Lightswitch trashed in only a few weeks... - Page 1
Forum section: electronics / general

author: peteb2
author_rank: Regular Contributor
timestamp: August 22, 2022, 03:38:46 am
content: So the pic is a standard PDL fully taken to bits to reveal the arc damage that occurs now that the ceiling full of room flouro lights have been substituted for multiple standard batten LED. There seems to be a huge a back emf upon lights switchoff as the (i'll assume) buck converters in each light that produce the 400VDC plus from Mains 230VAC to feed the large cob strip of LEDS. Anyway, the poor lightswitch soon gives up the ghost & the dissection reveals the destruction. Back in the day flouro lights would put a toll of the switch too when their ballasts or starters went sick. Can anyone suggest a fix ie an X or Y class capacitor & maybe a resistor in one leg of the capacitor across the Line & Neutral to snub out the arc potential & let a switch last a bit longer? TIA!

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: August 24, 2022, 01:13:34 am
content: Use a higher current switch. (Much higher)Jeff

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: August 24, 2022, 10:26:00 am
content: You could use a quencharc device, these include an RC network with the appropriate mains ratings and are specifically for this purpose.https://www.cde.com/resources/catalogs/Q-QRL.pdf

author: Whales
author_rank: Super Contributor
timestamp: August 24, 2022, 10:47:36 am
content: Quote from: peteb2 on August 22, 2022, 03:38:46 amHuh? All of the power supply schematics I've ever seen start with a fuse, perhaps some protection components, perhaps some filtering (X & Y caps, maybe a common mode choke) and then a diode bridge (or active bridge) before that 400V caps. That shouldn't act very inductive overall or be able to backfeed the contents of the 400V cap, ie it shouldn't fry the switch on turn-off. If it did then every switchmode power supply of similar construction would fry your switches, including computer power supplies.EDIT: But perhaps if they have bad diodes (or equivalent active parts) the DC could backfeed? That would wear an AC rated wallswitch out pretty fast if timed correctly.Are you still running the odl ballasts in series with the new power supplies? ie was it a replace-tube-and-remove-starter conversion to LED, or were the ceiling boxes taken down and the ballasts bypassed/removed too? I wonder if that situation could cause your problem... perhaps the LED PSUs bypass these big ballast spikes via their protection components and then the switch contacts take the hit instead? But why would these spikes be bigger than with the original tubes -- super low impedance when activated style protection?

author: wraper
author_rank: Unknown
timestamp: August 24, 2022, 10:53:56 am
content: I rather suspect this switch was at deaths doors already and changing the lights was a coincidence. Back EMF from LED bulbs is basically impossible FWIW. But with inductive ballasts of fluorescent bulbs this should be possible.

author: Someone
author_rank: Super Contributor
timestamp: August 24, 2022, 11:16:46 am
content: Good theory to start with if you have a noticeable pop/flash when turning off the switch. Next step is to capture/measure it and track it down!

author: wraper
author_rank: Unknown
timestamp: August 24, 2022, 11:45:54 am
content: Quote from: Whales on August 24, 2022, 10:47:36 amIf there is a bad diode which conducts in opposite direction, something will blow up immediately, a fusible resister or other weakest link.

author: tom66
author_rank: Super Contributor
timestamp: August 24, 2022, 12:42:35 pm
content: Agreed, this is not back EMF.Perhaps the problem is the LED lamps do not have sufficient inrush limiting, and while the current isn't enough to trip a breaker, it applies excess wear to the switch as the switch bounces at turn on.Still, this was probably more a switch at the end of life, than directly related to the LED install, I can't see even the most optimistic assumption about inrush burning a switch out in a few weeks.

author: sokoloff
author_rank: Super Contributor
timestamp: August 24, 2022, 03:30:23 pm
content: If it wasn't a new switch (that it's total time in service was only a few weeks), then I'd first just replace the switch.If it was a brand-new switch, I'd still just replace it with new and commit to destroying and disassembling the new one in 3 weeks' time to see if it's repeatable.

author: peteb2
author_rank: Regular Contributor
timestamp: August 25, 2022, 05:36:14 am
content: Most certainly no remnants of flouro lighting, all fittings were removed... (I have obviously made an error explaining the effect as a back EMF).It's as if the (i assume) buck module foreachfittings mass internal COB LED strip that uses Mains 230Vac to produce around 415Vdc for the appx 6ft long COB strip simply hates being powered off. In that mill-second as the Mains goes away the circuit regulates like mad trying to maintain its feed to its load & keep running. Thus it draws an increasing current even though it's collapsing away though & with multiple fittings it all adds up to an arc across the opening switch contacts.... Well at least that's what it appears like.In simple terms we're burning up the switch worse than when the same device was switching old flouros on & off....

author: capt bullshot
author_rank: Super Contributor
timestamp: August 25, 2022, 07:44:49 am
content: The problem is the inrush current of all these LED power supplies, not arcing when turning off.Each time the switch closes the circuit, the DC bulk capacitors charge and cause a rather high current surge across the switch. Sometimes this just welds the contacts together (it's difficult to switch off then), sometimes it quickly degrades the contact materials, as to be seen here.One could use the switch to control a contactor that in turn switches on the LED lights. The contactor would have to be appropriately rated for the rather large inrush, otherwise its contacts could stick together, too.

author: Whales
author_rank: Super Contributor
timestamp: August 25, 2022, 08:51:03 am
content: Quote from: peteb2 on August 25, 2022, 05:36:14 amHmm OK.The 400V cap will be sized to be able to survive at least 10msec of no power (this is the distance between peaks at 50Hz). If it couldn't do that then it would have horrible flicker. Actually many LED fixtures do seem to flicker madly, so you could be onto something hereTheoretically speaking 10msec should be enough because the switch will probably stop arcing after around 10msec (due to the zero crossings). But if it's already got too small of a capacitor (and is hitting it with lots of stress current) then this might make sense. Not sure.

author: wraper
author_rank: Unknown
timestamp: August 25, 2022, 09:28:10 am
content: Frankly this switch looks like something from 70 years ago.