Please help me identify this component

[freman]

Hi,

One of my neighbors had the remote for his remote for his snow machine let out the magic smoke.

It looks like this component got a little hot and bothered, the charcoal mark is kinda a give away even if half the case wasn't missing.

But what is it?

It's coming off the +v from the full bridge rectifier </electroboom> and runs to one leg of the coil for the relay and a bunch of other components (it's clearly part of the dropper/power supply)

It looks like no resistor I've seen (it only has 3 "bands" that I can see, black on each end and a white one towards one end) I'm wondering if maybe an inductor of some variety? It's a dual footprint component too, there's pads and holes for a smaller one.

[DavidAlfa]

Just a resistor, but if it burned like that, it's probably a consequence, something else went wrong and overloaded it.
Post pictures of the bottom to see the traces.

[freman]

Except for some rather expected heat damage the bottom looks pretty good, I was going to check the caps and diodes, the electrolytics look ok but there's some ceramic caps on the bottom, tho I would expect them to show some signs of getting hot as too.

a bit of bat of the napkin math says this is a 5v supply with the 820nf capacitor (Also, I cheated, the chip that hasn't had it's markings removed is a syn590r and it's a 3-5v chip). I could try booting everything beyond that resistor on my bench supply with the current limiter turned on and grab my thermal camera

I've got no way of knowing what the rating on that resistor was meant to be tho.

Edit:
ok, my napkin math was wrong, there's a tiny little surface mount 5v regulator on there for the syn590r
I ran the thing at 8v (looks like 12v is what it wants but 8 works) and there's no issues, everything past that resistor works, nothing but the regulator gets warm and the regulator is just being a regulator it's only a few degrees above ambient. With the relay triggered it's pulling 20ma.

So, a 2 watt resistor, I wonder what value... any guesses?

[Swainster]

For a capacitive dropper supply like this, that burnt component would typically be something like a fusible resistor in the 100 ohms range. The capacitor provides current limiting so the only ways that I can think of to cook the resistor would be:
1) Failure of said resistor - if its resistance goes up (under specc'd?) then it will dissipate more power, which could inturn cause its resistance to go up some more, leading to it cooking itself
2) Short failure of the big film cap
3) Short failure of the bleed resistor across the big film cap.

I dont think anything after the dropper (i.e. in the "low voltage" part of the circuit) can cause a significant increase in the power dissipated in the resistor.

BTW, the relay coil is 12V

[freman]

Thanks, I figured the relay coil out earlier but I didn't spot the tiny regulator for the 5v IC so I couldn't work out wtf.

I've tested the LV side it's happy as larry,

Testing in circuit the film cap is pretty close to on the money, reads 839nf
The bleed resistor is reading 1 meg ohm (I misread the brown bar as a discoloured purple bar, it's definitely brown in better lighting.
The bridge rectifier before the dead resistor checks out too

So maybe just an unlucky fusible resistor? beats me otherwise.

Go find myself a random fusible resistor? Maybe replace the film cap out of an abundance of caution?

[mikerj]

Testing in circuit the film cap is pretty close to on the money, reads 839nf

How about breakdown voltage?  It may be fine at a volt or two but fail under line voltage.

[freman]

How about breakdown voltage?  It may be fine at a volt or two but fail under line voltage.

Absolutely, I'm just going to replace it to be safe.

[DavidAlfa]

Is D1 ok? Try injecting 10V (To avoid gettign too close to the zener voltage) while measuring the current.
This diode and the nearby resistor have suspicious black spots:

[freman]

Is D1 ok? Try injecting 10V (To avoid gettign too close to the zener voltage) while measuring the current.
This diode and the nearby resistor have suspicious black spots:

Took it all the way up to 12v just because, when nothing is happening it uses 0ma, when I trigger the relay it uses 5ma.

I think the black spot is a combination of the black spot to indicate polarity, the glue, and poor lighting.

Also, the soldering on this is atrocious.

[DavidAlfa]

But is the circuit working with external power?

The problem with this type of PSU is the resistor and the zener will take all the excess power.
Ideally you must adjust it so when the circuit is consuming the most the zener does close to nothing, but sometimes it's oversized and the heat will slowly cook things up.

820nF will have a impedance of 3.9K (50Hz), so 240V/3.9K=61mA, which seems a lot if the highest consumption is 5mA.

I'd try lower capacitor values:
220nF: 15mA
330nF: 24mA
470nF: 35mA


About the burned resistor, try to measure it, might give some hints of its original value (getting ~half of its nominal value):

[wasedadoc]

That type of PSU is not universal 100-240 volt.  Check that it really is intended for use on Australian 230 volts rather than North America 110.

[DavidAlfa]

Thought the same but forgot later on.
How long did it last?
I mean, if it was designed for 127V, it would live for very short time at 240V.
But if it worked for months/years, then definitely was not the problem.

[freman]

Thank you all for your tips, especially the one to probe the resistor - I will be trying that in the morning.

This is actually the "rf remote" for one of those fog/bubble/snow blower machines. My neighbor has melted his way through the wired version already too.
I have the same machine and it's not having this problem so far (I guess my time with my old fog machine taught me not to hold the button down)

Tho I don't know where he got the remote from he had it already.

Seems to be switching the entire load of the machine through this, the buttons are only rated to 1 amps and it's a 900 watt machine - this guys slightly larger brother https://www.jaycar.com.au/600w-snow-machine/p/AF1221

Terrible concept imho.

I didn't get the wireless remote with mine, I think I'll patch in a sonoff rf instead, it's got a lovely little psu.

edit:

So I checked, couldn't sleep. Approximately half way across that resistor is 25 ohms, so if we guess it was a 50 ohm resistor.
I threw all the values into a couple of calculators (cos it's like 2 am)
 * http://www.nomad.ee/micros/transformerless/index.shtml
 * https://daycounter.com/Circuits/Transformerless-Power-Supplies/Transformerless-Power-Supplies.phtml
And assuming C1 isn't pooping itself at line voltages (I've ordered an X2) it should be good, R1 sees less than half a watt, D1 sees about 3x as much as that.

Just in case everything goes to poop, I also found a tiny little 3w potted power supply module, put 240 in, get 12v out... can't be much less safe than this setup. Tempted to put it *in* the machine and make this whole remote business a LV affair.

[DavidAlfa]

If you have a spare 12V adapter lying around, don't lose more time with it.
Connect the adapter, remove the zener (So it doesn't consume any power if the adapter gives 12.5V instead 12.0), close it, go have a beer.

[freman]

I ordered up a 12v one of these modules https://www.aliexpress.com/item/4000422022985.html it's just it won't get here till well after Christmas.