Blew my Hakko FX-951 by touching live mains with tip - Page 1
Forum section: electronics / repair

author: rszimm
author_rank: Newbie
timestamp: January 05, 2023, 12:06:59 am
content: So BANG! Touched my tip to 120V live. It blew the circuit breaker. When I regained my composure, I started up my iron...and it worked ?!? However the tip to ground resistance was infinite. I opened the case and saw that there was a lot of black around a surface mount R52 (which had ceased to exist). Anyone have any idea what that value should be?Incidentally, shouldn't the tip to ground resistance be something like 20M to prevent something like this from happening in the first place?

author: rszimm
author_rank: Newbie
timestamp: January 05, 2023, 12:11:18 am
content: Just in case you're curious about where the pads go on that R52, I probed around and..sure enough...the left pad connects to the tip, and the right pad connects to that green grounding wire. So it's likely that someone with an FX-951 could tell me the value of R52 by simply running a resistance check between their tip and ground.

author: james_s
author_rank: Super Contributor
timestamp: January 05, 2023, 01:12:55 am
content: According to Hakko, the tip to ground resistance should be 2R or less. A flameproof resistor of 1-2 ohms is probably a good choice.https://hakko.com.sg/blogs/tune-in-with-hakko/measuring-tip-to-ground-resistance

author: rszimm
author_rank: Newbie
timestamp: January 05, 2023, 01:26:45 am
content: 2ohms?! Can that possibly be right?? I see that it says that in the document, butreally? Heck, if I hit a capacitor with even just 5V on it that's 12.5 watts of power sinking through that resistor. No small little SMD resistor is going to stand up to that... Why even bother with a resistor? Just put a fuse there (or a jumper)...

author: jonpaul
author_rank: Super Contributor
timestamp: January 05, 2023, 02:46:17 am
content: Regulations for safety required 25 ohms or less from châssis, tip etc to earth ground.Résistance 2 oms is sacrificial, like a fuse for exactly the situation you have.jon

author: james_s
author_rank: Super Contributor
timestamp: January 05, 2023, 02:54:06 am
content: It's pretty common to use a low ohm fusible resistor as a fuse. In general you shouldn't be soldering anything that is live, even a charged capacitor should have no potential relative to ground if the device you are soldering on is unplugged.

author: Damianos
author_rank: Frequent Contributor
timestamp: January 05, 2023, 03:44:25 am
content: Quote from: rszimm on January 05, 2023, 12:06:59 amThink a little about it. What would happened if it was a high value resistor? Then you would ask (if you had survived!): why they don't make the solder wire of an insulating material? The resistor is there and has these characteristics to prevent more destructive matters and to "inform you" that something wrong happened.

author: rszimm
author_rank: Newbie
timestamp: January 05, 2023, 05:14:49 am
content: Quote from: jonpaul on January 05, 2023, 02:46:17 amEveryone has been super helpful and informative. Thanks. I do have to wonder about the practice of using a low value resistor instead of a fuse. A fuse would blow and be easy to replace as it’s designed to do so. A resistor, especially an SMD, takes the pads with it (as it did in this case).

author: james_s
author_rank: Super Contributor
timestamp: January 05, 2023, 08:33:23 am
content: I think the thought is if you blow it up that badly, you throw it away and buy a new one. Maybe I've been lucky but in ~30 years of soldering I have never hit a live mains wire with a soldering iron.A fuse may be more likely to blow (or even just fail) silently resulting in no earth ground so perhaps that was a consideration. A fuse also would have cost more and again this is not something that is ever expected to be blown.

author: Damianos
author_rank: Frequent Contributor
timestamp: January 05, 2023, 12:28:27 pm
content: Quote from: rszimm on January 05, 2023, 05:14:49 amA fuse will disconnect the ground before the trip of the breaker, leaving it connected to mains... The resistor there is also keeping the peak current in somewhat controlled value, so not to explode the entire device. A fuse is blown without producing sound and smell!If you decide to repair it by yourself, keep in mind that, except the resistor, there may be other things damaged, that affect the safety of the device.Edit: As you mentioned the case of a charged capacitor, a 2Ohm resistance will discharge a 10mF capacitor in less than a millisecond, so the resistor will be safe, giving you a spark to notify the bad matter.

author: rszimm
author_rank: Newbie
timestamp: January 05, 2023, 05:25:28 pm
content: Quote from: james_s on January 05, 2023, 08:33:23 amI'd like to think I've never been this stupid either, but for a large part of my soldering career I used just an old handheld weller soldering iron with a two prong plug and thus zero grounding on the tip, so even if I had touched mains, nothing would have happened. So maybe I have been this dumb before (?).OK, so I contacted Hakko tech support. Apparently there's a 1M resistor for ESD purposes and a zero ohm resistor, which is the one I blew. Seems the 1M is intact. I asked why they don't just make it a fuse, and was told that it's a UL requirement. My best guess is that it's what damianos mentioned, that a fuse would likely blow before the circuit breaker and leave the dangerous situation unresolved.Thanks everyone for your help.EDIT: For the record, Hakko support is amazingly quick and helpful. I was figuring I'd get a "we don't give out that information....just buy a new station" type response, but instead got a guy who took apart an iron in the lab to figure out what the values were in those locations. What company does that?

author: james_s
author_rank: Super Contributor
timestamp: January 05, 2023, 05:47:50 pm
content: That is impressive, though I'm surprised they had to take an iron apart, you'd think the company would have internal schematics for their own products but maybe that's all in their head office in Japan.

author: rszimm
author_rank: Newbie
timestamp: January 05, 2023, 06:00:22 pm
content: Yeah, probably one of those "it'll take me 30 minutes to identify the right board and then find the right rev of the schematic, or I could spend 5 minutes and measure the things directly"

author: factory
author_rank: Super Contributor
timestamp: January 05, 2023, 06:30:46 pm
content: Both the older Antex & Weller soldering station at work used double-insulated transformers and two core mains cord, some had optional connectors for ESD grounding, but not all. The Pace desoldering base station had a 1M resistor for the iron, this is marked on the front panel, it's replacement (if it ever gets ordered) will be a Metcal, pretty sure the datasheet said it was grounded to the iron.You shouldn't be working on powered equipment, but there are occasions where you need to be aware of grounding, i.e the back-up batteries in some equipment, you want to avoid grounding the battery through the solder or the iron, a few on here have cleared cal data on some equipment by mistake, when fitting replacements and accidentality grounding it.David

author: rszimm
author_rank: Newbie
timestamp: January 05, 2023, 06:50:43 pm
content: When I say "an old weller" I mean something like the iron in the top of this listing:https://www.ebay.com/itm/175088201324I likely bought it at Radio Shack around 1985. When dealing with ESD sensitive components I had a little clip that went through a 1M resistor to one of those snap button connectors that usually go to grounding wrist straps. Worked great for decades, but eventually I decided I wanted something that didn't take 15 minutes to warm up.

author: Miti
author_rank: Super Contributor
timestamp: January 07, 2023, 01:29:37 am
content: I have a Hakko FX-951. I know the question has been answered but just to add a picture to the thread for reference.

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: January 07, 2023, 05:03:57 am
content: I have a different board spin, those resistors are further away from the board edge on mine and labelled R15/R20.Can't really be zero ohms, can they

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: January 07, 2023, 05:06:00 am
content: Quote from: rszimm on January 05, 2023, 05:25:28 pmYou wouldn't have noticed anything making contact with the iron, but you would have had a pretty big surprise when you made contact with the solder.