I knew that. 
Help me identify mystery box
- Thread starter rossjamesparker
- Start date
Sponsored Links
Yes, it might be 3-core. Those photocells do require a neutral (and PIRs, which this item obviously isn't, will inevitably always require a neutral).
Kind Regards, John
Intriguing - thanks. Unless they contain a battery which charges whilst the load is drawing current (or let a little current flow through the load even when it is 'off'), I can't for the life of me see how they could power the electronics when the load was 'off'! (and, even with that, the battery would eventually go flat if the PIR were never activated - maybe a solar-charged battery?)
Kind Regards, John.
Sponsored Links
Havent really looked close but they do have some sort of battery,
There is a small dip switch which you have to use to switch it on after installation
There is a small dip switch which you have to use to switch it on after installation
There is a fairly simple trick they can use. If you delay turning on the load until the voltage has risen to a few volts, then you can use that to power the electronics (through the load to neutral). The effect would be to reduce the lamp brightness very slightly, though probably not enough for anyone to notice.
The standard method/'trick' for getting power for the electronics with '2-wire' devices is to stick a small resistor in series with the load - thereby, as you say, 'stealing' a small amount of the voltage, which thej gets rectified to power the electronics. However, that only works whilst there is current passing through the load. Hence, if the load ever gets 'completely switched off', the electronics would lose their power (and therefore not be able to switch the load back on again), unless there was a battery.
Kind Regards, John
That makes sense. In the absence of a neutral, there really are only two possibilities - either a battery or else some current flowing through the load even when it is 'off'. However, as I said before, if it's a battery, it will go flat if the PIR rarely, if ever, activates - unless there is a solar charging facility.
2-wire dimmers are obviously different, because their electronics is not required to function when the light is turned off.
Kind Regards, John
Commonly done, which is why it's common for them to be unsuitable for fluorescents, LEDs etc. And why people sometimes report such lamps flashing.
That makes sense, but the one to which you linked says "16 Amp max load regardless of load type including resistive loads and incandescent/fluorescent lighting". If it works 'regardless of load type', I would have thought it would be difficult to arrange for a small current to reliably flow through the (any) load when it was 'off', wouldn't it?
Another point, not insoluble, is that it would have to switch to a different electronics-powering circuit when the load was 'on', since the 'switching on' of the load would effectively short the series resistor which had been used to derive the power when the load was 'off'.
Kind Regards, John.
I think you missed what I'm describing.
Firstly, consider a standard light switch and light. When the switch is off, there is the full mains voltage across the switch - and if your electronics run on little enough power then you can use that to power it. This does mean that the power used by the circuit flows through the load when it's off - hence the "flashing CFLs or LEDs" mentioned by BAS.
Now replace the switch with a dimmer. At the end of each half cycle the electronic switch (usually a triac) turns off. On the next half cycle, the mains voltage starts rising which means that there is now a voltage appearing across the dimmer. After a delay, the triac is turned on, the voltage across the switch drops to 'nearly zero' and the mains voltage is applies across the load. The longer you delay turning on the triac, the less light you get from the light.
During the initial "off time" at the beginning of each half cycle you have the opportunity to charge a capacitor (through the load) to power the electronics for the rest of the half cycle until you can get another quick charge. Doesn't need adding resistors across the load or anything like that - just a means of picking up the voltage across your switch at the start of each half cycle.
Errrm - I'm not sure that it is I who is doing the 'missing'! We were talking about PIRs, not dimmers, and I've already said that there is no comparable issue with dimmers.
With a dimmer, one can do as you suggest (charging the power supply's capacitor during 'off' parts of the cycle), or put a small resistor in series with the load (and do the charging during the 'on' parts of the cycle). What one doesn't need with a dimmer is for the electronics to be powered when the load is switched off. With a PIR, one obviously does need the electronics to be powered all the time - so, in the absence of a battery, the only option is to have a very small current flowing through the load even when it is 'off'.
Kind Regards, John
Whoops
For "you", please read "333rocky333"! However, my point about what it says remains.Kind Regards, John
Without wanting to get into an argument, you have still missed the point. I introduced the dimmer to explain the operation - bear with me here.
Consider we've got a dimmer, and it's set to "almost" full brightness, so we have a short period 100 times a a second where we can collect a bit of power. With me so far ?
Now remove the "variable" bit from the dimmer - so we have a "switch" which is either "off (so we have the full mains voltage to play with), or it's "on" which corresponds to the situation described (we get a short period to collect power 100 times a second). Now operate that "switch" with a PIR sensor and timer - hey presto we have a two wire PIR switch which needs no separate power supply.
Delaying the turn on of the triac needn't be complicated - all we have to do is wait until the voltage across the device reaches a certain level (which will have charged our supply) before triggering it.
Also, bear in mind the level of power consumption the boffins have got electronics down to these days. I've just fitted an electronic thermostat for a friend - it's a 2 wire device and uses batteries for it's own supply. The destructions say the batteries should last for at least a year - that's 2x AA cells, over a year. That's some fraction of a mA average maximum power usage at 3V.
An alternative would be to have a larger reserve of power - there are "supercaps" which can store phenomenal amounts these days. I suspect with the right electronics (see above about power consumption) these could provide power for several hours (or longer) of device "on" operation. Should you need longer, then a very brief "off" period (could be just 1/2 a cycle or less with a triac switch) would recharge it - probably without the user detecting it.
And finally, have we completely discounted the possibility of the electronics being powered by the sensor itself ? Back when I were a lad, I recall my dad had a light meter to go with his camera - no batteries, just a meter powered by a small "solar cell" of some sort. The more light, the more power from the cell, and the more the meter moved.
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
Sponsored Links
