A LIGHT POWERING CHALLENGE

Thread Starter

Hugh Riddle

Joined Jun 12, 2020
55
An engineer friend of mine wants to construct a mains fed stairwell 8-15W LED ceiling light fed by the existing mains lighting circuit through its wall rocker light switches. He is, however, set on incorporating dim-up and dim-down behaviour and on powering the light through the dim-down phase following switch-off from an 18650 lithium battery . I anticipate criticism on using an 18650 but, if the switched mains constraint is accepted, the general idea seems to present a challenging engineering problem for which I can see no practical solution. It would be so interesting to hear your comments and any suggestions for other ways of meeting the requirement.
 

ronsimpson

Joined Oct 7, 2019
938
I am playing with leaving power to the light 100% of the time. To turn on/off/dim you send commands to the controller.
Some smart homes; all switches have addresses and all lights have addresses. Each switch is told what lights they control. There is not direct connection. There is not lithium battery to power the light during power down. The light sits on the power line all the time and you just tell it to dim to zero.
 

Thread Starter

Hugh Riddle

Joined Jun 12, 2020
55
Thanks for your replies ronsimpson. On the points you make:
"I am playing with leaving power to the light 100% of the time. To turn on/off/dim you send commands to the controller. To turn on/off/dim you send commands to the controller. "
That would avoid storing energy for the dim-down phase and standby (light-off) consumption could be kept very low. But my friend insists on using his existing mains lighting circuit and its wall-switches, a major constraint that makes energy storage unavoidable. IMHO it comes down to finding a safe and reasonably simple way of ensuring enough stored energy is available to power the dim-down phase whenever the light is switched off. I find that pretty difficult.
 
If you are dead set on making it dim using existing wiring (why?), consider a small supercapacitor in lieu of a chemical battery. Much longer shelf- & cyclic lives and less of a fire hazard should you realize any flaws in your design or execution. Or at the very least spend the extra money for an internally protected cell, some overvoltage protection and a properly sized DC-rated fuse.

If you go with the supercapacitor route, be sure your fuse has sufficiently high interrupting rating for the available fault current. Note that even small supercaps can kick out several kiloamps through a low impedance fault.
https://www.maxwell.com/images/documents/K2Series_DS_1015370_5_20141104.pdf
 
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Thread Starter

Hugh Riddle

Joined Jun 12, 2020
55
If you are dead set on making it dim using existing wiring (why?), consider a small supercapacitor in lieu of a chemical battery. Much longer shelf- & cyclic lives and less of a fire hazard should you realize any flaws in your design or execution. Or at the very least spend the extra money for an internally protected cell, some overvoltage protection and a properly sized DC-rated fuse.

If you go with the supercapacitor route, be sure your fuse has sufficiently high interrupting rating for the available fault current. Note that even small supercaps can kick out several kiloamps through a low impedance fault.
https://www.maxwell.com/images/documents/K2Series_DS_1015370_5_20141104.pdf
Thank you for your reply, Just Another Sparky. I think my friend wants to avoid altering his lighting circuit or its switching and having to provide means for remote turn-on. I accepted that as a challenging design problem and posted it in case others here see it similarly. I agree on the importance of overvoltage protecting the battery and am sure he'd agree that any 18650 he uses should be of internally protected type.

Using a supercapacitor instead of a battery seems a very interesting idea which I'd like to explore in detail. Would it be connected directly across the LV DC supply? Where exactly would the fuse(s) you advise be located in the circuit?
 

Thread Starter

Hugh Riddle

Joined Jun 12, 2020
55
I was hoping for an interesting discussion because there are other applications, some foreseeable for consumer products, which need simple low-cost local energy storage to power completion of some final operation and/or effect an orderly shutdown, after mains-off.

Just Another Sparky suggested using a supercapacitor for the local energy store. Connecting a 5.5V rated supercapacitor directly across the regulated 5VDC output of a switchmode mains-fed wall power supply or charger could provide a beautifully simple solution - with a few 'buts'.

At mains-on the supercapacitor will overload the LVDC supply for much longer than normal - as it re-charges to 5VDC. The LVDC supply must withstand these repeated overloads by electronically limiting the output current (without folding-back). I imagine most switchmode mains AC-LVDC convertors do that quite well but unless a specific (expensive) type is stipulated, a PTC could be wired into the mains feed - as a self-resetting fuse to cover switchmode failure.

The supercapacitor might almost completely self-discharge over a long period of misuse, introducing an extra, most likely acceptable, delay as it first re-charges to the system start-up voltage.

The problem seems 'simple yet complicated' so I'm pondering re-posting it as a more general (not lighting) topic and wondering which forum might be best.

Very glad to hear any further thoughts you may have about it.
 
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