One for the technical brain cells. I bought a LED light bulb from Pick & Pay. It claims to provide light for up to 4 hours in case of power failure. It’s got a battery build in that charge while the power is on. So far so good. It uses the normal switch to switch it on / off, but if the power should fail, it stays on or you can switch in on during the blackout. My problem is how does it know that the power was switched off as opposed to a power failure? It works as advertised.
My guess would be capacitive sensing of the circuit it’s connected to. It can probably baseline on what the capacitance of the wires are when the switch is in the off position vs. what it is in the “on” position.
Only four hours? LED lights typically last around 70 to 90 hours on common dry cells.
Instead of a simple explanation, we get this. Never even heard the word, nevermind understanding what it means. Think it’s maybe time to go back to my cave now. ; ???
It’s actually simpler than that, but it will only work in a circuit that operates a single bulb. The LED bulb manufacturer specifically mentions this single-bulb proviso: “It works for 2 way and 3 way switches but a switch can control only one bulb in the circuit.”
In such a single-bulb circuit, when the power to the LED bulb unit goes off, it tries to send a small current through the power supply line. If the current flows, it means the light switch is on and a power failure has occurred, in which case it switches over to drawing power from the battery. If the light switch is off, no current can flow, and the unit can thus tell the difference between a power failure and being switched off.
'Luthon64
Hi Tweefo. What needs to be remembered is that all plugs or lights in a house have two power wires connected to them. The live and neutral. When the live is connected via the switch the light or plug is “On”. When it is disconnected via the switch the power is “Off”. What many fail to remember is that the return conductor, the neutral, always has a voltage of 220V. An electrician will easily measure which is the neutral in a plug by measuring the earth contact to the power contacts. When off the voltage will register if the neutral and earth are measured. This is how the light “knows” that there is still power in the system and there is not a power failure. During a failure the voltage between the neutral and earth will be zero.
I’m afraid the above is incorrect. If you were to measure the voltage between earth/ground and neutral, you will measure only a small potential difference of less than 3V, which is due to stray self-induction from AC current flow in other parts of the house. With shielded cables throughout, you’d measure 0V. Measuring between the mains live and neutral terminals, or live and ground/earth, you’ll measure around 220V. You can confirm this yourself with a suitable multimeter that can handle AC.
In fact, except for earth leakage detection, the neutral plug connection is actually redundant in single-phase mains electrical supply because you could run any AC appliance by connecting it to live and earth, rather than live and neutral (try this only if your house doesn’t have earth leakage protection because it’ll trip immediately). At the power station’s generators themselves, neutral and earth are identical: A giant electrode sunk into the ground. Again, with a suitable multimeter, you’ll measure around 220V between the mains live terminal and something grounded, such as a water tap fed by steel or copper piping. (The resistance of the multimeter when measuring potential differences is so high that only the tiniest current will flow, and hence the earth leakage won’t trip.)
Also, if the passage cited above were correct, the single-bulb restriction wouldn’t be necessary.
While this stray self-induction may indeed present a possible mechanism for differentiating between being switched off and a power failure, it’s far too unreliable. With no other appliances drawing any power in the house, it’s likely to misidentify such a situation as a power failure.
'Luthon64
The occupational hazard of associating with clever peiple!! Did you understand Mefi’s post?
It’s every bit as simple and straightforward as the specifications for the turboencabulator…
It’s a tough word out there. Way back when, if a guy walks on water or changes water into wine, you just assume he was a god. Now we want explanations for everything but first, you have to go to school and all that, to understand said explanations. Better to just call her (Mefi) god, cut off a chicken’s head in her name, and pray to her if the light does not go on. However, if she can walk on wine I will have to spell god with a capital G.
It’s a tough word out there. Way back when, if a guy walks on water or changes water into wine, you just assume he was a god. Now we want explanations for everything but first, you have to go to school and all that, to understand said explanations. Better to just call her (Mefi) god, cut off a chicken’s head in her name, and pray to her if the light does not go on. However, if she can walk on wine I will have to spell god with a capital G.
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Yeah, but verily, remember that she can smite thee too. In fact, you sound like you’re a bit smitten.
Beyond a certain point on wine, I tend to stagger a bit. 
'Luthon64
Mefi, with all due respect, I have been doing this for 30 years and more. Open any plug in your house. Now, with a multimeter set on AC Volts scale above 220V:
- Ensure the switch on the plug is off.
- Measure the contact in the plug from neutral to earth. 220V right?
- Measure the contact in the plug from live to earth. 0V right?
- Measure the contacts in the plug from neutral to live. 0V right?
- Switch the switch to on.
- Measure the contact in the plug from neutral to earth. 220V right?
- Measure the contact in the plug from live to earth. 220V right?
- Measure the contacts in the plug from neutral to live. 220V right?
Anyone checking if a system is live always measures all conductors to earth, never from live to neutral or just between phases ignoring the earth. I have worked on systems with 1000’s of volts. If I was wrong I would have been fried.
Well foo, here I was thinking you were asking the question because you were electrically minded, but was willing to explain my theory further. However my theory was incorrect so blegh.
Here is a (higher grade) explanation of exactly how it works. In short (pun intended) it is indeed trying to send electricity through the circuit (house wiring) and if anything else in the cicuit (other lights, appliances, anything else plugged in lets even a teeeny little power flow it means the light cannot be switched off, since it’s still connected to the circuit (the switch is still on). I expect this also goes for any transformers on the supplier (Eskom) side.
Even easier explanation: It’s checking if anything else is connected to the power line it’s connected to. If so, the switch is not off, and since it’s not getting power… it means there’s a power failure.
SKEPRat, with all due respect for your decades-long experience, my SO just performed your stepwise protocol with me observing (because we have nothing better to do of a Sunday night are obsessive fact-checkers). I encourage others on the forum to do likewise so that they can ascertain for themselves what’s true. Here are the results:
Done.
Incorrect: 0.168V measured. (In fact, this simply can’t be right because it would mean you can be shocked by sticking, say, an iron nail into the neutral terminal of a plug that’s been switched off.)
Near enough: 0.172V measured.
Near enough: 0.023V measured.
Done.
Incorrect: 0.273V measured.
Almost exactly, as expected: 220.6V measured.
Again almost exactly, as expected: 220.1V measured.
The erroneousness of points 2. and 6. individually and collectively refute your claims, and are in line with what I wrote before. Ergo, I stand by my earlier assertions.
'Luthon64
Im in awe of seeping intelligence… I often imagine whilst conversing with a uniqely intelligent person that their heads explode and the brains splatter across the walls. How do they hold that much knowledge in the confines of a humble (or perhaps not so humble) package of grey matter encased by bone?
Bit crude, but thats my brain!
Thank you all, There is now a glimmer of light in the back of my cave. 8)
Hi Mefiante
The 0.168 Volt or similar on a modern electronic digital display multimeter is normal for that instrument. This is why aspirant electricians are first taught how to use and interpret a digital multimeter before using it. If you had used an old moving coil type multimeter you would have seen 0 Volt, as the above 220V scale would never allow you to read 0.172 Volt or similar.
I can only assume that the measuring points were incorrect on the points you did not get the required results on or your technique was incorrect. Get any electrician to measure those points as I stated and you will get the results I stated.
There is a vast difference in results if the experiment is performed incorrectly.
You can be shocked by inserting a nail into the neutral socket with the plug switched off. My exact point in answering this post. The neutral is always at a 220V potential above earth if the power is supplied or if you earth yourself in any way. Eg. Standing in a pool of water or touching a brass window handle on a steel window frame. Or touching the earth socket on a plug. The exact reason the SA and British plugs have a barrier to inserting anything into the sockets. Never assume your earth leakage protection is working either.
Also don’t assume the wiring in your plug is correct. A fatal mistake for any electrician. The colour codes may be swapped. Try the experiment assuming the codes are incorrect. Never take anything at face value with electricity - always assume that it is deadly and check. The erroneous points 2 and 6 indicate incorrectly coded wiring or swapped live and neutral connection points.
Another fact is that a meter never measures the highest value of the Voltage in the circuit. It measures the Root Mean Squared (RMS) voltage which is 0.707 of the peak voltage of the sine wave.
Not so, as all four other readings agree, near enough, with what both your protocol and I said they should be.
Clearly, this isn’t true, since in both cases we measured a voltage of less than 0.3V between E and N.
Agreed—which is why we took all of the other measurements as well and reported them here.
Again, not true, since each of the other four measurements is in agreement with what we both claim they should be.
I understand RMS quite well in the context of wave phenomena, among which are AC measurements and voltage/current specifications: You integrate the square of the voltage amplitude over a full wave period, divide by the length of that period, and take the square root of the result. For a sinusoidal wave as is AC, this works out at 1/√2 of the magnitude of the peak amplitude, and 1/√2 ≈ 0.707. If you have three-phase power in your home, the RMS voltage between any two of the three phases is nominally 380V, whereas it’s 220V between any one phase and neutral (380/220 is near enough a factor of √3, which also isn’t a coincidence). But this fact alone already seriously challenges the claim that “The neutral is always at a 220V potential above earth if the power is supplied…” since the three phases are shifted by 120° relative to each other.
Here’s a pair of photos showing what was measured at a different plug in respect of the two contested readings. The other four readings were, again, in agreement with what we saw before:
In each case, you can see whether the plug is switched on or off, which poles are being measured, and the reading on the multimeter’s display. It will be noted that the readings are in “mV” or millivolts; in volts, that’s the reading divided by 1,000.
Consequently, I still remain at the point where I began.
My suggestion in order to put this matter to rest is that some other forum regular performs these same measurements and then reports their findings here independently.
Will someone please step up to the plug? I mean “plate.” Anyone? Please?
'Luthon64