Resistor insulation problem - Page 1
Forum section: electronics / general

author: skjafar
author_rank: Contributor
timestamp: March 17, 2013, 10:23:21 am
content: Two weeks ago I was working on an 800V power supply when it suddenly started burning input fuses. It would directly burn the fuse just as i connect it to power.Of course the usual diagnostics to checking shorts, DMM checks and then general components checks, no good, then I went and checked all the components (not so many) still all were looking ok. This was when I was really getting frustrated, it was an old linear regulator, so what I did was connect an external DC power to the regulation circuit with current limit to see if I can check the problem live, the bloody thing was working ok, of course I was working on 20V and it originally has an 850V input, then one of my colleagues suggested that it might be insulation problems, I tried to check it without power I was not able to, then I took the gamble of having the board directly in my view then plugging it in, I saw a small spark, it was in a middle point between a transistor and a resistor, so I instantly thought it might be sparking between the leads, nope, after actually having to remove both the transistor and the resistor I found out that the insulation layer above the resistive film in the resistor has decayed in an area as big as a hair pin, and this has happened only on the bottom side of the resistor, with the ground plain underneath all the components 850V on the resistor was enough to create a spark there.Simple problem of changing the resistortook one whole day

author: David_AVD
author_rank: Super Contributor
timestamp: March 17, 2013, 11:47:02 am
content: Sounds like a clearance issue. I don't think I'd have 850V on a resistor that was pressed up against the 0V plane.

author: G7PSK
author_rank: Super Contributor
timestamp: March 17, 2013, 12:21:40 pm
content: I would have put a sleeve onto the resistor as the insulation cover on the board would be damaged as well. The insulation on resistors is not much more than paint and should not be relied on at high voltages.

author: skjafar
author_rank: Contributor
timestamp: March 17, 2013, 12:27:13 pm
content: Quote from: G7PSK on March 17, 2013, 12:21:40 pmWhat I did is raise the resistor off the board. Good idea though.

author: Len
author_rank: Frequent Contributor
timestamp: March 17, 2013, 03:03:37 pm
content: You need to change your sig now.

author: skjafar
author_rank: Contributor
timestamp: March 17, 2013, 04:58:46 pm
content: Quote from: Len on March 17, 2013, 03:03:37 pmIt'sMy Signature.So I don't care what you think.

author: Balaur
author_rank: Unknown
timestamp: March 17, 2013, 06:50:05 pm
content: Quote from: skjafar on March 17, 2013, 04:58:46 pmLen was joking. Based on this thread's subject, it looks like you also need to check resistors in faulty test gear.

author: emc2
author_rank: Newbie
timestamp: March 18, 2013, 01:54:34 am
content: Quote from: Balaur on March 17, 2013, 06:50:05 pmwhat i was thinking

author: skjafar
author_rank: Contributor
timestamp: March 18, 2013, 05:13:28 am
content: Quote from: Balaur on March 17, 2013, 06:50:05 pmIf it's a joke then I apologize for coming too harsh.

author: David_AVD
author_rank: Super Contributor
timestamp: March 18, 2013, 08:02:26 am
content: Ahh... the difficulties of Internet humour....

author: larry42
author_rank: Regular Contributor
timestamp: March 18, 2013, 11:43:24 am
content: without a schematic it's hard to envisage it completely, but you know that resistors have a max. voltage rating that is independent of their power and resistance rating, right? Special resistors are required for 300V operation. I know, I ordered ones for 750V AC.

author: David_AVD
author_rank: Super Contributor
timestamp: March 18, 2013, 12:34:26 pm
content: The working voltage (from end to end) is also a different parameter to the insulation (coating) rating too I believe.

author: Len
author_rank: Frequent Contributor
timestamp: March 18, 2013, 05:28:30 pm
content: Quote from: David_AVD on March 18, 2013, 08:02:26 amYes, I've added a smiley which I should have put there.Sorry if I gave offense!

author: SeanB
author_rank: Super Contributor
timestamp: March 18, 2013, 07:30:54 pm
content: The funny thing as well is that the common white ceramic boat style resistor actually is not an insulated resistor ( strange that, it is in a ceramic boat and held in with cement) but is actually rated as an exposed element device. This is because it can run so hot ( bright cherry red on overload no problem) that the ceramic gets conductive, not an issue with a sub 1k unit, but enough to cause problems with leakage current to adjacent conductive substances touching them. you have to go to the metal clad ones to get a proper case to substrate insulation rating.

author: skjafar
author_rank: Contributor
timestamp: March 18, 2013, 08:24:48 pm
content: Quote from: Len on March 18, 2013, 05:28:30 pmNo harm done mate, I am new in the forum, maybe other guys here know you more so they knew what you meant.Now we can be friends

author: David
author_rank: Frequent Contributor
timestamp: March 18, 2013, 09:57:07 pm
content: I remember working on a similar problem with some resistors on a power supply. The insulation was changed by the manufacturer without telling anybody and they had "pulled it back" from the leads which caused arcing. Got to play with a flash tester which was quite fun though!