Do dimmers still blow their triac with crap 60W bulbs (x3)? The triac used to be quicker at terminally disconnecting faults before the cpd did (somewhat less dramatically :-)
If so is it worth locally protecting the dimmer with a local fuse? eg, grid K4501 + grid fuseholder with 1A fuse.
CPD on the final circuit is 6A Type B so 5*In or 30A will disconnect a fault in
Not had any problems with recent ones like the varilight touch dimmers etc. I had an older touch dimmer that blew its own internal fuse on a bulb failure once - but after replacement of the fuse the triac was still fine.
There's a trade-off in cheap dimmers, and it's between having a low minimum load (which requires a small triac die with a small holding current, but can then drive a single 40W lamp) and having the capability to handle the high fault current resulting from a flashover as a filament breaks (which requires a large triac die to absorb the heat pulse without melting, but can't drive a load as small as a single 40W lamp).
The other option is a more expensive dimmer circuit which uses a hard firing technique for the triac (which requires an integrated circuit rather than just the simple resistor, capacitor, diac circuit), in which case the minimum load problem goes away, and a large die triac can be used which will both survive the high fault current for long enough to blow a fuse/mcb, and can also drive low power loads.
I have an expensive remote controlled DIN rail mounted dimmer which I use in one location. It probably would survive a flashover, but as belt-and-braces protection, I added a £5 B3 MCB on the rail next to it -- that will trip faster than most 1A fuses on a fault current -- it can quench the arc mid-half-cycle, which many fuses don't do, so you get faster disconnection time.
That is what I recall.
PWM for lights?
Fuses are slow.
So I gain nothing by a 1A fuse next to a K1501 (iQ) dimmer?
If so it might be better to do grid dimmer+switch with a picture light, that way there is still functional lighting in the room if after cpd reset the dimmer is sat dead in the water. It is for an elderly relative who irritatingly wants a low light to draw curtains or refuses to put it on - no doubt followed by falling over in the room.
It's PWM identically in both cases. Hard firing the triac just means that a gate current is supplied continuously when the triac should be conducting, rather than just a pulse to start the conduction and relying on the load current being > than the triac's holding current to maintain conduction to the zero crossing point. With pulse firing and a small load, the current drops below the triac's holding current well before the zero crossing point, which causes premature cut-off before the zero-crossing point. Furthermore, triacs are not manufactured to have well matched holding current in each direction, as that's not a property of a triac which is generally expolited. This means you get 50Hz flicker at low light levels because of the uneven cutoff in
+ve and -ve going mains cycles, and it screws up the RC network timing for the next half cycle by prematurely starting to charge the capacitor before the zero crossing point, and that can make the light output go very unstable, flickering badly. Hard firing avoids the vaguries of the holding current and uses much better timing circuits. This is easily integrated into a small monolithic IC together with things like soft-on/soft-off control and/or touch control. Mullard/Signetics/Philips were producing such ICs over 30 years ago.Depends so much on the fuse, and on the prospective short circuit current at the lampholder. It won't do any harm, except it might needlessly blow every time the lamp blows.
Some assistive technology schemes do away with light switches and use occupancy sensors instead, and this is because an elderly person might otherwise get up in the middle of the night and not switch on the lights, before moving around and falling over something.
I had noticed bad flickering on the old dimmer when set low.
Indeed catch-22.
I'll retain the wall light on a dimmer. However I will add a ceiling occupancy sensor & picture light, easy enough.
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