Can I replace a fusible mains input resistor with a standard resistor and fuse? - Page 1
Forum section: electronics / repair

author: theleakydiode
author_rank: Regular Contributor
timestamp: November 20, 2022, 08:30:14 pm
content: Got a small switching wallwart 5.5v 800mA for a Karcher window vac and the inrush limiting 10 ohm 2w fusible resistor has gone open due to what looks like the PCB spark gap firing (near the blue neutral wire in the image).Can't seem to get any replacement matching fusible resistors cheaply in the UK without ordering a big reel of them so can I just use a regular 10 ohm 2 watt resistor in series with a glass fuse? Mains is 240v and the PSU draws 200mA according to the nameplate, the resistor doubles as the mains fuse and I do need there to be some fusable element due to UK ring mains.

author: bdunham7
author_rank: Super Contributor
timestamp: November 20, 2022, 08:55:04 pm
content: You probably could putjusta fuse in there. Perhaps a 315mA slo-blow wire-leaded version would be handy.However, a better solution would be a new fusible resistor, install a MOV where it says "MOV" and clean up/eliminate the spark gap. You could use a 3.3R 1W resistor if they are easier to get since it says so right on the board, or you can find your 10R 2W fusible WW at Farnell.https://uk.farnell.com/bourns/fw20a10r0ja/res-10r-5-2w-axial-wirewound/dp/2299885

author: wraper
author_rank: Unknown
timestamp: November 20, 2022, 09:01:39 pm
content: Quote from: bdunham7 on November 20, 2022, 08:55:04 pmNope, unless you want to destroy a rectifier and capacitors. There will be no inrush current limit beyond fuse and L1 resistance, and ESR of said capacitors.QuoteEven if you install a MOV, there is no point to eliminate a spark gap.

author: theleakydiode
author_rank: Regular Contributor
timestamp: November 20, 2022, 09:07:17 pm
content: Quote from: bdunham7 on November 20, 2022, 08:55:04 pmThanks. Problem with places like farnell is the delivery costs, £12 for a 26p part. I don't do enough electronics these days for big orders either, only other option is ebay. Regarding the 3.3 ohm 1 watt option, could that be for 120v? The adaptor is marked 100-240v however.Which type of MOV would you suggest?Quote from: wraper on November 20, 2022, 09:01:39 pmSo would a fuse + regular resistor in series be good enough?

author: bdunham7
author_rank: Super Contributor
timestamp: November 20, 2022, 10:30:57 pm
content: Quote from: wraper on November 20, 2022, 09:01:39 pmI understand all that but think that it would likely work just fine anyhow.QuoteThe point is to remedy its current degraded condition, whether by restoring it to its original glory or just widening it to the point where it doesn't matter.

author: bdunham7
author_rank: Super Contributor
timestamp: November 20, 2022, 10:37:45 pm
content: Quote from: theleakydiode on November 20, 2022, 09:07:17 pmFuse + resistor is fine but you don't want to make things worse if there isn't physical space to put them in neatly and properly. First choice IMO would be replace the original resistor with the same thing.MOV, whatever fits and is rated for usage at 250VAC or a bit more.https://uk.farnell.com/littelfuse/v275la20cp/metal-oxide-varistor/dp/1703839

author: theleakydiode
author_rank: Regular Contributor
timestamp: November 21, 2022, 02:02:04 am
content: Just ordered one of these from ebayhttps://www.ebay.co.uk/itm/360623557353I didn't want to resort to ebay but shipping costs and minimum order quantities have become a problem in recent years with the legit distributors.

author: Gyro
author_rank: Super Contributor
timestamp: November 21, 2022, 09:48:17 pm
content: It seems a bit of an assumption that the spark gap flashed over and took out the fusible resistor. You didn't say anything about what checks you've carried out. Have you checked for shorts after the resistor? The bridge rectifier might be shorted for instance, or maybe the switching FET.It would be a shame if your new resistor immediately joined its predecessor.

author: lowimpedance
author_rank: Super Contributor
timestamp: November 21, 2022, 10:37:00 pm
content: Quote from: Gyro on November 21, 2022, 09:48:17 pmIt looks like the spark gap in parallel with the MOV was supposed to protect from surges, shame they cheeped out on the MOV.I agree, definitely check the bridge etc.

author: theleakydiode
author_rank: Regular Contributor
timestamp: November 22, 2022, 08:59:27 pm
content: Quote from: Gyro on November 21, 2022, 09:48:17 pmQuote from: lowimpedance on November 21, 2022, 10:37:00 pmEverything seems to check ok, actually there was a blob of solder rattling around in the case so maybe that caused it. Also the room it lives in is subject to large daily temperature swings and condensation at night so that could of caused the spark gap to fire too.I've found some MOVs but am unsure of which is safe to use across 240VAC UK mains (via that fusible resistor), "07D361K" "07D431K" "471D07" "7D471K", which would be best suited to this circuit? These were the only ones I have that are small enough to fit on the PCB.I'm leaning toward 07D431K since the mains can reach 250VAC here on occasion, but maybe 471 would be better? Also these are pulled from old equipment and I understand MOVs degrade with age.

author: bdunham7
author_rank: Super Contributor
timestamp: November 22, 2022, 09:30:53 pm
content: There's no need to cut it close--the 300 or 320VAC rated parts are fine to use. Even higher is probably OK.

author: theleakydiode
author_rank: Regular Contributor
timestamp: November 22, 2022, 09:42:59 pm
content: Thanks, I'll use the 471 then. Also what's ebay like for specific resistors? This is the one I orderedhttps://www.ebay.co.uk/itm/360623557353and it's taking about a week for it to arrive so I assume it's one of those bulk order sellers who'll order from RS, Farnell etc once an order has been placed with them (minimum order quantity and shipping were too expensive for me to order direct).This has implications for the MOV as if the fusible resistor is fake then the MOV will basically be across the mains with no legit fuse protection.

author: Gyro
author_rank: Super Contributor
timestamp: November 22, 2022, 10:33:08 pm
content: Yes, pcarena-bristol is one of those UK sellers (there are a few) who buy stuff from RS and Farnell (not sure about Digikey) and then part them out for a handsome profit. The lead time includes the distributor delivery time. As in your situation, they are sometimes worth the large mark-up if you just want a single item rather than being able to reach the free delivery threshold.Actually I sometimes find them useful for finding out whether RS or Farnell actually stock a item, rather than using their website search functions!

author: theleakydiode
author_rank: Regular Contributor
timestamp: November 22, 2022, 10:42:48 pm
content: Quote from: Gyro on November 22, 2022, 10:33:08 pmThat makes sense then! Yeah I agree the markup is insane for a single resistor, but the adaptor is for a device we use everyday during winter and it's still cheaper than buying a new charger. Remember the days when RS let you buy single items with free delivery no matter how little the total was? Shame they came to an end.

author: Gyro
author_rank: Super Contributor
timestamp: November 22, 2022, 10:45:10 pm
content: Yes, it was reliable next day delivery too! I guess the writing was on the wall.

author: theleakydiode
author_rank: Regular Contributor
timestamp: November 22, 2022, 10:56:11 pm
content: YepAlso single items don't appear as common now either, everything seems to have higher minimum order quantities which further sucks for the occasional hobbyist or beginner. Years ago you could order a bunch of single capacitors for a recap job totalling a couple of pounds/bucks including delivery, now I'll have to use ebay bulk resellers.

author: bdunham7
author_rank: Super Contributor
timestamp: November 22, 2022, 11:00:47 pm
content: Quote from: theleakydiode on November 22, 2022, 09:42:59 pmThe 10R still limits the peaks and will eventually fuse at some point, although if isn't fusible or flameproof that process may be messy. But those MOVs are used across the line without any resistor or fuse (or at least they were--fusible versions are now common) so I wouldn't worry too much. But I would want a proper fusible one in there so hopefully your seller is reliable, especially for that price.

author: Gyro
author_rank: Super Contributor
timestamp: November 22, 2022, 11:04:22 pm
content: Quote from: theleakydiode on November 22, 2022, 10:56:11 pmCPC are about the lowest threshold at (only) £20. They're an offshoot (acquisition) of Farnell but often have lower pricing for the same item. Their component selection is a lot more limited though.

author: theleakydiode
author_rank: Regular Contributor
timestamp: November 26, 2022, 08:48:41 pm
content: Update: It's alive, I also populated the PCB MOV footprint with a 471D07.

author: Gyro
author_rank: Super Contributor
timestamp: November 26, 2022, 10:57:19 pm
content: Thanks for reporting back. Too many don't.