Ripple control is a technique used by electricity companies to control loads. Its rather primitive, but that’s because it dates back to Before Computers, and its still done today with ingenious electromechanical hardware. The idea is that the electricity company can reduce peak loads by broadcasting signals over the mains wiring, that switch off consumer’s appliances. This may sound like a Bad Idea – why would anyone agree to it? But there are some sorts of quite substantial loads which can be switched on or off by the electricity company without causing any grief, the prime example being household water heating. The thermal mass and insulation of your hot water cylinder mean that it doesn’t matter if the power is switched off to it for an hour or so. So all hot water cylinders are required to be on a seperate ripple-controlled circuit, which is what one of those mysterious boxes in your household fuse box is doing. And every day, the electricity company sends out signals to turn it on and off. They also send out different signals to turn on or off various industrial loads, street lighting, and so on.
So far, so good. The electricity company gets to reduce peak loads, which is important to them as peak loads define how much they need to have of everything – generators, transmission lines, the lot. However, the design of the system is, as I said, a bit primitive. The signals that they superimpose on the 240V 50Hz mains have to be (a) low enough in frequency that can be detected by electromechanical means and (b) small enough they they don’t interfere with the operations of normal appliances. This means frequencies down in the 100-1000Hz range, which also happen to be audible frequencies. Now, whatever signal they use is going to attenuate as it travels through overhead wires and transformers and so on to get to you, so to get a reliable minimum signal to their furthest consumers, they have to apply a rather larger signal than the minimum at the injection point, which will typically be in a substation somewhere.
The upshot of this is that depending on how close you are to a substation – and a few other factors – you may well be able to hear the ripple control signals, as the transformers or motor windings of your switched-on appliances resonate with them. If you hear a series of buzzing or humming noises that switch on and off at intervals of a few seconds and occur at regular times of the day, you are probably hearing ripple control signals.
The reason I’m posting about this is that ripple control noise can actually be rather annoying. In our case we had installed a ceiling fan over our bed, which was great, until it became clear that if we left it on overnight, it resonated quite loudly in a silent bedroom when the ripple control came on. At 11pm. And 5am. And 6am. Unfortunately, I’m a rather light sleeper, and after being woken up a few times I was fairly motivated to Do Something About It.
Electricians – or at least the ones we have asked – look rather blank when asked about remedies for this sort of thing. There certainly aren’t any aftermarket filters for ripple control noise that can be fitted to vulnerable appliances. There are any number of mains filter devices, but they seem to invariably operate at radio frequencies, which are way above the audio frequencies used by ripple control.
If the noise couldn’t be filtered out, the only other solution would be to turn the fan off. We could have used a timer to do this, but that would require a suitable electronic timer that could do at least three “dead times” a day, and an electrician to install it into the fixed wiring. This would cost at least a couple of hundred dollars, and it assumes that the electricity company are going to do ripple control at the same times each day. It would still be prone to waking us up if some emergency signalling was necessary at, say, 3am. A much better solution would be to detect the signals when they actually happened, and turn the fan off. So I designed and built a device to do this, mostly using parts from my junk box.
The schematic and construction details of the device are here.