Gas Discharge tube based mains transient protector......upstream of mains fuse? - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: November 09, 2017, 09:16:00 pm
content: Hello,We have designed a 40W non-isolated , offline, 230VAC LED lamp.For transient protection, we have a 470V gas discharge tube (Bourns, 2051-47-SM-RP-LF) in series with a 270V “surge absorber” (Panasonic, ERZVF2M271).ERZVF2M271 datasheet:https://industrial.panasonic.com/cdbs/www-data/pdf/AWA0000/AWA0000C4.pdf2051-47-SM-RP-LF datasheet:https://www.bourns.com/docs/Product-Datasheets/2051.pdfThis transient “block” sits across live and neutral, just downstream of the fuse. The fuse is the first thing after the mains input connector.Our contractor tells us that because the Gas Discharge Tube will always fail open circuit, we should therefore move the fuse downstream of the Gas discharge tube based transient protector. –This way, when the gas discharge tube flashes over due to a transient, the fuse will not blow , and so the product will stay functional for longer.Do you agree? Should we do this?

author: Benta
author_rank: Super Contributor
timestamp: November 09, 2017, 09:29:49 pm
content: First, are you sure they are in series?I'd expect them both to be connected live - neutral.The surge absorber is supposed to take the first impact before the tube ignites and takes the brunt of the transient.Anyway, these devices and the input fuse serve two different purposes. One is transient clamping, the other is fire protection, so the fuse should be the first thing before everything else.

author: ocset
author_rank: Super Contributor
timestamp: November 10, 2017, 06:19:24 am
content: Thanks, yes they are in series

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 10, 2017, 06:54:26 am
content: I have torn down some old HP server PSUs and one generation had the MOV after the fuse while one newer generation had the MOV before the fuse. I suspect the reason is because surges were blowing the fuses and causing field returns.

author: Benta
author_rank: Super Contributor
timestamp: November 10, 2017, 01:54:15 pm
content: Quote from: treez on November 10, 2017, 06:19:24 amIn that case I don't understand the functionality, sorry.

author: Gyro
author_rank: Super Contributor
timestamp: November 10, 2017, 03:26:26 pm
content: QuoteThat configuration is normally used to protect high impedance instrumentation type signals...https://www.eevblog.com/forum/metrology/gas-discharge-tube-with-varistor-in-protection-circuit-why/msg1308573/#msg1308573I have absolutely no idea why someone would want to use it on a 40W mains lamp.I've also don't know why it would be worth risking it before the fuse on a lamp - It hardly sounds like a critical system, maybe it is???@treez:I think you need to share rather more about this project (it's presumably the one with the narrow 300V track spacings too?) if you want us to be able to give you sensible feedback.

author: Benta
author_rank: Super Contributor
timestamp: November 10, 2017, 03:55:55 pm
content: QuoteThat's what I meant.

author: stj
author_rank: Super Contributor
timestamp: November 10, 2017, 06:20:34 pm
content: for legal reasons, the fuse should always come first - even if it looks wrong from an electronic/functional standpoint.

author: oldway
author_rank: Super Contributor
timestamp: November 10, 2017, 06:51:47 pm
content: I think there are several points that deserve reflection:1) Is it necessary to provide protection against transients?The advantages and disadvantages of protection must always be analyzed because protection may become more dangerous or less reliable than lack of protection.It is therefore necessary to know the characteristics of the load .... It is useless to put a protection which limits the tension with 600V if the load is damaged with 400v ...It would therefore be necessary to know the circuit to be protected as well as the specifications of the components.On the other hand, transients are rare and many devices do not have such protection.The result in practice does not seem disastrous.One can therefore ask the question of the need for such protection.2) What is the nature of such a transient surge?An electrical network has a relatively low impedance, so a possible transient can have a high energy.Wanting to eliminate the transient of the network seems to me a design error because we can not put MOVs of sufficient size in a simple 40W LED lamp.We must think only to protect the lamp, not to clean the network.On the other hand, using a discharge tube alone seems inadequate ... that is why the discharge tube is put in series with the MOV resistor ... It is necessary to start the discharge tube in the first place.So, what I recommend is an input fuse, then a wire wound resistor of low value to limit the current in case of transient overvoltage, then the mov + discharge tube connected in series between live and neutral.

author: Gyro
author_rank: Super Contributor
timestamp: November 10, 2017, 07:57:21 pm
content: QuoteWhy use a discharge tube at all though? - that's very unusual for mains transient protection, it's usually only found in Telecoms gear and the like. Just a MOV L-N is the normal transient protection used in consumer SMPSs, I don't see why an LED light should be any different.

author: jmelson
author_rank: Super Contributor
timestamp: November 10, 2017, 08:26:36 pm
content: Quote from: treez on November 09, 2017, 09:16:00 pmDo not believe this. We have a gas tube suppressor on our phone lines, and they have to be replaced every 10 years or so, as they get enough vaporized material on the walls to start to conduct until the phone line senses that as "off hook".I can only imagine the assault of mains-level transients will be MUCH worse. When he says "always fail open circuit" I think he is referring to the case where the fault current blows the gas tube to bits. That doesn't sound like a good situation.Oh, and make sure your gas tube is rated to quench the discharge at peak mains voltage! Otherwise, when it gets triggered, it will maintain a short until it explodes.Jon

author: ocset
author_rank: Super Contributor
timestamp: November 11, 2017, 02:12:07 pm
content: QuoteThanks, this is a different project to the "300V" one.This is a LED driver like as follows...DT3001 controllerhttp://www.seoulsemicon.com/_upload/Goods_Spec/Acrich2-Applicationnote.pdf

author: Gyro
author_rank: Super Contributor
timestamp: November 11, 2017, 03:53:10 pm
content: Thanks, those look like interesting controllers.I notice from the document that their recommended protection networks are either VDR (after the fuse) followed by an RC network or VDR (again after the fuse) followed by a TVS diode on the DC side of the bridge rectifier. To me, these look to be better advice than GDT+VDR.