Magnetic base LED machine light – WARNING MAY BE UNSAFE

[SillyOldDufferModerator @sillyoldduffer]

Posted by duncan webster on 29/06/2016 16:42:03:

I ask merely for interest, as mains electrical safety is out of my pay grade. As the supplied 2 pin plug isn't handed, you can get live on the side with the 100 ohm resistor in series, so the + output to the LEDs can be at mains potential? Or have I got it wrong?

If I'm right would we be better chopping off the supplied plug and connecting to a 3 pin, which is handed? Always supposing you can work out which is which.

Beyond my pay grade too Duncan so I risk being blasted by the professionals!

The circuit operates whichever way it is plugged in and you are quite right – it is possible to imagine a fault condition that would cause live mains to appear on the DC side. That sounds dangerous but a precondition of this type of circuit is that the wiring must be sealed inside the appliance.

Being sealed makes it rather less dangerous than a good old glass filament lamp. If you break the glass bulb of a filament lamp you expose a live conductor that's connected directly to the mains. Even a lighting circuit can deliver a good few amps. In comparison the capacitive divider always has components in circuit that tend to reduce the current that can flow. So it's rather safer than an ordinary light bulb.

That said, Mike P's lamp was unsafe because it wasn't "sealed" – that metal neck should never have had volts on it.

Cheers,

Dave

[Mike pParticipant @mikep]

Hi all, I did not keep the remains of this lamp but from memory the item marked R2 on Bandersnatches picture had flashed over to the switch and the end of the flexible neck. Whilst appreciating all being said about RCDs and breakers I speak from live experience not theory and the shock I received was not theoretical but real that is why I posted to warn others of the danger. Mike p

[Enough!Participant @enough]

The way I interpret Duncan's comment is that it wouldn't take a fault condition for the mains potential to appear at the output.

The plug (on mine at least) is 2-pin and not polarised in any way. The blades are identical. In fact there is no way to determine which is the "correct" (intended) way round to plug it in. Under those conditions the switch might be in either the live or neutral lead with the other lead unswitched.

Same disclaimer as Duncan, but it seems to me that the live side of the mains could then (in normal use) be present on the LED output terminals/wires (via the 100 ohm resistor and a diode). If they shorted somehow to the metallic flexible neck on Mike p's sample, he may indeed have had a mains short to ground.

[MWParticipant @mw27036]

You should always be wary when you're handling electricity of any sort, as it's always potentially lethal. Be sure everything is grounded and isolated properly and the right sort of wiring is used. And before trying anything on the mains try it on an RCD first.

Having said that, treating it like the monster under the bridge shouldn't stop you from learning about it.

Michael W

Edited By Michael Walters on 29/06/2016 22:30:35

[Neil WyattModerator @neilwyatt]

Posted by Mike p on 29/06/2016 21:47:21:

Hi all, I did not keep the remains of this lamp but from memory the item marked R2 on Bandersnatches picture had flashed over to the switch and the end of the flexible neck. Whilst appreciating all being said about RCDs and breakers I speak from live experience not theory and the shock I received was not theoretical but real that is why I posted to warn others of the danger. Mike p

Th whole of that PCB needs to be regarded as being 'Live' from a design point of view, none of it is isolated from the mains and the end of R2 is only 100 ohms away from the mains when plugged in so white is live. The bare tip could easily deliver a dangerous shock.

If the design is such that anything on that PCB isn't effectively isolated form metal parts I wouldn't touch one of these with a bargepole.

It is an acceptable design for sealed inside a non-conductive envelope like many LED lamps (as pointed out no worse than a filament lamp) but not suitable for inside an unearthed metal case.

Can anyone post a picture of how these PCBs fit in the case?

Neil

[MWParticipant @mw27036]

Am i reading this right? An AC current with no earthing terminal?

Ps. if anything from the pcb circuit was in contact with the casing it could become a conductor. 

Michael W

Edited By Michael Walters on 29/06/2016 22:48:25

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Michael Walters on 29/06/2016 22:43:15:

Am i reading this right? An AC current with no earthing terminal?

.

Yes

[Neil WyattModerator @neilwyatt]

Posted by Michael Walters on 29/06/2016 22:43:15:

Am i reading this right? An AC current with no earthing terminal?

Michael W

More common than not these days for consumer goods. Double insulation is considered 'inherently safe', the problem with this design is that it appears it may not be double insulated.

Neil

[MWParticipant @mw27036]

Sounds unusual… against my learning anyway. I recently bought a product from Taiwan, it came with a tiny moulded 3 pin plug, no approval marks on the casing, i thought to myself "theres no way a fuse is in there" so i cut it off and bought a BS/CE approved plug from wilkos and sorted it. I wasn't going to risk using an electrical appliance without a fuse.

I've been known to experiment at times but never on safety.

Shoutout to Neil, I was listening to planet rock earlier and the host mentioned your name for a phone in request? any truth to that?

Michael W

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 29/06/2016 22:49:16:

More common than not these days for consumer goods. Double insulation is considered 'inherently safe', the problem with this design is that it appears it may not be double insulated.

.

It would certainly be interesting to see how they handle the insulation [if any] inside, and adjacent to, the [presumably metallic] gooseneck.

MichaelG.

Edited By Michael Gilligan on 29/06/2016 23:28:21

[duncan webster 1Participant @duncanwebster1]

OK despite my initial respones (top of page 2)  I'm convinced. Fortunately I have 40v on the milling machine for driving stepper motor, so I'm dumping the circular box of electronics and driving the LEDs direct from 40v with suitable dropping resistors. I'll report back when I've done it as to how well it works. Currently I've worked out that there are 2 parallel arms, each with 15 LEDs in series, but no way of ensuring each arm takes the same current, so I'm going to cut the appropriate track and put a small dropper resistance in each arm. Watch this space

Edited By duncan webster on 29/06/2016 23:30:13

[Michael GilliganParticipant @michaelgilligan61133]

Wise man, Duncan

You may deserve a pay-rise [ref. Your earlier post]

MichaelG.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Michael Walters on 29/06/2016 22:43:15:

Am i reading this right? An AC current with no earthing terminal?

Ps. if anything from the pcb circuit was in contact with the casing it could become a conductor.

Michael W

Edited By Michael Walters on 29/06/2016 22:48:25

An old joke in Information Technology is that "Fail Safe Systems fail by failing to fail safely" and earth systems are a case in point. They also have the potential to become dangerous.

A domestic single phase mains supply is actually part of a three phase distribution system. The earth / neutral / live arrangement reduces the risk of electrocution in the event that a fault causes a metal case to become live. But the arrangement we consider "safe" itself becomes dangerous if an earth or neutral fault occurs on the supply side. For example a neutral fault can cause the domestic live wire in an entire street to rise to 440V. This destroys equipment, blows fuses, and could cause fires.

The good news is that electricity providers take care to reduce the risk of earth/neutral faults. It does happen though: errors bonding transformers; an inadequate earth at the sub-station (perhaps caused by corrosion); sabotage; theft; and accidental physical damage to the network are all possibilities.

Risk is something to be managed. Ideally all electrical equipment should be double insulated and designed so that people and water can't get at the innards. As that's impracticable we just have to take care, bearing in mind that not everything on sale is automatically suitable for workshop use.

Cheers,

Dave

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 30/06/2016 10:09:41:

… not everything on sale is automatically suitable for workshop use.

.

Wise words, Dave

MichaelG.

[MWParticipant @mw27036]

Well, i guess i learn something new quite often..if not everyday.

Michael W

[Enough!Participant @enough]

Posted by Michael Gilligan on 29/06/2016 23:22:22:

It would certainly be interesting to see how they handle the insulation [if any] inside, and adjacent to, the [presumably metallic] gooseneck.

OK, some more pics (sorry about the quality – I didn't have much time). these show how the thing is assembled.

Couple of points:

– The round magnetic base containing the pcb is crowded – the switch has to sit well down between the higher components. And it's assembled pretty much blind (!)

– There appears to be no additional insulation inside the metal conduit (gooseneck). I don't think two insulated wires running inside could be considered double-insulated (could it?).

– The ends of the conduit sit in counterbored holes in the housings at each end which prevent it from protruding into the respective enclosures (pcb and led head). Nonetheless the bare metal is pretty close to the 40V terminations at the pcb and it's my opinion that it could still short if, say, the wires were stripped back a little more than necessary. It might also be possible for the wires to fray at the cut ends of the conduit.

[Michael GilliganParticipant @michaelgilligan61133]

Thanks, Bandersnatch

The Gooseneck is more tidily installed than I feared it might be … But the whole thing is too risky for my taste.

MichaelG.

[Clive HartlandParticipant @clivehartland94829]

Could the box not be filled with a sealant? this would prevent any short.

Clive

[duncan webster 1Participant @duncanwebster1]

 

I've modded mine to work off 40V DC. When I prised the back off the lamp unit I found tat red wire went to -ve and blac wire to +ve, which wasn't a good start. Doesn't actually matter as long as it's wrong both ends, which is was, but I've made it right. Then cut track and soldered on an extra lead. The existing circuit board is in the bin, replaced with dropper resistances to ensure current sharing. I used 2 off 100 ohm in parallel in each leg as I didn't have any 50 ohm. Now works off the stepper motor power supply, all it needs is an in line fuse, it wouldn't fit in the little box.

I'll just keep an eye out for another 40v transformer for the other one, not worth buying one, although with a lot more surgery I could make the LED board be 6 lots of 4 in series and run it off 12v. Life's too short!

Edited By duncan webster on 01/07/2016 12:48:58

Edited By duncan webster on 01/07/2016 12:50:37

Edited By duncan webster on 01/07/2016 12:51:36

[SillyOldDufferModerator @sillyoldduffer]

Posted by duncan webster on 01/07/2016 12:42:49:

I've modded mine to work off 40V DC. When I prised the back off the lamp unit I found tat red wire went to -ve and blac wire to +ve, which wasn't a good start. Doesn't actually matter as long as it's wrong both ends, which is was, but I've made it right. Then cut track and soldered on an extra lead. The existing circuit board is in the bin, replaced with dropper resistances to ensure current sharing. I used 2 off 100 ohm in parallel in each leg as I didn't have any 50 ohm. Now works off the stepper motor power supply, all it needs is an in line fuse, it wouldn't fit in the little box.

I'll just keep an eye out for another 40v transformer for the other one, not worth buying one, although with a lot more surgery I could make the LED board be 6 lots of 4 in series and run it off 12v. Life's too short!

I've put two photos in the albums, but for some reason it won't let me paste them into this message

Edited By duncan webster on 01/07/2016 12:48:58

Hi Duncan,

Your pictures are causing another mystery. I can't see them in your original post but as you see they appear when I quote it!

Anyway, looking at Bandersnatch's post and Mike P account of his blue flash moment, Sherlock Duffer concludes that MikeP's lamp must have had a bare wire inside the metal goose-neck. As the lamp circuit floats, it could have been either the plus or minus wire that shorted.

I wonder if you might need in-line fuses on both plus and minus? Depends on your stepper motor supply, which could be floating, or negative ground, or positive ground.

The extra fuse isn't a safety feature – it further guards against the possibility of a short damaging your stepper motor supply. (Perhaps!)

Dave

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Bandersnatch on 28/06/2016 22:45:39:

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 01/07/2016 13:04:15:

Anyway, looking at Bandersnatch's post and Mike P account of his blue flash moment, Sherlock Duffer concludes that MikeP's lamp must have had a bare wire inside the metal goose-neck. As the lamp circuit floats, it could have been either the plus or minus wire that shorted.

.

I hope you're right, Dave … But the other [much more severe] possibility is that the upstanding end of R2 somehow made contact with the metalwork [see earlier picture from Bandersnatch, above] … I still can't tell how plausible this is; but I definitely don't like the layout.

MichaelG.

Edited By Michael Gilligan on 01/07/2016 13:42:04

[VicParticipant @vic]

I'm late to the party on this thread as I bought an LED light from a reputable company and won't buy any more as I simply don't like the quality of light it gives off. I'll use the one I've got but won't buy any more for the time being. I very much prefer the halogen work light on my mill to any LED's I've seen so far.

There have been several reports of dangerous LED lights without proper earthing so to suggest that having a CE mark makes the item safe is simply incorrect. And thats without China's fake "China Export" mark on many items that end up here.

If it's very cheap there's probably a very good reason. Keep safe out there!

[duncan webster 1Participant @duncanwebster1]

Posted by SillyOldDuffer on 01/07/2016 13:04:15:

Posted by duncan webster on 01/07/2016 12:42:49:

Hi Duncan,

Your pictures are causing another mystery. I can't see them in your original post but as you see they appear when I quote it!

Anyway, looking at Bandersnatch's post and Mike P account of his blue flash moment, Sherlock Duffer concludes that MikeP's lamp must have had a bare wire inside the metal goose-neck. As the lamp circuit floats, it could have been either the plus or minus wire that shorted.

I wonder if you might need in-line fuses on both plus and minus? Depends on your stepper motor supply, which could be floating, or negative ground, or positive ground.

The extra fuse isn't a safety feature – it further guards against the possibility of a short damaging your stepper motor supply. (Perhaps!)

Dave

The reason you couldn't see the pictures was that I wrote the post, then couldn't seem to get the pictures in, so posted it so I wouldn't lose it, then found out how to do it and edited the psot.

Anyway, I don't know whether my power supply is earthed on the LT, but I know it's isolated (I made it). I can easily earth one side of the LT winding on the transformer. Is this the right thing to do? Seems to me that if it's floating I can't get any leakage to earth. There is already a big fuse to limit current from the power supply, I was just going to put the fuse in to protect the LED wiring, not me, I think I'm reasonably safe at 40v

Edited By duncan webster on 01/07/2016 13:51:10

[Neil WyattModerator @neilwyatt]

Posted by Michael Walters on 29/06/2016 22:56:52:

Shoutout to Neil, I was listening to planet rock earlier and the host mentioned your name for a phone in request? any truth to that?

Michael W

Just noticed this, It's possible Michael – I'm a regular listener (and emailer) to Planet Rock and Radio 4. Apparently I was on Feedback last week…

Neil

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