Just Replace the Fuse and You're Good!! - Page 1
Forum section: electronics / general

author: engrguy42
author_rank: Frequent Contributor
timestamp: May 21, 2020, 07:50:32 pm
content: Ok, so y'know how the dumbest thing anyone can say in the electrical world is "Oh, no wonder!! The fuse blew!! Just replace it and you're good to go!!"?(Of course the reason is that fuses don't blow all by themselves, they need an overcurrent first.)Anyway, I had a power meter I bought a while ago (the kind you plug into the wall outlet to measure V, I, Watts, etc.). And when I received it it was DOA. Nothing on the LCD display when I plugged it in. So I was gonna send it back, but decided with all this covid crap I'd hang on to it.So I took it apart, and first thing I tested was the 0.2 A fuse that feeds the measuring circuit. Son of a gun, it was open!!. So I'm thinking there's a shorted component from the factory, so I open it up and look around. Nothing obvious, so I give it 15 volts from my waveform generator and hook up my scope and it seems to be doing okay.So after some head scratching I figure, "what the heck, I'll just plug it into the wall, and instead of the fuse I'll run some wires to my ammeter and see what happens. I'll just quickly touch the leads to the wires and measure the current."And what do you know, it read only 40mA. And the LCD display came on, and was reading 121 volts.Maybe the fuse broke in shipping? Or it was bad from the start?This is all, like, weird and stuff.....

author: Messtechniker
author_rank: Frequent Contributor
timestamp: May 21, 2020, 08:13:12 pm
content: Of course fuses sometimes blow for no apparent reason.That's why I always have suitable spare fuses readywhen doing field recordings, for example.

author: engrguy42
author_rank: Frequent Contributor
timestamp: May 21, 2020, 08:15:07 pm
content: Quote from: Messtechniker on May 21, 2020, 08:13:12 pmYou mean, like, magic and stuff??

author: Gregg
author_rank: Super Contributor
timestamp: May 21, 2020, 08:19:25 pm
content: Note the very short lead on one side of the fuse. It probably weakened the fuse link when it was soldered and rattled open in shipping.

author: engrguy42
author_rank: Frequent Contributor
timestamp: May 21, 2020, 08:24:23 pm
content: The weird thing is it came in one of those formed, clear plastic covers. I'd think that would give a lot of protection inside the shipping box.I dunno. Maybe it's just one of those things.

author: Alti
author_rank: Frequent Contributor
timestamp: May 21, 2020, 08:26:39 pm
content: Quote from: engrguy42 on May 21, 2020, 07:50:32 pmIt might be this is a capacitive dropper, has a (1uF?) capacitor in series with resistor, fuse, bridge, zener and then tied to mains voltage. When you plug it near peak of sin(x), the current is only limited by drop resistor and ESR of a capacitor, resistance of fuse and inductance of mains wiring.Put a jumper over the fuse, take a scope with current probe (small shunt will do), sqrt(2)*110VDCand measure the integral of i2t (square of current * time) on inrush. Fast 0.2A fuses have melting i2t of around 0.02 A2s (Littelfuse 0217.200). If your i2t is over 50% of that value then this is not a coincidence.

author: engrguy42
author_rank: Frequent Contributor
timestamp: May 21, 2020, 08:46:04 pm
content: Quote from: Alti on May 21, 2020, 08:26:39 pmThanks. Yeah this is definitely a capacitive dropper, so I think you have a good point. Though I'm kinda surprised it would take the fuse out. I'll just replace it with a removable bayonnet one mounted to the case in case this happens again.I was just looking at another capacitive dropper supply for something a month or so ago, though I can't recall what it was......It sucks getting old

author: engrguy42
author_rank: Frequent Contributor
timestamp: May 21, 2020, 09:31:40 pm
content: I decided to do a quick reverse engineering of this thing in KiCad, and you can see the capacitive dropper. The bottom right 6 pin connector has a ribbon cable to the front panel, but I'm too lazy to reverse engineer that too.

author: SparkyFX
author_rank: Frequent Contributor
timestamp: May 21, 2020, 09:34:31 pm
content: Quote from: engrguy42 on May 21, 2020, 07:50:32 pmShipping means additional vibration, so yeah, could be. We don't know what their QA does.I can't see in the photo where or how the fuse broke, it might help finding if it mechanically or electrically broke.

author: engrguy42
author_rank: Frequent Contributor
timestamp: May 21, 2020, 09:56:49 pm
content: Quote from: SparkyFX on May 21, 2020, 09:34:31 pmYeah, it's really tough to tell. The freakin' thing is so small. And the glass is kinda thick so there's a lot of reflections when I look for the element inside. But I do see what appears to be an intact element that's like a human hair. Or less. But certainly nothing like a typical blown fuse.I'm guessing it's a mechanical break inside. I suppose I could snap the glass and see for sure......

author: engrguy42
author_rank: Frequent Contributor
timestamp: May 21, 2020, 10:07:35 pm
content: Wow, these smartphone cameras are amazing...I zoomed in on a 4k image from my phone, and you can see the apparently intact element running down the middle.

author: engrguy42
author_rank: Frequent Contributor
timestamp: May 22, 2020, 12:20:55 am
content: Quote from: Alti on May 21, 2020, 08:26:39 pmAlti, you sure about the inrush with the capacitive dropper? You're right, the capacitor is 1uF (I show it as 0.1uF), but I did some simulations in LTSpice with that plus the 100ohm resistor in series, and even with the source at peak of the sine wave I just show a 60mA constant peak current, no big inital peak. And the fuse is 200mA.

author: engrguy42
author_rank: Frequent Contributor
timestamp: May 22, 2020, 09:48:28 am
content: BTW, here's the screenshot of my LTSpice simulation of the circuit. Basically 60Hz AC, 170V peak (120RMS), applied to the capacitive dropper via a time controlled switch. The source AC (yellow trace) is shifted 90 degrees to ensure a voltage peak on closing. I even included some source inductance.As you can see there's a nice clean current with a 60mA peak. Now if you increase the C greatly you'll get some initial offset in current due to the LC stuff going on. But in this case I'm not seeing the big inrush.

author: Alti
author_rank: Frequent Contributor
timestamp: May 22, 2020, 10:24:42 am
content: Quote from: engrguy42 on May 22, 2020, 12:20:55 amYes, the discharged capacitor across power line is a dead short at t=0. That is why there has to be inrush current limiting resistor or adequate ESR in a cap. Otherwise inrush overwhelms a fuse. So it should show you 156V/100R=1.56A peak and a bit more if you include 10% mains voltage tolerance.Now, 1uF cap in series with 100R resistor connected to 110V+10% AC gives i2t of around 4e-4A2s and the fuse withstands 2e-2A2s so with 100R resistor it is 1/50 of a kill. So 200mA is just too big for 100R, lets hope it protects electronics adequately.

author: engrguy42
author_rank: Frequent Contributor
timestamp: May 22, 2020, 10:30:01 am
content: Alti, you're applying a DC to the capacitive dropper. Why? We're talking about AC.

author: Electro Detective
author_rank: Super Contributor
timestamp: May 22, 2020, 11:50:26 am
content: OP is another victim of.. FAKE FUSES!

author: engrguy42
author_rank: Frequent Contributor
timestamp: May 22, 2020, 11:57:28 am
content: Quote from: Electro Detective on May 22, 2020, 11:50:26 amAHA!!! Maybe you're right !!! Maybe the intact element I saw inside the fuse, which looks like a human hair, REALLY IS A HUMAN HAIR!!!

author: Electro Detective
author_rank: Super Contributor
timestamp: May 22, 2020, 12:09:45 pm
content: Fuse links all finished on the TooHungLow assembly line,is what can do if no more, and ship waiting at port?"Everybodies come here, some hair plz to finish this order.."()

author: engrguy42
author_rank: Frequent Contributor
timestamp: May 22, 2020, 12:10:24 pm
content: Another data point...I have a second, identical unit they sent me when I told them the one I received was DOA (gotta love that Amazon...it arrived the next day). And it works fine. So I tried plugging and unplugging it into the wall like 30 times to see if there was some inrush current that would blow the fuse. So far, nothing. And I'm sure it hit it at a voltage peak at least one of those times...

author: engrguy42
author_rank: Frequent Contributor
timestamp: May 22, 2020, 12:13:45 pm
content: Quote from: Electro Detective on May 22, 2020, 12:09:45 pmActually this one was made in Australia.

author: Alti
author_rank: Frequent Contributor
timestamp: May 22, 2020, 12:23:02 pm
content: Quote from: engrguy42 on May 22, 2020, 10:30:01 amTo be precise, I apply a step function. This is 0V in the past and 110VDC*sqrt2 afterwards. This is no different than sticking your meter at peak of sin(x) to the mains receptacle. If you want to, you can use sin(x) 60Hz, starting at peak but you won't see any perceivable difference of inrush during those 1.5ms

author: engrguy42
author_rank: Frequent Contributor
timestamp: May 22, 2020, 12:41:43 pm
content: Quote from: Alti on May 22, 2020, 12:23:02 pmAlti, yeah, you're right. This freakin' LTSpice was assuming the capacitor starts off fully chargedSo I added a shunt resistor to discharge it before applying the AC, and yeah, there's a super short current pulse (< 1mSec) to charge the capacitor. Maybe that would be enough? Though it seems in this case it was more of a mechanical issue. But that's interesting, I never realized it might be an issue. Thanks.EDIT:BTW, the correct way to set initial conditions on the capacitor is to CTRL-right click on the capacitor and in the "SpiceLine" cell set the value to "IC=0".

author: Alti
author_rank: Frequent Contributor
timestamp: May 22, 2020, 05:16:32 pm
content: Quote from: engrguy42 on May 22, 2020, 12:41:43 pmNo, it is not enough because even super-fast LRC Littelfuse would have survived 50x higher abuse (once). Whatever 200mA fuses you are going to get, these can only have higher i2t rating. As for mechanical failure - I really doubt it. It might be this was just damaged during production although it is hard to believe they do not test fuses for continuity.If you are interested to investigate further then I can recommend non-destructive fuse testing. You cannot check i2t this way but if they messed up and it was rebadged 32mA then this should be easy to show. Just gently crush the glass, get the wire out and test it.Just a side-note: a well designed gear should also consider a scenario when you unplug device near peak (+156V) and, with cap fully charged, plug it back 8.33ms/2 later at negative peak -156V. Then the current shuld double and i2t should quadruple, reaching 1.6e-3A2s which is still much below 2e-2A2s.

author: engrguy42
author_rank: Frequent Contributor
timestamp: May 22, 2020, 05:29:26 pm
content: Yeah, I agree it strange that they didn't seem to even plug the thing in prior to shipping to see if the LCD comes up. But anyway, it is what it is. I'll just install a bayonnet removable fuse so if the fuse blows in the future I'll just replace it.Regarding whether the fuse might blow in the future due to this inrush thing I think is almost irrelevant now since I can easily replace it. And these things have been on the market for years so I'd assume if it was happening a lot everyone would be complaining and they'd just re-engineer with a different fuse.I'll just mark this one as a bad mechanical connection in the fuse upon receipt and that's about it. Heck, it's only a $20 device and I have 2 of them, so no big deal if one fails. We can hypothetical-ize for the next 2 months about what could happen I suppose. And, of course, I can't do non-destructive testing on the fuse since it's broken/open inside.But thanks, I learned some stuff today.