gonna chide you for not knowing!

author: Psi
author_rank: Super Contributor
timestamp: June 19, 2012, 08:36:09 am
content: You could use a N-channel, but you'd be switching ground on/off, which is a bad idea.So yeah, as DaveXRQ said, It needs to be a P-channel

author: djeans
author_rank: Contributor
timestamp: September 16, 2013, 03:01:00 pm
content: quiet old topic, but still not really answered?you could use the output to drive your n-mosfet. Then it should work, if you have UGS(th) max as output voltage. or?

author: ncrowl
author_rank: Newbie
timestamp: April 08, 2025, 12:00:06 am
content: Hi Eevblog, my 1st post on this forum.I had the same problem, only having n-mos fets, no p-mos.Have redrawn (mirrored) and simulated 2 circuits, that have floated around there:https://tinyurl.com/25wkx2v5(the original resembled the one in the video)https://tinyurl.com/2c68n6qe(touchy even simpler)What I trying todo: want "soft latching witch switches" for lights (and usb-charger.. on a ebike, circuit should accept 42V supply.Screenshot:

author: Zero999
author_rank: Super Contributor
timestamp: April 08, 2025, 08:46:32 am
content: Generally, the same circuit can be made with N-channel, rather than P-channel. Swap the polarity of the power supply and all of the other polarised components i.e. diodes and capacitors. Change all N-channel devices to P-channel and P-channel to N-channel. Regarding BJTs: NPN should be swapped with PNP and PNP for NPN.

author: ncrowl
author_rank: Newbie
timestamp: April 08, 2025, 01:51:51 pm
content: Quote from: Zero999 on April 08, 2025, 08:46:32 amYea, I have several PNP BJTs instead, the jellybean BC557 should work, upto 45v.I found 1 error in my 1st link, forgot to moveabout the capacitor (some startup glitch pumped the charge over it to 2*Vcc), revived circuit (for 42v):https://tinyurl.com/2al5zpxgScreenshot:The cap needs tobe rather big, -feels odd to need to put an electrolytic in the middle of the circuit?

author: Zero999
author_rank: Super Contributor
timestamp: April 08, 2025, 02:10:41 pm
content: 5µF is not big and you'd use 4.7µF which is a standard E3 value.Why do you think it's odd?

author: ncrowl
author_rank: Newbie
timestamp: April 08, 2025, 02:31:01 pm
content: Quote from: Zero999 on April 08, 2025, 02:10:41 pmMy biggest ceramic I have is 0.33 uF, and it's yellow, probably tantalum, so it polarized also.(and I don't know what transients my ebike makes, rather blowup something sheep..

author: Zero999
author_rank: Super Contributor
timestamp: April 08, 2025, 02:48:31 pm
content: Quote from: ncrowl on April 08, 2025, 02:31:01 pm4.7µF is a common value of aluminum electrolytic capacitor.What's the supply voltage? Note your MOSFET's maximum gate-source voltage rating is only 20V.

author: ncrowl
author_rank: Newbie
timestamp: April 08, 2025, 03:29:48 pm
content: Quote from: Zero999 on April 08, 2025, 02:48:31 pmVery good point, "Vgs" can't exceed 20V (even for power-mosfets)!The circuit will work with the supply voltage of 36v ebike, so between 30-42V.Had to revise the circuit again (added voltage-divider for the Mos-gate):https://tinyurl.com/2aqapejnScreenshot:

author: ncrowl
author_rank: Newbie
timestamp: April 08, 2025, 11:45:18 pm
content: I have now bread-boarded the circuit, worked, sortof.BUT It was default "on", when I turned on my battery, it shouldn't.So I tested with another PNP-BJTs "BD140", but no, stayed default on.Tested to put an extra 1uF caps over the "Mos-gate" and "gnd" (that I made voltage-divider for), seems to have to have solved the "default on" problem.(Im not sure on any values here, maybe I should simulate it in Ltspice also..The number of component start to grow,, is there a correct way??(toasted a pc-psu that was connected in it 5v, with my 42v ebike battery, and a multimeter flatlined ..

author: KimLum
author_rank: Contributor
timestamp: April 09, 2025, 11:29:24 am
content: Try to put a Power-On Reset capacitor (aka. POR cap) on the second PNP transistor (VS+ -> cap -> PNP-base), as you have with the first PNP.

author: ncrowl
author_rank: Newbie
timestamp: April 09, 2025, 01:11:40 pm
content: Quote from: KimLum on April 09, 2025, 11:29:24 amYes, thank you, that worked. Now the mos-fet doesn't fade o&off.(circuit now more resemble a: flip-flop, SR, multivibrator -circuit)Not sure my voltage-divider as mos-fet-driver is suitable in high-switching..

author: Zero999
author_rank: Super Contributor
timestamp: April 10, 2025, 07:29:34 am
content: The circuit is a flip-flop.I strongly recommend you build this first, on a breadboard, before finalising the design. Simulation is good, but the generic models used by they online simulator will be very different to real life components. You could try a different simulator such as LTSpice or TINA TI which allow you to use models provided by component manufactures and also come with lots of models.

author: ncrowl
author_rank: Newbie
timestamp: April 13, 2025, 12:13:13 am
content: Hi, again. I have attempted to finalize this thingy, but have another issue, can't turn it off (it blinks though, -if switch is pressed).Eeh I soldered it up, but with something that more resemble the original design, with extra mos-fet at separate rail-rail (Because high/low load impacted on/off timing).I include picture,,, hope somebody have a clew.My guess is that the 1u capacitor, electrolytic, have some memory. something I thk I observed at room-temp with bigger caps..More or less the same circuit Dave had in his video, with an extra mos-fet separated:https://tinyurl.com/28yedwl6

author: wraper
author_rank: Unknown
timestamp: April 13, 2025, 01:05:20 am
content: Quote from: Psi on June 19, 2012, 08:36:09 amIt's not a bad idea as such. Just less versatile than switching positive rail.

author: ncrowl
author_rank: Newbie
timestamp: April 21, 2025, 04:59:49 pm
content: Quote from: wraper on April 13, 2025, 01:05:20 amI complicate things by using 42volt. Just need 5v output atm, so I could have used a linear-regulator, say lm317, instead of a bunch of voltage divider, will see (have a 100W LED that I wanto make a stobe of, in the future..And, there was further issues, the blinking wasn't consistent after a while. Needed to insure there never was never +5v on Vbe, soo 2 more resistor..(why it also was default off, on lower voltages, also)I include my final Ltspice-module "te_fix.asc"Also, I don't wana dedicate a box for it, just going to use electric-tape, see included picture.