Fuses, dimmers and energy-saving light-bulbs

[D.M. Procida]

I have a dimmer, with a slow-blow T1.6A fuse; the fuse blew when connected to a lamp with an energy-saving light-bulb.

Is that normal behaviour?

Daniele

[tabbypurr]

no

NT

[Huge]

Energy saving bulbs are not dimmable.

[Dave Plowman (News)]

What sort of energy saving bulb? Any LED will normally work on a dimmer circuit - but not dim unless a dimmable LED with compatible dimmer.

I've never tried a CFL.

[D.M. Procida]

I'm pretty sure this one isn't. I didn't expect the dimming to work, but I didn't expect a fuse to blow either.

Daniele

[Brian Gaff]

Who knows these days. You don't say what sort of one it was, but many lamps have inductors in the circuit so all sorts of stuff can happen. I don't believe a standard dimmer can work on most energy saving systems. Brian

[Brian Gaff]

The point is that dimmers work in one of two ways, they either reduce the duty cycle at 50 hz, or employ a more sophisticated system of doing things at the zero crossing point. I have no idea how the various power supplies in other forms of lamp other than filament might react in these situations. Brian

[Brian Gaff]

The early (heavy) CFLs tended to work like old fashioned fluorescents did, with a choke and capacitor etc, the newer ones have a switch mode psu instead basically. Brian

[tabbypurr]

If you connect a CR ballasted LED lamp to a triac or MOSFET dimmer, you'll get 100 large pulses of current a second. That will kill the LED lamp in short order, and might perhaps take out the fuse.

CR ballasted lamps are very dimmable, but not by chopping the waveform. A series capacitor is the easiest way to dim them. If you want several brightness levels, a switchbank and 2 capacitors would get you 4 levels + off.

NT

[NY]

I presume that dimmable LED lights (eg Philip Hue) are dimmed by varying the mark:space ratio of a constant-frequency square wave within the low-voltage DC circuitry (ie not by varying the pseudo-AC mains that is fed to the power supply).

It ought to be possible to see the effect of dimming LEDs by illuminating a desk fan. At full brightness the blades will be blurred (probably with sharp edges as the LEDs turn on and off) and the area of blurring will become shorter as the LED is dimmed and therefore turned on for a shorter period within each pulsing cycle. This is probably easier to see if the fan speed can be controlled (*) so it rotates at a sub-multiple of the pulsing frequency and therefore becomes stationary.

I've seen interesting effects with fluorescent lamps and a desk fan: a sharp blueish image caused by the visible components of the discharge and a more blurred yellowish after-image caused by the decay of the fluorescence from the phosphors.

(*) Either by varying the supply voltage to the fan or else by breaking H&S rules and removing the guard so you can apply friction to the hub of the blades and so slow it down a bit.

[D.M. Procida]

Thanks, that's probably what happened.

I think I'll just buy a new bulb! I was only testing a newly rewired lamp.

Daniele

[NY]

wrote in message news: [e-mail]...

How are fluorescent tubes dimmed continuously (ie not in noticeable steps)? I remember our lecture theatre at school (new in the late 1970s) had banks of 2-tube lights which could be dimmed over a period of a few seconds from full brightness to barely-lit, with no jumps in brightness apart from between the dimmest brightness and fully-off. I remember there were racks of electronics in the projection room which buzzed as the lights were dimmed (but were silent at full brightness), though I can't remember now which racks were used for dimming the fluorescent house lights and which were for dimming the tungsten theatrical lights (I imagine the latter were conventional triac dimmers).

As an aside, I also helped with the stage lighting for some of the school plays in the main hall (as opposed to the lecture theatre which wasn't much use for plays because it didn't have a proscenium arch and wings for the off-stage actors to wait in) and the hall had a huge dimmer box with a creaking rotary wheel to dim the tungsten house lights, and a rack of sliding wire-wound dimmers for the various stage lights (as opposed to a console of little sliders for the lecture theatre). I remember being warned of three things: 1) dim the house lights *quickly* and do not leave them on partially-dimmed for any longer than necessary, to avoid the dimmer overheating; 2) when operating the master kill-switch for all the stage lights (when an instant blackout was needed for theatrical effect), always hit the switch with the wooden mallet provided, never with your hand, because the switch gave off some fearsome arcs; 3) when turning on the stage lights, always dim them from off, never by reversing the kill-switch or you'll blow every bulb. A coordinated dim of multiple lamps involved a piece of 2x1" wooden batten to move multiple faders in sync; the various lamp circuits were wired by a patch panel so lights that those which needed to be faded in sync were always on adjacent faders for that reason. That was real heavy-engineering with everything big, hot and sparking :-) I remember the carefully-balanced bucket of water above the stage, operated by a rope from the lighting gallery, which was rigged as a surprise for the cast during the curtain-call on the final night of one Gilbert and Sullivan opera - the headmaster professed to be "not amused" and "took a very dim view of the antics" afterwards when he bollocked us all :-)

[Dave Plowman (News)]

We had dimmable fluorescents too in our school hall. Someone's pet project, given how little that facility was used. In the '50s.

The fluorescents in my kitchen dim. Special Osram electronic ballasts. The dimmer control just a simple pot.

[tabbypurr]

That sort of pre-war stuff makes variac dimming seem high tech. There are various ways to dim fluorescents, most require a dimmable ballast but variacs and series caps can be used to a limited extent on old iron ballasts. But haven't been in many decades.

NT

[Martin Brown]

It might be if you are out of luck, it is the wrong sort of leading edge dimmer and you hit the peak voltage in a cycle at first switch on. The worst case peak inrush current might be enough to take down a nominally slow-blow fuse. I'd expect it to last a few on off cycles though.

LED bulbs are tetchy about which dimmers they will work with and older dimmers can fail at least two ways - they don't like the much lower load presented by the LED bulb (minimum load for stability not met) or they cut the wrong part off the mains waveform leading to component stress.

Worst case the LED control electronics ends up drawing ever more current from the dimmer to try and maintain constant brightness. This may well blow the fuse for some partial brightness setting on the dimmer.

Even quite expensive dimmers that claim to do it right seem to be prone to premature failure from what I have seen. A friend with some expensive ones has got through 3 or 4 under warrantee now. The makers admitted some sort of batch fault so maybe he was just unlucky.