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AlphaGeek
08-24-2010, 07:40 PM
I did not realize that LEDs have an explosive failure mode until a few minutes ago. Here's the cautionary tale:

I've been wanting a bit more area lighting in the area under the spindle of my G0484 mill (Grizzly's RF45 clone). My inner cheapskate perked up when I saw this project, which repurposes a ring-shaped 6V LED tent light:

http://www.boysungrain.com/led-ring.htm

Finally, I thought, a quick project modifying an inexpensive stock assembly and requiring no new tooling. I sourced an appropriate light on Amazon for the princely sum of $10.00, with "free" shipping thanks to my Amazon Prime membership:

http://www.amazon.com/gp/product/B001P4Q5QI

All times in PDT.
16:15 Packaged arrived
16:18 Package opened (Yes, I was a little excited)
16:20 Product internals inspected. Noted that all of the hold-down screws for the LED strips were notably loose.
16:22 4 fresh AA batteries installed (correctly)
16:23 Noted that only inner ring of LEDs illuminated.
T+10 seconds: Several LEDs from inner ring go out. Light put down on work table, LED-side down.
T+20 seconds: Light starts to make astonishing and alarming hissing noise (~50dBa)
T+23 seconds: Made sure area around light on table was clear, backed away in a hurry
T+23.5 seconds: LED fails explosively with a crack loud enough to startle me

As I joked to my wife, I may have just set Amazon's record for the shortest time between product delivery and product-failure refund request.

It looks to me like the full voltage and current of the 4 AA batteries was going through the inner ring of LEDs, which was more than double what they were meant to handle in volts, amps, or both.

Perhaps I'll take another look at the LED illumination projects which use decent-quality LED strips and grids instead of a $10 battery-powered tent lamp. :)

-AG

10KPete
08-24-2010, 07:50 PM
I'd hate to have that happen in a tent!!

Pete

AlphaGeek
08-24-2010, 07:54 PM
I'd hate to have that happen in a tent!!

Pete

Yeah, we'd have to wash at least one of the kids' sleeping bags after an incident like that.

-AG

Weston Bye
08-24-2010, 08:07 PM
Noise Emitting Diode.

CCWKen
08-24-2010, 08:10 PM
You mean you read the reviews and still ordered it? :rolleyes:

A tent light that bursts into flames. Marvelous.

lwalker
08-24-2010, 08:11 PM
Ha! I still remember the day our shipper/final test person came running up the stairs to tell me about the LED that exploded when she plugged in the power supply and barely missed her face.

I think anything that can generate heat has a potential explosive failure mode :-)

Odds are that you bought one of those cheap LED lights that use the battery's internal resistance to limit current. Doesn't always work to plan if the batteries you use are better than spec!

AlphaGeek
08-24-2010, 08:17 PM
Ha! I still remember the day our shipper/final test person came running up the stairs to tell me about the LED that exploded when she plugged in the power supply and barely missed her face.

I think anything that can generate heat has a potential explosive failure mode :-)

Odds are that you bought one of those cheap LED lights that use the battery's internal resistance to limit current. Doesn't always work to plan if the batteries you use are better than spec!

Heh. Great story.

I'm pretty sure you're right about the electrical "design" since I didn't see any components other than LEDs and a switch attached to those circuit boards. I figure the inner ring of LEDs was getting >2x nominal current since the outer ring wasn't soaking up any juice.

-AG

AlphaGeek
08-24-2010, 08:22 PM
You mean you read the reviews and still ordered it? :rolleyes:

A tent light that bursts into flames. Marvelous.

To be fair, I would never have actually used this inside any tent or residential area. :)

And yes, I knew this was a crapshoot after reading the reviews, but I was planning on tossing the switch and wiring anyway. I just didn't expect it to be such as POS that it would blow up the one time it would ever be asked to run on batteries before being re-engineered into a mill light...

-AG

Evan
08-24-2010, 08:25 PM
There is a joker in the deck. I have noticed lately that many types of LEDs have been designed to fail short circuit. When used in a circuit where the current is limited only by the load and external resistance that means the current per device goes up if any device fails. This produces a runaway effect that eventually placed just one working device on the entire supply. Boom!

What is happening here is that those ring lights have been around for years and were perfectly safe with the old point contact LEDs that almost always fail open. When they failed it simply went out. The new LED designs fail short but are intended to be used with a constant current supply. A constant current supply automatically adjusts the voltage to maintain the proper current on a string of series LEDs so if one fails the rest still work.

BTW, I have some new High Power LEDs to play with. They are 12 volt running at 1 amp and put out 800 to 1000 lumens. They are only 1.5" long strips about .25" wide.

squirrel
08-24-2010, 08:27 PM
Would you please post a pic of that. I am very curious about what happened.

AlphaGeek
08-24-2010, 09:12 PM
Here's a link to the 4-photo set on Flickr:

http://www.flickr.com/photos/ag/sets/72157624801299190/

Two LEDs had visible signs of failure (one burned out, the other one exploded internally) and I posted close-ups of both.

-AG

squirrel
08-24-2010, 09:59 PM
That is interesting, looks like a good example of thermal runaway.

arcs_n_sparks
08-24-2010, 10:23 PM
I am with Evan. Most LED driver arrangements provide constant current across a string. Of course, that takes parts. Going the cheap route invites this kind of failure.

J Tiers
08-24-2010, 10:58 PM
Maybe someone back in china got a good deal on the LEDs......

Not ALL LEDs fail short..... but some are apparently indeed designed to..... perhaps so the whole string of Christmas tree light don't go out.

Wait until you have 6800 uF of capacitor, out of a group totaling over 20,000 uF @ 90V, develop an internal short.....

I never saw it happen, but I was shown the dents in the test area ceiling..... And when complaints were made, I asked why the fixtures did not have covers, since that test was the first time the boards were powered up, and anything could happen.

After I produced a small but fairly comprehensive paragraph in legal-sounding language covering all possible hazards I could imagine, and sent it in office e-mail, covers appeared rather quickly.

The chinese purchasing agent no doubt got a very good deal on those LEDs, and some poor sap approved the change. That's assuming, of course, that the third or fourth tier manufacturer in question HAS anyone approve that stuff.

gnm109
08-24-2010, 10:59 PM
I've used dozens of LED's on my model railroading for the past 25 years. I run them on an 8VDC power supply and use scaling/coupling resistors to hold the current at 25 ma. I've yet to have one fail.

I wasn't aware that they could short out if they were to fail. That wouldn't be good.

Evan
08-24-2010, 11:04 PM
There is no question looking at those LEDs. They are the new style self shorting type, not standard old style 5mm single whisker electrode type. Failing shorted is an advantage as long as the power supply is current regulated. But that is a very old design lamp and they have substituted the new LEDs that are not appropriate for the application.

I would guess that if you look very closely you will see at least two shorter bonding wires entering the conversion phosphor blob (on the die), possibly more.

J Tiers
08-24-2010, 11:12 PM
I've used dozens of LED's on my model railroading for the past 25 years. I run them on an 8VDC power supply and use scaling/coupling resistors to hold the current at 25 ma. I've yet to have one fail.

I wasn't aware that they could short out if they were to fail. That wouldn't be good.

With resistor limiting, nothing much bad should happen..... it's close to current limiting. Unless, of course, you use a bunch in series...... then current might go up quite a lot due to low series resistor.

gnm109
08-24-2010, 11:17 PM
With resistor limiting, nothing much bad should happen..... it's close to current limiting. Unless, of course, you use a bunch in series...... then current might go up quite a lot due to low series resistor.


Well, the power supply is one that I built myself many years ago and it's well-fused in case something like that could happen. I've got a lifetime supply of LED's at this point so I doubt I'll ever need any of the new type in any case.

I built an 8 VDC supply to run my home-made switch machines. I used slot car motors to operate them and they run at a very realistic speed on 8 VDC. Some of the LED's are used as position indicators for the switch machines, which are hard to see from the other side of the room.

.

KIMFAB
08-24-2010, 11:27 PM
I used the exact same light setup. I bought 3 for $18.00 ruined one and successfully bored another out to fit my Laguns quill.
I then purchased a wall wart universal transformer from the local Radio shack for $9.50 and got it to work.

I went back for a second transformer and found out I had purchased the last one, but they could order another for $44.00
I'll have to watch out for the explosion, thanks for the info.

Too_Many_Tools
08-25-2010, 12:09 AM
Darn good discussion topic.

Many years ago I hooked a LED up to a 9v battery....*BANG*,

It happens.

I agree that covers should be in place any time a new circuit or changes to a circuit are powered up.

TMT

winchman
08-25-2010, 02:44 AM
I'd chalk the failure up to unintended use of the product. Anyone using a tent light would normally only turn it on when it's dark, but you were in a lighted shop.

HAP
08-25-2010, 04:53 AM
Here is what I did to give a little more light.

http://bbs.homeshopmachinist.net/showthread.php?t=41613&highlight=cheap+mill+light

HAP

Circlip
08-25-2010, 05:29 AM
Although you say new AA cells, they weren't rechargables??

Regards Ian.

DFMiller
08-25-2010, 10:09 AM
After looking at the pictures. Those LED are hooked in Parallel. They would have to fail open for continued operation.
Dave

Deja Vu
08-25-2010, 10:28 AM
After looking at the pictures. Those LED are hooked in Parallel. They would have to fail open for continued operation.
Dave
Also, although not necessarily a cause for the type of failure experienced, those hold-down screws mentioned as being loose might have been as such for a reason. The screw heads look to be shorting across the circuit traces on the outer ring.

EVguru
08-25-2010, 10:52 AM
A work collegue once connecected an ordinary LED to 240Vac.

He invented the FED.

He was also the same person who said 'Look, this EPROM has a power light built in!'. He'd put it in the wrong way round and part of the die was glowing.

Dragons_fire
08-25-2010, 12:44 PM
He was also the same person who said 'Look, this EPROM has a power light built in!'. He'd put it in the wrong way round and part of the die was glowing.

I did that about 2 years ago with a 8SOIC switching regulator. i plugged the battery in backwards and it started glowing.. I pulled the battery, let it cool off, and it still runs as good as it did when it was new. Its been going on and off for the last 2 years and doesnt seem to care that it was ever plugged in backwards.

Evan
08-25-2010, 01:03 PM
After looking at the pictures. Those LED are hooked in Parallel. They would have to fail open for continued operation.


Exactly the same thing happens in parallel as in series if they fail shorted. The current is normally divided between all the LEDs in parallel so if they fail open the current to the remaining leds is increased. If they are designed to fail shorted the failed LED presents a short circuit and blows up since it is designed to be wired in series as is the case in many current LED lighting products. Once one LED fails catastrophically the current to the rest is increased and they fail progressively.

AlphaGeek
08-25-2010, 05:09 PM
Also, although not necessarily a cause for the type of failure experienced, those hold-down screws mentioned as being loose might have been as such for a reason. The screw heads look to be shorting across the circuit traces on the outer ring.

That occurred to me as well -- but the traces are on the side of the circuit board facing away from the screw heads. Otherwise, loose or not, it would have short-circuited immediately, since the back side of the circuit board was touching several of the screw heads.

-AG

AlphaGeek
08-25-2010, 05:14 PM
I'd chalk the failure up to unintended use of the product. Anyone using a tent light would normally only turn it on when it's dark, but you were in a lighted shop.

I'm glad I wasn't drinking anything when I read that. LOL indeed.



Although you say new AA cells, they weren't rechargables??

While I power nearly all of my stuff that requires AA batteries with low-self-discharge Sanyo Eneloops these days, I do in fact keep a supply of alkaline AAs on hand for infrequently-used and loaner/throwaway devices. Hard to believe, but they haven't outlawed alkaline batteries here in CA yet. Probably waiting to spring that one on us until after the incandescent bulb phase-out. :)

-AG

AlphaGeek
08-25-2010, 05:45 PM
Here is what I did to give a little more light.

http://bbs.homeshopmachinist.net/showthread.php?t=41613&highlight=cheap+mill+light

HAP

Nifty. Thanks for posting the link.

I used to live in Charlotte. Either you have an air-conditioned workshop or you're far more heat-resistant than I am, as I think I'd melt into a puddle trying to use that setup during a North Carolina summer. :)

-AG

MickeyD
08-26-2010, 01:02 AM
I make a lot of enclosures and heatsinks for constant current power supplies used in high output LED lights and they are not real cheap if you do them right. Trying to use the resistance in the battery or a cheap chinese power supply is not going to give good long term results.

HAP
08-27-2010, 12:59 PM
Nifty. Thanks for posting the link.

I used to live in Charlotte. Either you have an air-conditioned workshop or you're far more heat-resistant than I am, as I think I'd melt into a puddle trying to use that setup during a North Carolina summer. :)

-AG

It does get a lttle warm, not too bad though. Bottom line, it throws down some light when I need it, and it was free :). Is this the same mill you have?

Regards,
HAP

AlphaGeek
08-27-2010, 02:46 PM
It does get a lttle warm, not too bad though. Bottom line, it throws down some light when I need it, and it was free :). Is this the same mill you have?

Regards,
HAP

Yep, I also have the G0484. Noted a couple of minor differences between yours and mine -- you have a nicer DRO, different (nicer) X-travel limit stops on the front of the table, and yours is clean, assembled and ready to use. :)

Mine currently has all of the gearbox internals removed, as I've had to get the majority of the gears replaced by Grizzly under warranty. I'm also replacing all of the bearings. The whole sordid tale is being documented in this thread:

http://www.chaski.org/homemachinist/viewtopic.php?f=43&t=87435

-AG

HAP
08-28-2010, 08:16 AM
AG, sorry to hear about your mill problems. I did notice that my High/Low gear selector handle is on the oposite side from what the owner's manual show. Also, ther is no mention of the verticle travel colum stops on the head unit. Had to figure out how they worked to get the head to move. I bough mine March 2010. Perhaps the changes are design improvements based on detailed feedback from your experience. Did you post any pics of your efforts yet?

Paul Alciatore
08-28-2010, 02:59 PM
On the exploding LED, it is no real surprise. Many electric or electronic components can/will explode if too much current is suddenly passed through them. This is one of the reasons why there are fuses. Unfortunately, the fuses do not always blow fast enough. Old saying, "The transistor will always blow fast enough to save the fuse."

I once witnessed a pole pig explode. I was a half block away and that was as close as I would ever want to be. I still could have been hurt. I had a closer call some years ago while working on an old, tube type video tape recorder. A 400 or 500 Volt power supply used a bank of regulator tubes and some large can type capacitors. I was leaning over it and looking down into it when one of the capacitors decided to blow. It was upside down so the base was pointing directly at my face. Only the mounting rail saved me from eye damage, but I really needed to change my shorts.

I have seen many smaller parts blown apart: capacitors, resistors, transistors, IC, etc. Almost any component can explode if the current surge is high enough and fast enough to eliminate melting. Something on the inside vaporizes and boom.

Evan
08-28-2010, 03:20 PM
I once witnessed a pole pig explode. I was a half block away and that was as close as I would ever want to be.

Back in the 70's when we lived in Edmonton we were stopped at a red light in a thunderstorm downpour. As we waited lightning struck the transformer directly above us. Blue flaming burning oil rained down all around and on us but fortunately the rain was so heavy it washed it all off the car almost immediately. I got us out of there as fast as I could, red light be damned.

Paul Alciatore
08-28-2010, 03:23 PM
Yep, I also have the G0484. Noted a couple of minor differences between yours and mine -- you have a nicer DRO, different (nicer) X-travel limit stops on the front of the table, and yours is clean, assembled and ready to use. :)

Mine currently has all of the gearbox internals removed, as I've had to get the majority of the gears replaced by Grizzly under warranty. I'm also replacing all of the bearings. The whole sordid tale is being documented in this thread:

http://www.chaski.org/homemachinist/viewtopic.php?f=43&t=87435

-AG

I also have a Griz G0484 so I read your thread with interest. I don't seem to have the play you mention but it does sound a bit too loud to me when I run at medium or higher speeds. I will open the gearbox and inspect.

I am awaiting the results of your adventure.

Thanks

Paul Alciatore
08-28-2010, 03:25 PM
Back in the 70's when we lived in Edmonton we were stopped at a red light in a thunderstorm downpour. As we waited lightning struck the transformer directly above us. Blue flaming burning oil rained down all around and on us but fortunately the rain was so heavy it washed it all off the car almost immediately. I got us out of there as fast as I could, red light be damned.

OK, it figures you would one-up me.

Why aren't we in our shops? Why?

Evan
08-28-2010, 03:40 PM
Because my wife hasn't left the house yet and I am supposedly waiting for the grout to dry in the shower before I can proceed with the next phase of the repair.

madman
09-16-2010, 05:47 AM
Was it Chinese or Iraqi? IED ?