Hi,
I since I like "low power" I frequently use resistor in MOhm range. Mains hum often causes problems for me. Now I got an idea how to "strike back"and use it for good cause - I would like to use it as a clock source (I also thought about using it as power source but I doubt I could get enough power from it - or maybe...). So I did some experiment and it looks promising. But after Googling I found little to no such projects. So I would like to ask if there is some catch - such as mains hum is present only when "something" close is drawing power (and turning off light and computer will remove it) or glitches are so common and hard to remove that mere watch crystal is more reliable?
Mains voltage is an AC signal so all you have to do is get the voltage down to something save and put it through a Schmidt trigger and you'll get a nice 50 Hz block signal, which should work fine as clock signal.
Hi,
I think you will find that although the HUM is at mains frequency, it will not always be there, of consistent amplitude to give you a reliable clock pulse.
If the source is a machine or electrically noisy device, you may find that harmonics and noise may contaminate the waveform.
Unless you have the hum coming directly from a known consistent source.
If you have the time go for it, worth the experiment.
Tom... 
Certanly in England the mains is a very accurate and reliable frequency source.
To use it I'd suggest an X2 rated capacitor (0.05uF?) in series with a largish (2 x 100k - 1W in series ) resistor driving two 4.7v zeners in parallel ( in case one fails - my 'intrinsic safety' background ) as suitable bit of signal conditioning.
Observe 4mm spacings between traces/wires.
Allan
I don't want to connect to mains directly - I would like to sense the AC wirelessly and as part of a portable something. I have no concrete plan yet but I think it would be nice to have "free" very (long time) precise time source. When I get some time I will look at this and see if it is possible.
You can also get the time free from GPS. The big advantage to GPS time (or internet time) is that if you loose the power or the time-signal, it can be automatically re-set... When the battery in your watch dies you have to re-set the watch after replacing the battery. When the battery in your phone dies, you don't have to re-set it after re-charging.
Regular electric clocks (the kind with a motor) use the power line frequency. In modern countries it's super-accurate and the correct it over time so your clock should be "perfect" as long as you never loose power.
Hum (electromagnetic radiation) is going to vary depending on what's powered-on around you so it's going to be unreliable but the frequency will be stable (except for other electromagnetic noise). When you say "portable" the AC radiation becomes more unstable/unreliable.
I would like to sense the AC wirelessly and as part of a portable something. I have no concrete plan yet but I think it would be nice to have "free" very (long time) precise time source.
Although the energy consumed from the power line will be immeasurable, you will be consuming energy and the source is the power line. You'll have an (inefficient) transformer or capacitor.
I did a quick-and-dirty (and possibly inaccurate) estimate of 120VAC across a 1M resistor and I got a little more than 100 Watt-hours per year. That should cost less than 2 cents. You could use a 10M or 100M resistor to reduce that further. And, you'd still need to power the clock.
allanhurst:
Certanly in England the mains is a very accurate and reliable frequency source.
I expect its the same across the whole of the UK.
It's really mains frequency versus battery/crystal. If the mains power goes out, then the mains frequency method (regardless of direct connection method or wireless method) is out of action, so would need to fall back on the usual method anyway (eg crystal).
Yes, the mains frequency is very stable. The continuity of the mains power is not stable over time. There are various switching transients that may cause a dropout for several cycles. In the US, a shorted power line somewhere may cause a circuit breaker to try a different connection to restore the power 3 times. If that doesn't work, then your power is out for the duration.
Paul
Hi,
I think you need to re-read the original post and take it in the spirit it is intended.
The OP has an idea he want's to pursue for personal interest, it doesn't involve connecting to the mains, he has a very noisy environment and want to see if he can exploit that phenomenon in his interest in low power devices.
More power to him (pun intended), if he can get a good reliable signal that can be used in his situation good, if not the just as good.
In either case he will have learnt something in his endeavors.
Tom...
Certanly in England the mains is a very accurate and reliable frequency source.
And indeed a lot of Europe. We have undersea power lines across the channel to use France's large nuclear power sources from time to time. It's all phase-locked through the Europe-wide grid system.
But, of course, not totally reliable . There can be local switching transients and power cuts.
Maybe a RTC as backup?
Allan
Hi,
allanhurst:
And indeed a lot of Europe. We have undersea power lines across the channel to use France's large nuclear power sources from time to time. It's all phase-locked through the Europe-wide grid system.Allan
We have an under sea power link between Victoria and Tasmania and it is a DC Bus, to prevent cable losses if AC is used.
Tom..
I looked - that's a serious project !
How is the ac -> dc 400kV achieved ? - and - more interestingly - the 400kVdc -> 50Hz ac at the other end ?
I don't expect they use arduinos.
Just a guess
Allan
TomGeorge:
Hi,We have an under sea power link between Victoria and Tasmania and it is a DC Bus, to prevent cable losses if AC is used.
Basslink - WikipediaHVDC Cross-Channel - Wikipedia
Tom..
We have a similar set up here between the Dalles dam on the Columbia river and Los Angeles. This runs at 250,000 volts DC. They designed it to run at 500,000 volts DC, but had to use the ground for the return line. Raised hell with all the pipes in the ground and all the electric fences between Oregon and California, so limit is 250kv.
In the 1970's, a group from the Oregon Amateur Radio club had a tour of the Dalles converter station. At that time, we got to go walk through the actual array of mercury vapor controlled rectifiers. Huge devices. Now all solid state.
Then it was controlled on both ends by a DEC minicomputer. I don't know what they use today. Power could be reversed and sent back from LA to Oregon.
The operation was begun at the 3-phase dynamos in the Dalles dam. Six of the dynamos were phased to produce an AC of 18 phases, then through transformers as applied to the rectifiers to produce the DC.
On the LA end, the DC was converted to AC using the controlled rectifiers to switch the DC input to transformers.
The computer control could switch directions of the power in just a few seconds, and also was able to shut the whole operation down in just a few seconds during an earthquake in LA.
The link from Australia to Tasmania is under sea, so a single set of wires with ground return would not cause a problem. Perhaps they have good enough insulation to allow a complete wired circuit. Must be many wires of huge surface area.
Paul
If they're using ground return for 1/2 the dc, what's the electrolytic erosion at the electrodes like?
Allan