SELV voltage transiently overshoots safe level? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: April 29, 2023, 10:47:41 am
content: Hello,We are making a “finger heater surface plate” which heats up peoples fingers when they lay their fingers on it.They just lay their fingers on it and switch it on with the other hand.The element is a “snaked up” region of thin resistive conductor trace, which lies beneath a wafer thin bit of plastic, on which the fingers rest. A voltage of 40V drives current through the element.Power to the element can be switched off at any time. The (NFET) switch is low side, so the element is still connected to the 40V rail, even after the element is switched OFF.When the element is suddenly switched OFF, theoffline isolated SMPS that delivers the 40V, suddenly gets no loaded, and so its voltage transiently goes up to 48V, before settling back down to 40V.This is above the SELV limit level of 42.4V. As such, does this mean our product will fail safety regulations?And how high could the voltage rail that drives the elements be, before EU electrical safety regulations are failed? (EN32368).Eg , could we use 65V for the element voltage?

author: tggzzz
author_rank: Super Contributor
timestamp: April 29, 2023, 11:00:22 am
content: Quote from: Faringdon on April 29, 2023, 10:47:41 amYoucoulduse 1kV for the element voltage.Apart from that, you have four options:read and understand the safety regulations, and expect them to be applieduse a different PSUuse a lower voltagechange your design to avoid falling foul of safety regulationsAny responses you get in this thread are, ultimately, worth what you paid for them. That's important when it comes to safety and regulations.Youare the engineer, so youwilltake responsibility for the design.

author: Faringdon
author_rank: Super Contributor
timestamp: April 29, 2023, 11:06:40 am
content: Thanks, what is the law on transiently overshooting the SELV voltage limit?

author: tggzzz
author_rank: Super Contributor
timestamp: April 29, 2023, 11:19:44 am
content: Quote from: Faringdon on April 29, 2023, 11:06:40 amThat is, of course, specified in the standards.

author: wrangler
author_rank: Newbie
timestamp: April 29, 2023, 10:29:09 pm
content: What is your plan for safety if the SMPS stops regulating well entirely, and makes 100V? Or it has an isolation failure to mains potential?Try and consider the adverse conditions you've not yet seen.

author: tggzzz
author_rank: Super Contributor
timestamp: April 29, 2023, 10:36:24 pm
content: Quote from: wrangler on April 29, 2023, 10:29:09 pmAs I put it: amateurs think about how things work, professional engineers think about how things fail.If you are new here you are probably unaware of treez/ocset/faringdon's posting history. (It ain't pretty)

author: wrangler
author_rank: Newbie
timestamp: April 29, 2023, 11:12:34 pm
content: Quote from: tggzzz on April 29, 2023, 10:36:24 pmAsks better questions than some of my engineers.

author: Gyro
author_rank: Super Contributor
timestamp: April 30, 2023, 05:02:24 pm
content: Quote from: wrangler on April 29, 2023, 11:12:34 pmHoly sh*t, youhaveto improve your recruitment procedures!

author: wrangler
author_rank: Newbie
timestamp: April 30, 2023, 06:09:49 pm
content: Quote from: Gyro on April 30, 2023, 05:02:24 pmI'll take any advice you've got. I'm in the United States, and I'm having a heck of a time finding competent people. The local university isn't in my good graces. I occasionally get lucky with a hire, but what's coming in the door is no good, especially with the youngins. I'm playing the role of teacher, mentor, parent, and boss, all at the same time.

author: mzzj
author_rank: Super Contributor
timestamp: April 30, 2023, 07:47:03 pm
content: Quote from: wrangler on April 29, 2023, 11:12:34 pmOh, I find many of his questions perfectly relevant.It is just his apparent inability to learn anything from the given info or think on his own when in the same time he likes to act like SMPS guru bashing on chinese products and attempting to copy them in the same time. Drives many people nuts but seems harmless as long as his power supplies don’t kill somebody.Looking back on his aliases posting history is a wild ride so I’d guess he has learn’t something along the way. His modus operandi just seems to be ”ask first, ask second time, ask third time with slighly different wording”.. and maybe half year later engage to think.

author: Gyro
author_rank: Super Contributor
timestamp: April 30, 2023, 08:11:34 pm
content: Quote from: wrangler on April 30, 2023, 06:09:49 pmYes, that's a big problem here too. The only things you can really do to attract decent people is to ensure that you offer a very competitive package and working conditions. Candidate wise, ensure that you ask plenty of technical questions and make them work hard at interview, and look for leadership potential - you can't sustain being a full time teacher as well as parent whilst being effective as a mentor and boss. With the likes of Faringdon, you can spot the warning signs by thehugenumber of previous employers on their CV and lack of references.I'm glad to say that those days are now in my past, but it's hard work building and sustaining a good team. With a less than good team, a stressful and thankless task - it certainly makes you hanker for the days when all you had to worry about were your individual timescales and deliverables.