PSU Power Button: Soft or Hard? - Page 1
Forum section: electronics / projects

author: fc3000
author_rank: Regular Contributor
timestamp: August 31, 2023, 06:57:04 am
content: Hi all.I am designing a Bench PSU.Many modern PSU have the output controlled by a 'Soft' button.Many older PSU have the output controlled by a 'Hard' switch.What are the benefits of either of them?Which one is better?I would like you to help me choose.Regards

author: RoGeorge
author_rank: Super Contributor
timestamp: August 31, 2023, 07:21:54 am
content: I know that differently. Newer models include a soft button, but they all still keep their hardware OFF capability, which is usually by a long press on the power button. That long-press power-off is entirely hardware.Aside from those two, more expensive designs also add a mechanical switch for mains power, which is good to have for 2 reasons: so you won't need to unplug the device from the outlet, and to be sure it's off (most power supplies still draw a few watts while they are, because off usually means standby voltages are still on in the power supply.

author: fchk
author_rank: Frequent Contributor
timestamp: August 31, 2023, 09:25:34 am
content: Most Desktop PCs have both: a mechanical switch integrated in the ATX power supply for safety and maintanence and a soft switch controlled by the main board for day-to-day use.I'd suggest such a module that copntains IEC socket, fuse, power switch and mains filter like this. You will need at least fuse and IEC socket and perhaps the mains filter anyway.https://www.distrelec.de/en/power-inlet-with-filter-10a-250vac-schaffner-fn284-10-06/p/16965941?pos=2&origPos=2&origPageSize=50&track=falseOn the controller side you can then add soft power with auto power off etc.fchk

author: fc3000
author_rank: Regular Contributor
timestamp: August 31, 2023, 10:00:29 am
content: Quote from: RoGeorge on August 31, 2023, 07:21:54 amThank you for the reply.I have already allocated a mains switch on the front panel.My question was related to the power-out button.Is it better, for example, to have a soft button that shuts down the regulator,or a mechanical switch that disconnect the output?Regards

author: LordXaos
author_rank: Contributor
timestamp: August 31, 2023, 10:09:52 am
content: I'd guess the main benefit of a soft button (though regardless whether it controls some solid state or mechanical switch in the end) is that it can be remote controlled by software.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 31, 2023, 10:10:24 am
content: Quote from: fc3000 on August 31, 2023, 10:00:29 amAvoid the contacts ( you can always unplug it, if you really need abrupt on/off)Soft button allow normal V ramp up and down, which is what most end uses do anyway.Breaking contacts can cause overvoltage ringing with lead inductance and ceramic caps.

author: fc3000
author_rank: Regular Contributor
timestamp: August 31, 2023, 11:30:04 am
content: Thank you for your comments.I'll go for the soft option...pun WAS intended.Regards

author: Swake
author_rank: Frequent Contributor
timestamp: August 31, 2023, 06:42:54 pm
content: What design is the control going to be, old fashion analog or micro-controller digital?Make sure you'll still be able to verify the set voltage when the output is off.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: September 01, 2023, 07:23:21 am
content: Soft power button. It will be very hard to have a hard power button that does not bounce, and will switch on/off cleanly. Using a soft button and mosfet and some capacitors, you should be able to cleanly turn your power on/off.

author: coppercone2
author_rank: Super Contributor
timestamp: September 01, 2023, 07:57:39 am
content: I vote hard switched. I have seen a emergency cutoff switch (big label too, big red button, with the words "emergency off" on it), been implemented with a soft switch.Guess what happened when the equipment locked up (possibly from low power blue tooth?)? It did not respond to this button. output went to whatever it felt like, and there was no response from the emergency off buttonwhat a joke. not cheap either, american and worth 20k.. less then 5 years old too. with the correct options I believe the output is something like 400V.Because there was no dangerous problem I spent a good five minutes pressing various buttons, probing the output, videoing it, etc. nothing.nice to be able to turn off malfunctioning equipment without trying to reach for cords.Nothin like a false sense of security from bull shit features. The button does work, so long there is no emergency.I call it safety theater. The button should say "reach for the cord if there is a actual problem". Nothin quite like fake safety buttons to make you feel better about your self

author: JustMeHere
author_rank: Frequent Contributor
timestamp: September 02, 2023, 12:22:36 pm
content: Quote from: coppercone2 on September 01, 2023, 07:57:39 amNothing wrong with a "master" hard switch, but circuit on/off can't be done cleanly this way.

author: coppercone2
author_rank: Super Contributor
timestamp: September 03, 2023, 01:23:48 am
content: but for the love of god don't call it "emergency"and you can do it cleanly, with a relay, or a normal switch controlling a relay. then its repeatable and you can design for it better. Or a e-switch controlling a relay tool. But that is exactly what the 'emergency' switch I saw controlled.like a e-switch that does the soft switching followed by a relay. Then you have isolation but also controlled parameters. That is IMO higher quality for a lab supply.all these approaches have some benefit to them IMOfor instance benefits to true mechanical isolation of precision supply : you don't have any bias current charging the input caps to whatever. This will lead to higher accuracy for circuit 'turn on' during testing. Like if you leave a low power device connected to the supply while 'off' between tests. In some cases its serious business.It might not be commercially viable for anything but the best devices, but if you DIY its actually not that much over head to add this stuff.

author: Swake
author_rank: Frequent Contributor
timestamp: September 03, 2023, 06:20:28 am
content: What would be the sequence?off- e-switch off -> relais off (how much time between both and why?)- relais off -> e-switch off (timing?)- both off at the same timeon- e-switch on -> relais on (timing?)- relais on -> e-switch on (timing?)- both on simultaneously

author: coppercone2
author_rank: Super Contributor
timestamp: September 03, 2023, 06:43:03 am
content: for turn on you close the relay with eswitch off and then bring it upfor turn off you bring the e-switch down and then turn the relay off.timing is like generous?not 100% sure on the relay closing with no current though. There may be exceptions there based on the kind of relay.just get like a good contact plating material and it should be fine. it might get hard if you need like really super duper low and high end behavior from the same supply