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Low voltage power from a UK live only light switch

Discussion in 'Electrics UK' started by Lewishw, 13 May 2015.

  1. plugwash

    plugwash

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    The trickier problem is the wireless receiver itself. With many wireless chipsets merely listening for incoming packets takes nearly as much power as actively transmitting or receiving does.

    Theres a reason low end wireless burgular alarms are usually one-way communication.

    You have to read the datasheets carefully though. For example on the PIC18F45j20 I used in a low power battery powered projects you only get the lowest power sleep mode (which the headline figures reffer to) if you have the main memory disabled (so you basically have to reboot when you wake up) and all the timers disabled so you can only wake up on external triggers.

    Microwatts is more typical for a useful sleep mode with main memory retained
     
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  3. bernardgreen

    bernardgreen

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    Designing a unit with a wireless receiver close to a microprocessor will involve careful thought about how to prevent the inevitable electromagnetic radiation from the processor affecting the wireless receiver. A simple receiver ( essential for low cost and low power consumption ) may be swamped by the EMR from the processor.
     
  4. Lewishw

    Lewishw

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    I have made a small test circuit powered by a 2600mAh rechargable battery pack which lasted just over 24 hours. The coding is far from perfect at the moment so with sleep modes could probably get 2-3 days out of it. This means that battery power is not an option as I do not want to be changing batteries every week.

    Just to confirm, this is a project that I would like to turn into a retail product in the future so it is essential that it works with one install i.e. Changing the light switch to the new controller.

    It is a tricky problem as I know problems occur when devices are wired in serial but that is the only way I can see it working. Maybe I need to get hold of a LightwaveRF switch and take it apart to see how they do it, or find a schematic for it.
     
  5. JohnW2

    JohnW2

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    In that case, I would imagine that you have little choice than to do something 'slightly complicated' like SimonH2 suggested - when the switch is 'off' use the voltage across it (going to neutral through the load) to charge a battery or capacitor and when the switch is 'on' (assuming an 'electronic' switch), steal a millisecond or two at the start of each half cycle to charge the battery/capacitor. In that way, the battery/cap would always be getting charged, whether the switch was 'on' or 'off', and therefore (if adequately designed) should never go 'flat'.

    Kind Regards, John
     
  6. plugwash

    plugwash

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    I think the honest answer is you are way out of your depth. Designing a power scavenger is difficult, ideally it needs to have two modes, a high voltage low current mode and a low voltage high current mode*. If it only supports one or the other then you will have problems if the light is left in one state for too long. The high current mode is also weird because rather than having voltage externally determined and current determined by your devices power needs you have current externally determined and voltage determined by your devices power needs.

    This isn't impossible but it's not easy either. While I generally consider BAS's approach of "throwing a pile of books at people" to be counterproductive I really don't think designing such a supply is a good first task for someone new to power electronics.

    You also need to get your power consumption down as low as possible. The lower your power consumption the lower the chance you will have problems with lights flickering. That means you really need to design your system so the receiver can spend most of it's time asleep, learn a lot about the sleep modes of your devices and very carefully design the circuit so it doesn't throw power away.

    * Using the words high and low here in their usually everyday sense, not their BS7671 voltage bands sense.
     
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  7. ban-all-sheds

    ban-all-sheds

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    [​IMG][​IMG]
     
  8. JohnW2

    JohnW2

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    Indeed, but they may well not be discussing fully optimised, commercially appropriate/viable, 'two wire' products to replace light switches. As plugwash has said, if someone trying to develop such a commercial product has to ask in a DIY forum about designing an optimal power scavanging system for it, then something is probably less-than-ideal.

    Kind Regards, John
     
  9. plugwash

    plugwash

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    I wonder if a better approach would be to put the receiver at the light (and if you still want a switch on the wall use the switch drop to connect it to a control input on the receiver).

    Downside is it's a bit more work to install than simply replacing a switch (though a lot less work than replacing the switch drop). Upside is easier to design and no flicker problems.
     
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  11. JohnW2

    JohnW2

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    That would certainly make life much easier for him. However ...
    Kind Regards, John
     
  12. endecotp

    endecotp

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  13. Lewishw

    Lewishw

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    Thank you so much for all of your help. I have been following this discussion and I am considering one of the following design changes in the circuit as well as moving over to Philips Hue and LivingWhite smart bulbs:
    1. because of the smart bulb, the circuit would be on all of the time, would this make it easier to have a power supply in series or will it depend on if the light constantly uses power? I suspect that the power drop in the circuit would make this difficult.

    2. This has been mentioned earlier but where would I stand on changing the switched live wire to a live and neutral (230v) going to the light switch socket? would an electrician have to do it, or inspect it? would it cause problems if this becomes a retail product? would it be better to send a low voltage down the wires (i.e. have the transformer in the ceiling rose). Would I be putting people in danger by having a high voltage live and neutral behind a switch (instead of live and switched live)
     
  14. Lewishw

    Lewishw

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    Just to put your mind at ease, it is currently 100% DIY at the moment with a distant hope of the possibility that one day it could become a commercial product with plenty of professional help and testing. Also I don't want to break too many regulations or put anyone in any danger.
     
  15. Taylortwocities

    Taylortwocities

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    If you need to place your product where there is live and neutral present then you have an issue. There are several ways of wiring up lighting circuits and you will have a problem finding a "one product suits all" offering.

    Some of the versions (and your challenges) are:

    In the (usual for now) normal lighting set up, this is called "loop in" at the light fitting. In this, the place that you will find live and neutral is at the ceiling rose. At the switch there is only live and switched live, if you need a neutral there too, then your luckless customer will have to dig out his walls and install 3-core and earth cable back to the light fitting.

    In some installations the loop-in is at the switch itself. This will be fine for you as L, N and SWL are all there. But the box will be mighty crowded already.

    In some installations (older ones and some with more complex lighting set-ups) there will be a lighting junction box remote from the light fittings. In theis box there will be L, N and SWL. At the light there will only be SWL and N, at the switch there will be L and SWL. So the product would need to be connected at the junction box, which may be buried under a floor, carpet, laminate, tiles etc…

    Have a look at the lighting wiring diagrams in the WIKI, and imagine how your average B&Q customer might install your product on a wet Saturday afternoon..
     
  16. ban-all-sheds

    ban-all-sheds

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    In a state of ignorance.
     
  17. endecotp

    endecotp

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    How much power does the "smart bulb" take when it is "off"?

    How much power do you require?

    If the latter is substantially less than the former, then you might have a chance.
    More likely you will need more than the bulb does, in which case you're doomed.
     
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