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An outdoor PIR switch that popped a fuse, and when a new one was put in it glowed orange inside before blowing the new fuse.
This unit is an excellent example of design by someone with no real experience of PCB design involving 120/240V. The one routed anti-tracking slot is almost a token gesture inclusion to distract from all the other places tracks with high voltage difference between them pass close together. Tracking is when moisture or other contaminants cause a carbon track to gradually form across a surface between tracks and pads. Once it reaches a significant level it can start conducting current and flash over.
It's also an excellent demonstration of why prominent branded units often include a thermal fuse in the vicinity of relay contacts to kill the unit if any significant contact arcing occurs.
This is especially important with small relays with low contact gaps. Some loads, especially inductive ones, can cause sustained arcing between open contacts.
In the wrong circumstances, this unit could have caused a fire. Especially if protected by a high value fuse.
The dropper capacitors in these units tend to degrade over time and reduce in value. This one was a 560nF capacitor that had dropped to 400nF. It was probably still enough to do the job, but would have reduced the current available to the circuitry, possibly affecting the relay's operation.
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It's our favorite type of electronics, it's dead electronics, particularly with hot melty bits and a real electronic stink. So this is a passive infrared detector, typical type you might find the garden and it comes the story: hey clive, this pir operated a water feature. The water feature stopped found the mains three amp fuse blue. In the connection unit.

I put a new fuse in looked at pir and i'd lit up in the top left corner. I marked in purple, so it was actually glowing in here arcing. Presumably, it lasted about six seconds in total, obviously, something inside burning uh, which can't see, but on the top, has a melted. Okay, let's explore.

So this is a typical detector. It's got three wires: brown, blue and uh red, usually brown blue, are live and neutral and red is normally switched feed. Let's get just lop these right off and get them out of the way. I shall reduce the size of the unit a bit so after those because i don't think this is getting used again.

It looks as though it's gone smokey. It's also fetching shade of black, which is never terribly convenient for uh, visible things. Oh so the first thing i'm noticing is that normally there's a couple of screws holding the bottom here on. Oh, it is stinking so much you if you this was fitted with smell-o-vision you'd, be getting this right now, because this is stinking.

Oh, this is glued together. Let's get the vise of knowledge in and we'll squeeze it until it pops, so i shall try squeezing the base, not that i'm trying to like keep anything intact because, let's face it, it's dead, oh, that is, that is actually quite tightly together right. I'm going to have to i'm going to. I know what tool to use here.

One moment please pipe grips they're so destructive. Let's put it in here and do a squeeze that's making crunchy noises. These will open most things destructively. I mean: let's face it's not going back together again.

Is it oh that sounded reassuring in a sort of destructive way, let's stab a screwdriver into it, without stabbing a screwdriver into myself. This is not always guaranteed. Try and open this up a little tad more. Oh, this looks promising, so the inside is coated with oh, the smell, the smell is absolutely terrible.

It is stinking no okay, it's the really right. Okay, i have to get you a closer look at this relay one moment. Please right. This is complex uh because the fault could have started in three places, but i think it started in the relay down to the fact that there wasn't enough clearance in this contact.

This is a side view of the relay it's very, very burnt. The side is physically burnt off from the little fluffy fibrous stuff here, i'd say it's probably glass, reinforced plastic. The really has a little contact bounces up and down here, and that is this bit here. It goes up touches the contact here which wasn't in use uh, and it also has another contact which was the normally closed con.

The normally open contact, the one that would actually bridge when it comes down, and it's here and i get the feeling that it's arced between the common contact and the normally closed contact and that arcing has been sustained. Even the relay has possibly been off uh because it's just not been given enough clearance to break the load. There is a snubber network, uh i'll show you the the remnants the snubber network as well. But let's take a look, that's the capacitor.

It was right next to the relay they've, it's very hard to say, which went first, but this one is a little a little adorable little hole in the side where it's obviously shot a flame out. Whether it was this that set fire to the relay, or vice versa, is very hard to say it's one of these things that if the i'd got a verb, one that had just started flickering or going wrong, i would have been able to tell a lot more Uh, but this is uh - this is toast, as you may see, i shall put this out the way. It's also extremely stinky, because that uh plastic is very burnt and crusty right. The circuit board, quick, summary of the circuit board and we'll marvel at their token gesture.

Um, andy tracking slot, there's the token gesture anti-tracking slot. It doesn't really do an awful lot. Let's zoom down this a bit. So here's the front of the circuit board - and here is the back of the circuit board just cause.

You want to trace things out yourself. It's all a bit black down here, things worth of know before we go any further. There is the token gesture and the tracking slot between live and uh. The coil in this instance here plus also the track wise.

It's the live and neutral, but look here, they've actually just run it right. Next to the the neutrals just pass right next to the live, with a very thin track that has done its best to blow up, but the neutral continues along and uh. It's quite interesting. Their use here why they've continued neutral down here, because in reality they could have done it differently, but it's actually complicated by led loads.

That's not clearing things up. Is it let us uh move that out the way, but there oh there's the snubber network by the way, a resistor of unknown value, because it's cremated and the capacitor, which is 22 nanofarad, which is quite low for us, never network. Here's the circuitry a quick summary of the circuitry itself, a capacitive dropper, a bridge racked fire underneath a zener dart, probably 24 volt clamp, smoothing capacitor voltage regulator, feed a biss-0001 generic passive infrared chip, which has the light sensing facility uh and timing with these capacitors and Also, the uh, the actual pass infrared sensor, which just puts a low level signal output that gets amplified by this chip, and it looks for undulations as people walk by and then triggers when it reaches the threshold. It triggers a timer.

Everything is built into this chip by changing component values. You can tweak and adjust your heart's content. The unit turns on the chip turns on what the smoking remnants here with this transistor a 10k resistor in series of the the base of this transistor, and then it switches the coil. This little diode here is across the coil to protect that transistor.

The relay here's the other bit where it could have failed. It's possibly the heat that came down this track. They've er, the other, normally uh closed contact, they've just cut it off the bottom of the relay switch just sat on top of the other side of the circuit board, but this area here has a very obvious sign of tracking. Let me just zoom into this bit, because this is a bit we're looking at now.

I shall make sure i'm still focused on that. This is where all goes wrong with it. Suddenly, it's not in focus, so here is the uh feed it to the middle pin. But that's the common contact of this really and here is the normally open one that then feeds over to the output and there's also that little snubber network with a capacitor from here to here and then the smoke remains a resistor over to there with tracks blown Off and everything so here's the other possibilities that could have the possibilities that could have failed.

We've already had the relay kind of burned up inside. I think that's a prime suspect uh we've already had the uh, the snubber network capacitor could have failed, but it since sears this resistor and that should theoretically have kind of blown clear like a fuse. But now we've got the uh the neutral that was brought over for that snubber network is right next to the switch live, and there is very obvious signs of the pitting and etching into the circuit board. It's been tracking so that, while that was on uh, it may have actually just continued to burn, particularly if humidity got involved, um but then finally, just actually flashed over and when it flashes over.

That's when it will blow the fuse, sometimes it'll just arc and fizz. For ages before doing that, and sometimes you put the fuse in, it will have blown itself clear and it will just start fizzing and arc in the background again until it flashes over later. On that happens, a lot um, so let the snubber network. Let me just zoom back out again and we'll talk about the snubber network.

I shall bring in a notepad. Quite often when you have a relay contact. Let's uh just draw it like this. Quite often, you will have a snubber network which is capacitor and a resistor across that, and the point of that is that uh this would typically be 100 ohm and this would be 100.

Nanofarad is very typical. You actually get dedicated snubber networks. I was just looking for one here lying next to the bench: no, it's not going to be there. It's not going to be there when i need it.

I kind of want it, but right one moment please a snubber network. This one is 220. 9500. 100 ohm.

It's just a capacitor like this inside in series with a resistor all just built into one package, and this is normally wired across contacts and contactors, and here's the idea when this contactor, when this contact opens. Sometimes you get a little bit of an arc occurs between the two contacts and because the capacitor uh has it happily, the higher frequencies it'll actually pass them easier. It effectively provides an alternative path for that, and it takes a lot of the current away and lets that arc extinguish the resistors here to limit the amount of current can flow through the capacitor effect in that way. Also it's to limit having a capacitor across the contacts, because otherwise it can cause pitting.

So it limits the current. What they've actually done? Supposing this is live and it was going out to the load and there was neutral going straight out of the load load, neutral load live. What they've done is they've put that capacitor across there instead and the reason they've done. That is because, if you have a standard snubber network across a contact, switching an led lamp that 100 nano far 195 capacitor will easily pass enough current to keep that lamp glowing or make it start pulsing.

So that's why they can't really do that with a with very sensitive loads like compact, fluorescents and leds. So that's why they've put that there other things worthy of note. Quite often, if you have a circuit board, that's designed properly and it's got the relay on the board to switch a load to an output. You'll often find that there's a little thermal fuse mounted right up.

Next, the relay sometimes actually stuck onto the side of the case. The reason for that is that, when something like this happens, let me just grab the charred remnants of that when something like this happens, and the case starts burning up, because the the contacts are arcing inside or just make a bad connection result in the arcing. This fuse will break the circuit and uh, if that's say, for instance, leading to the common connection in here, then it just kills that part of the circuitry, sometimes it just kills the whole unit completely. When that thermal fuse goes that's when things are properly designed, this design is so perplexing, because it's got it looks like someone's copied a design or just based on a standard design, but their grasp of the vicinity of things like neutral and live is just not there.

They've not had that understanding. Likewise, the really just uh, it just seems so small, a lot of stuff you get from the grey imports from ebay, uh you'll find these relays are not rated even for in the case the uk 240 volts they're only rated for about 125 volts um, and they The current ratings are always ambitious. You know they say things like this. This can switch 10 amps.

In reality, it's not going to switch 10 amps in the case of some of the modules, but interesting, so without actually catching it earlier. I can't tell if the fault originated in the relay, i'm going to say that i think it is arcing the relay or if it's a little snubber capacitor, which has kind of blown out the side here and cracked. So it's obviously been pretty unhappy. But then it's been next to something that was on fire or this tracking down here, but this tracking could have been initiated by the heat from this pin getting red hot uh, causing heat to travel across and burning the actual circuit board itself.

The underside of that coil is pretty well, it's pretty black. The whole lot's been going up, but that's fascinating, interesting stuff. It's all always worth looking at designs like this and how they've failed, because it shows you what can go wrong in this case. Just for that tiny bit of extra space that little thermal fuse, even if they butted the thermal, fuse up the end here and moved the snubber elsewhere.

That could have actually saved this whole thing from happening. But there we go an autopsy on a completely smoked: um password infrared detector.

17 thoughts on “Faulty pir switch meltdown”
  1. Avataaar/Circle Created with python_avatars Chris Robinson says:

    The vise of knowledge should be paired with the hammer of curiosity for the really tough jobs.

  2. Avataaar/Circle Created with python_avatars voltare2amstereo says:

    it's an Active Infrared Switch, nothing passive going in in that anymore

  3. Avataaar/Circle Created with python_avatars Geoffrey Waldo says:

    Always promising when Big Clive has to bring out the Clamp Of Persuasion!

  4. Avataaar/Circle Created with python_avatars Scott Strehlow says:

    Those pipe grips are the best nut crackers you can get.

  5. Avataaar/Circle Created with python_avatars Mark Warburton says:

    It's always very instructive to see failures and sleuth the cause of their demise. Thanks!

  6. Avataaar/Circle Created with python_avatars Simeon Walker says:

    Looks just like one I got from Lidl a few years back. A few months back I had to disconnect it as it started turning the light on and off constantly. Typical tat!

  7. Avataaar/Circle Created with python_avatars Cameradoctor says:

    I've often wondered about heat build-up in the coil of relays that are held active for extended periods – especially in Automotive circuits – what are your thoughts Clive ?

  8. Avataaar/Circle Created with python_avatars No no says:

    The real problem with having such inferior products in the market is they create a demand for cheap goods, which creates pressure on otherwise safe product manufacturers to cut corners. On top of that is the fact that its impossible to tell whether something is safe or unsafe without doing an expert teardown. (Price is a good indicator but insufficient, because nothing stops a disreputable vendor from marking up the unsafe cheap products to the same level as the safe expensive ones.) We are going to have a number tragedies reminiscent of 19th and early 20th century West before this phenomen get fixed.

    Which is why BigClive's channel is a service to humanity, if you think about it!

  9. Avataaar/Circle Created with python_avatars Kevin Hardisty says:

    So close to a decent product. Interesting teardown and analysis, as always

  10. Avataaar/Circle Created with python_avatars Keith Lambell says:

    Maybe we should introduce a "BCA" marking on units that are "Big Clive Approved" !

  11. Avataaar/Circle Created with python_avatars Dave Seddon says:

    mmm – a little more care in the layout and carnage would have been avoided. There are guidelines for track spacings (voltage vs current), always a good idea to adhere to them!

  12. Avataaar/Circle Created with python_avatars Stephen Eyles says:

    Always grateful for someone who suffers that terrible smell and shows us what has happened – thankfully Smellivision is still a long way off!!

  13. Avataaar/Circle Created with python_avatars Gregory Thomas says:

    I try to assume that any relays that are not from reputable sellers are going to be no more than 12.5% of the rated value…I used to assume 25% but to my detriment ๐Ÿ™

  14. Avataaar/Circle Created with python_avatars Wim Widdershins says:

    There's a security light that pops and fizzes on the wall outside my unit.

  15. Avataaar/Circle Created with python_avatars Grant Rennie says:

    Oh I've seen an expensive (overpriced) fancy brand subwoofer speaker (ยฃ1500) totally burned out by a flamed relay, whole lot burned to nothing ๐Ÿ™„

  16. Avataaar/Circle Created with python_avatars Grant Rennie says:

    Thanks for another great video Clive ๐Ÿ™‚

  17. Avataaar/Circle Created with python_avatars AMDRADEONRUBY says:

    Yeah this system is well used pretty reliable most of the time.

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