Why AC Digital Clocks Gain Time

[Wizard_Of_Oz]

I have seen a number of comments on this site about AC operated Digital Clocks gaining time. I was perplexed about this for several years. I bought different clocks, and some had the problem, and some did not. I began to suspect something with the power line.

About two years ago, my electric meter was replaced with the "Automatic Reading & Reporting" type. The problem seemed to coincide with that change.

I took several of the problem clocks to my cabin on the lake where it on a manually read meter. Those clocks perform flawlessly at the lake cabin. I brought one back home and it misperformed in the same old way.

I have concluded that some of the clocks do not have the necessary filtering to remove the digital pulses that the "Automatic Reading & Reporting" meters send back to the electric company. These additional data pulses cause the clocks to gain time........

 

[CocaCola]

More likely they are being timed directly off the AC frequency and there is a minor variation in the frequency between your two locations...

From http://en.wikipedia.org/wiki/Utility_frequency

Regulation of power system frequency for timekeeping accuracy was not commonplace until after 1926 and the invention of the electric clock driven by a synchronous motor. Today network operators regulate the daily average frequency so that clocks stay within a few seconds of correct time. In practice the nominal frequency is raised or lowered by a specific percentage to maintain synchronization. Over the course of a day, the average frequency is maintained at the nominal value within a few hundred parts per million.

Read further on that page and it goes into more details...

I certainly can't speak for every utility company, but I'm willing to bet that several of them are less then diligent on those 'corrections' since digital clocks with crystals are more common today...

 

[(*steve*)]

You might also find that some clocks are more sensitive to powerline interference than others. noise at a high enough magnitude will appear as additional clock pulses.

 

[Harald Kapp]

More likely they are being timed directly off the AC frequency and there is a minor variation in the frequency between your two locations...

While it is a matter of fact that many clocks are timed off the mains frequency, it is unlikely that variations of the mains frequency are the culprit for clocks advancing too fast (it also doesn't explain why some clocks do better than others at the same location). Unless the locations are very far apart and on different grids. The reason is that within one grid the frequency has to be the same for all locations. If there were variations in frequency this would lead to uncontrolled power flow through the grid due to the accumulating phase difference between the locations.
On short terms there are fluctuations in frequency, but again they are the same throughout the grid.
Generally, in the long term the utility companies usually try to keep the mean frequency very stable. They do this by increasing or decreasing the frequency for some time such that over a period of e.g. a month the mean frequency is rather exact.

What is likely is that one location has a "cleaner" mains than the other. The mains voltage ideally is a pure sine, but in reality it shows distortions (transients, voltage dips, high freqency components etc - look up "power quality" if you want to know more about this). At your cabin at the lake the mains supply probably come via long distance lines and transformers which act as low pass filters, thus letting the clock "see" mainly a quite clean 50 Hz or 60 Hz signal.
At your main location - I guess in a populated neighborhood - the mains is probably distorted by lots of transients (e.g. every time the refrigerator starts or stops). The clocks may count these transients as additional clock pulses and thus advance faster than they should.
This also explains why some clocks do better than others in the same location: they simply have better filters in the clock circuit to filter out these transients.
Adding a suitable filter between the clock and mains should improve the accuracy of the clocks.

An interesting site is here: http://www.powerstandards.com/PQube.php#outputs
Click on "test drive a pqube". From the map select one of the PQubes showing a green light (preferably select the one from PQube in Ontario). You may now look at actual values from that particular PQube, including stats and trends in csv format.


Harald

 

[David Smith]

My friend John has a clock that on one side of the house it gains time. But on the other side of the house, it works fine. I think he needs to check the line voltage with an oscilloscope for noise.

 

[davenn]

this is a very old thread that you responded to @David Smith

The OP never did come back

thread closed