On Wednesday, May 1, 2013 4:26:14 PM UTC+1, John Williamson wrote:
whisky-dave wrote:

On Wednesday, May 1, 2013 2:54:09 PM UTC+1, wrote:

On Wednesday, May 1, 2013 1:54:36 PM UTC+1, whisky-dave wrote:

But you need a dedicated clock you can;t use the average MPUs clock as it's too inaccurate, you need a proper RTC, you do know that computers keep pretty crap time unless they get it from a server.

You wouldn;t use that method anyway as you need to mearuce the actual frequency of teh main not simulate it.

and time the period from local zero crossing to mains zero crossing, and log the latest value to see how far it moves.

Not necessary.





All one need do is



a) xtal control the cpu clock, an xtal with no oven is more than accurate enough to read 50Hz +/- 0.1%.



XTAL DO NOT READ FREQUENCIES and niether do CPUs.



No, but you can use even the cheapest CPU to count clock cycles with a

smidgeon of machine code.

Why count clock cycles ?


Add an external circuit to flag zero crossings

of the mains waveform,

why would you need that ?


and it is then trivial to calculate the mains

frequency. It is also trivial to write a prgram and design a circuit

that will automatically turn off a load if the frequency drops below a

certain level.

yes it is but is that what you want.., probbaly not you'd want flexability if you're controling things with smart meters.


A cheap resonator clock is well within the accuracy meeded for this

application.

Yes I know but that's not what required yuo need contol or rather they do.

What they also what which no on eseems to realise is have the ability to turn off yuor aplinces should you not pay yuor bill, legally I don;t think they cvan just cut you off without going through a lot of hassle but if you don;t pay and they switch yuor kettle down to half power ........



b) count a divided version of the clock from one mains zero crossing to the next. Problem solved.



That's not the way to do it, you don;t need zerop crossing wither until you decide to switch something even then zero crossing has nothing to do with it.



Wrong. Counting zero crossings and comparing the timing to a refrence

frequency, such as the clock controlling a microprocessor is the easiest

way to work out the frequency of a signal,especially the relativiely

pure sine wave that comes down the mains supply.

Good you know it's a sine wave which will change in amplitude as well as fequency which makes zero crossing more difficult and less useful.



The slight frequency spread of the xtals prevents all appliances going off at the same time.



That's not really relible, XTALS have a very high accuracy when made, but temperature and series as well as parellel capacitance play a part.



Wow, you got something right. Even after aging and cooking, cheap

crystal and ceramic resonator frequencies are accurate to within not too

many parts per million. You're trying to check frequency stability of

the supply to within parts per thousand.



If yuo really need to switch things off then measure the frequency of the grid and if it's below what you see as a miniumium then send a single to turn off particular devices that are in yuor home that the smartmeter recognise.





So you're proposing that appliances have 2 way communications,



No they don;t need two way communication, any more than your TV does.



They do, otherwise how does the smart meter know what's switched on to

turn off?

You send an off signal, same as they do when BBC3 goes off line, I don;t need to send a signal back to the BBC.


Minimum is a "Hello, controller, I am XXXXXXX" when switched

on, and "Goodbye from XXXXXXX" when turned off manually, and the

capability to receive "XXXXXXX Turn off now" and "XXXXXX Turn on now".

Not needed.

Why would yuor kettle need to tell anyone it's turned off or on. ?




and that smart meters talk to each appliance in each house. Doable of course, but its more money, so not attractive.



Why is it more money ?



More complex circuitry. the frequency measuring can be done for about a

pound a unit by the maker,

So ONE POUND for the whole house as it will be put in the smart meter.
If we know the national grid is runnoing at 49Hz why both measuring it in each aplince in each house in the country ?


and if there's already a microprocessor in

the unit, it comes down to a few pence once the design costs have been

written off, the communications requires about a fiver.

Not evenm that as USB or TCP/IP cound be used.




Be silly to have to measure the frequency of every device.





Either you measure locally or decide centrally and communicate. Do you think 2 way communication's cheaper?





Yes.





I'm struggling to see how enabling 2 way comms and playing with MACs is cheaper than a divider in the chip.

Because everything is already in place and dessigned.



What do you mean by a divider in the chip, which chip are you refering to ?



The cheap, 4 or 8 bit CPU in the appliance. A new use for a Z80 at last.....

Why a divider what would that acheive ?



Measuring low frequecies accuratly is more difficult than you think.

What you have to do is count cycles in one second yuod get perhaps 47 or 53 so you need to average it out especailly in times of high or low demand.

So you need to sample it for at least 10 seconds to giove you 0.1 resoultion and in that time the situation can change.





Its not hard at all, just time the number of clock cycles between 2 successive crossings.

Which would be about 1 but maybe ther;e s two zero crossing per cycle or period depends how you choose to count them.





Two crossing of what ?



Zero voltage of the supply in this case.

Two during 1 cycle or 1 per cycle ?




You could average lots of counts but I don't believe there's a need to, a small percentage of mistimes is immaterial in practice. Simply averaging several yes or nos would be trivially easy.


Yes ot no's to what exactly .



Yes, the frequency is too low, no it isn't.

yuo'll have to count it first and that takes time at such low frequencies.



Anyway it would be cheaper to have just one device measuring frequency rather than every electrical device each reading it. If I were designing such a thing it'd use teh smart meter to measure it rather than install the ciruitry in my kettle and fridge.freeze, TV cooker shower , washing machine, coffee maker storage heaters fires hair dryer etc.....





but you'd still need to install everything in the appliances,



Yes of course unless you turn the whole suply to tteh hosue off.



?\The unit would be installed in the factory,

what unit what factory ?

with an option to fit it

to older machinery as well. To retrofit it, the only work needed in the

houseis to unplug the appliance. Your way means installing a new meter,

which *does* require disconnecting the supply.

Yes the meter people come around and install a new meter free of chrge.



idf all 2KW+ kettles had a 1/2 power switch then my first try would be to turn those kettles is 1/2 power them, I wouldn't turn off motors or mmoving things such as food blenders.





I'd agree there. There are many things can be turned off or down if its ever worth it. Today its mostly not though.

To who....
I'm not sure it'd be worth doing this sort of thing, but some think the smart meters are goong to contol everything even toothbrushes, my toothbrush is wireless BTW, I'd like to say b;luetooth but I'm not totally sure it uses that methode, if it's cheap enough to put into a toothbrush why not a kettle or a TV or fridge freezer ?


I agree so why bother ?

As I said before better to turn off scotland with all those deep fat fryers ;-)



You'll have Scotty on you before you know it if you suggest that.

Lots of sciotties I'd assume although I doubt they'd catch me





--

Tciao for Now!



John.