I have had a number of small domestic light controllers fail.
In each case the failure occurred concurrently with the "normal" failure of the lamp.
The very high current that typically can flow when a lamp fails is literally blowing part of the small triac fitted within the controller away - resulting in the failure of the controller.
These are only 100W lamps, on a UK domestic ring main proteected by a 5A wire fuse. The triacs apparently blow before the fuse can blow and clear the fault current.
Now, some points I would welcome your views on:
Firstly, why does such a high current flow when a lamp fails? Are there incandescent lamp types that don't (eg would industrial long life lamps be better)?
To me, this is a design fault in that the device is not being kept within its SOAR by protective device(s) within the controller. The designer should have allowed for the normal and failure mode of the load.
Now I can fix the things by fitting much larger triacs. I have doubts that any protective device could reliably discriminate between the inrush current of the lamp and the fault current, quickly enough to protect the small triacs fitted as OE.
Any thoughts? Is this a common problem with commercial domestic lighting controllers? This is the first time I have actually bought in controllers rather than designing and building my own - they may be cheap but they certainly aren't cheerful..
If your lamps are standard tungsten filament lamps they are what is known as 'coil in coil'.
This means that the tungsten is first coiled very tightly and then the coiled wire is coiled again less tightly. So the actual length of the wire is much longer than you can see in the lamp (don't know the exact ratio).
It's a method of fitting 'say' a two metre length of resistance wire in a three cm space (domestic lamp).
If you're lucky when the lamp fails it will fail open circuit. If you're unlucky when the lamp fails you short out many turns of the filament. The fault current is a little 'pot luck', it may be high or it may be low dependent on how much filament is shorted out. So as you know sometimes the main protective device has to step in to protect the circuit and other times the lamp will do the job sufficiently.
This is the reason that nearly all domestic lamps have internal fuses. (SEE POSTING 'BALLOTINI' BY RUDE MR PILGARLICK) Lamp fuse disconnection time is long to allow for sufficient discrimination with the installations protective device this results in your devices attempting to feed a load that is far in excess of its design brief.
Some of the better lamp controllers have features such as 'lamp saving' circuits. These will ramp up the voltage and maintain the voltage at a pre-set level to prolong the lamps life, this will reduce 'in-rush' to negligible. These controllers normally have current limiting circuits to protect the power electronics they contain (A feature that can cause a bit of head scratching occasionally). Industrial units also have out put fuses for ultimate protection. The better quality domestic halogen lamp drivers have some of these features and are usually these days based on 'switched mode' drivers.
Switched mode drivers are far superior to old fashioned transformers and triacs.
As this device is series it will see all of your fault current, and no standard circuit thermal protective device will protect power electronics, the protection must come from the electronic device itself (unless the device is ten times larger than it needs to be).
This might sound a little bizarre but have a look at the lamp orientation, it's unusual for this problem you're having to be so predictable, I would also measure the supply voltage at the lamp terminals and consider protection between the controller and the lamp.
The lamp with the most floppy filament has to be RUDE MR PILGARLICK's 15w lamp, just pick one up and look at the filament,you can well imagine the filament bouncing around after its failed (everybody knows the old trick of thumping your car headlight to make it work again).
When RUDE MR PILGARLICK's 15w lamps were used as signal lamps in old machinery such a diesel generators for example they caused a lot of problems like this, (thank heaven for LED's 'fit 'n' forget'). Even when the lamps failed open they had to be removed immediately because they were still a danger. This why you should not remove lamps unless you know the circuit is dead. As the lamp is disturbed the filament can fall across the supply terminals in the lamp and at best give you a fright or at worst explode!.
See my detailed description in the recent "Ballotini Fuses" thread, news:
No.
You might find halogen lamps less likely to flash-over when a filament breaks because they have a much higher pressure gas-fill, but they cannot claim to be completely immune from the problem.
Triacs are normally oversized to help cope with this, but there's a trade-off in that the minimum lamp load the dimmer will drive increases with larger triacs, which have a higher holding current. For all intents and purposes, if the lamp flashes over when the filament breaks, it is pretty much a short circuit, so your triac needs to be able to handle a short circuit current (i.e. limited by the supply impedance) for the length of time it takes the fault current protective device to operate. Even the fastest fuses will continue to conduct until the next zero-crossing point, and might take several half cycles before they have melted, vapourised, and the arc been extinguished. If the fuse you are referring to is a BS1362 plug top fuse, these are quite slow blow anyway. Some MCB's used in UK (EU) are capable of tripping in significanly less time using the magnetic (fault current) tripping element and methods to quench the arc without having to wait for the next zero crossing point. These can be a pain in lighting circuits where they often trip when a lamp blows, but might operate fast enough to save your triac. This is also a function of your supply's prospective short circuit current, and yours might be higher than is common.
------- They may melt before the first peak but they will clear at the next current zero (or later) on AC - There is no attempt made to "chop" the arc-which is actually a bad thing. Andrew is correct.
The literature for Buss current limiting fuses has illustrative figures showing the fuse clearing well within 1/2 cycle.
"Current limiting fuses can restrict the short-circuit current before the first damaging peak current is reached and can clear the circuit in one-half cycle or less." Electrical proteciton handbook Bulletin SPD-78.
One design for current limiting fuses is the silver-sand type. This has multiple relatively fine silver wires in parallel surrounded by sand. Using silver allows a finer wire because of lower resistance, also probably higher melting temperature. Multiple wires produce smaller arcs. They are extinguished by the surrounding sand and do not require a zero crossing.
Limiting the peak current reduces magnetic effects that can tear appart switchgear. But the heat energy that components (inclcding semiconductors) are exposed to is proportional to current squared times time (I squared t). Minimizing the clearing time reduces this energy.
Buss also makes fuses to protect semiconductors that are "ultra-fast acting".
I should have said 1/2 cycle instead of 1/4 cycle. Also, the fuses have to be interrupting a large enough current to be in their current limiting mode to clear the circuit in less than 1/2 cycle.
Bud--
D>>"Current limiting" fuses will clear within 1/4 cycle (if the fault
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