Time Glitch
November 8, 2007 12:13 PM   Subscribe

How can a digital clock run fast?

I have two of these digital clocks, exactly the same, and one has started to run fast. I've set it twice in three days...After about 24 hours it's an hour and a half ahead. ??? I'm already disoriented from daylight savings. What's up?
posted by cometwendy to Technology (13 answers total) 2 users marked this as a favorite
Digital clocks rely on a crystal or some sort of timer that generates pulses at a known frequency. The crystal can go off or if it's based on a capacitor that can go off too. If the pulses are short then the clock runs fast.

Read up on astable multivibrator circuits. Note that the frequency is dependent of the resistor and capacitor values. These are analog components at their core and their values can drift over time due to manufacturing defects. Even better circuits like a 555 timer or a crystal oscillator can suffer this fate.
posted by GuyZero at 12:21 PM on November 8, 2007

read up on Oscillators.
Specifically, Crystal Oscillators. Perhaps the crystal in your clock got damaged somehow? Or there is a short or disconnect in the circuitry? Or they didn't spring for the added cost of a crystal, and just went with a 'sloppy' circuit, which has gone south for whatever reason.
posted by blenderfish at 12:25 PM on November 8, 2007

The digital clocks we had at one of my old jobs would be fine until we switched over to the backup generator in the summer when power was at a demand. They would start drifting and by the end of the day be almost 30 minutes fast.
posted by Captain_Science at 12:41 PM on November 8, 2007

Not all digital clocks use crystal oscillators. Some cheap AC ones count the cycles that appear on the AC wall voltage which is supposed to be a steady 60Hz in North America. These are notoriously inaccurate since the frequency can vary quite a bit. Ane of your may be of this type.
posted by chairface at 12:42 PM on November 8, 2007

Actually, counting the wall frequency is extremely stable. It's the reason that an electric (i.e. not electronic) clock based on an AC motor is so accurate. They're multiphase motors and use the wall frequency as their timebase.

Probably the problem with this digital clock is that the crystal has gotten bollixed, as others have said. It almost certainly would cost more to repair than to replace.
posted by Steven C. Den Beste at 12:44 PM on November 8, 2007

Some digital clocks are based on an assumption of 60 cycle AC as their power source. If that varies, they get off.

In college dorm-dwelling days, I had a digital clock that persistently went fast when I was at school, but didn't have a problem at home. I eventually figured it out: The school used a campus-wide system to synchronize all the wall clocks, and the way it worked, believe it or not, was by raising the line frequency briefly at 5 minutes 'til the hour, as determined by a central clock. When the other clocks (which, of course, were specially made for this system) detected this signal, they reset themselves to the appropriate time. (It caused a noticeable little hum or clatter that people eventually developed an almost Pavlovian response to.) This change of frequency was enough, over time, to make my clock gain a noticeable amount time.
posted by Wolfdog at 12:47 PM on November 8, 2007 [2 favorites]

Counting wall frequency can be stable, but isn't necessarily.

Here in Orlando, Florida, our power co. counts the number of cycles over the course of the day, and if there's a surplus or deficit, then it spends the last few minutes of the day shifting phase to make up for it. They thereby ensure that the number of cycles in a 24 hour period is very near 60*60*24.

I get the impression from my informant that it was novel and rare.
posted by cmiller at 12:56 PM on November 8, 2007

Captain_Science: mains-powered clocks use the AC as the oscillator. Though there may be small short-term fluctuations, in the long term the power company makes sure it evens out and you get exactly your 50 or 60Hz. Your generator will approximate your local AC frequency, but because it does't have an accurate timesource, it won't do nearly as well.
posted by fvw at 12:58 PM on November 8, 2007

I've had clocks do similar things when the batteries were very low. Was a mechanical, not crystal problem.
posted by Mr_Crazyhorse at 1:51 PM on November 8, 2007

Two identical clocks, plugged into the same main, and one of them starts to run fast cannot be attributable to variation in the AC source power's frequency. They're both plugged into that source and if the power frequency changed they'd both change. Use your head, people.
posted by ikkyu2 at 2:45 PM on November 8, 2007

Is one clock cheaper than the other?

"> New street lighting systems in an East Midlands village have been playing
> curious games with alarm clocks, causing them to race up to four hours ahead
> while their owners slept.

"The electrical grid provides a kind of time/frequency service in addition to power---the frequency is held to 60 Hz (or 50 Hz) over the long run, and simple clocks count AC mains cycles.

"The problem with electronic clocks is that, if they are poorly designed, they can have much less noise immunity than their mechanical counterparts. The clock on the classroom wall won't care about a 10 microsecond spike on the power line, but a "red" clock may see it as an extra cycle. Lots of spikes, and you have a clock running amok. The new streetlights were probably injecting noise onto the power lines. (Notice that the clock always runs fast---a noise source can't remove cycles, only add them.)

"The solution is either a crystal-controlled clock, which must be periodically trimmed to the correct time, or a noise-immune cycle detector. (The synchronous motor in the mechanical clock is such.) . . ."

"> [ Can anyone explain why "red" clocks should be more susceptible to this form
> of interference than "green" clocks? ]

"I'm going to guess that the street-light timers in question might be using some form of RF carrier for coordination and control. The timer might be using a technology similar to the X-10 over-the-power-wires system sold here in the U.S., in which short bursts of low-frequency RF are transmitted over the power lines. Or, it might simply be a matter of some electrical interference (noise) being emitted by the timers.

"The X-10 signals are transmitted only during the zero-crossing portion of the 60 Hz cycle. Noise can occur at any pointin the waveform, but certain forms are more likely at the zero-crossing point. Now... most inexpensive AC-powered digital clocks keep track of the time by monitoring zero-crossings in the AC cycle... the long-term accuracy of the AC is better than any inexpensive quartz-crystal oscillator, and it's easy to implement the zero-crossing detector using a Schmidt trigger circuit. My guess is that the signals (or other interference) transmitted by the street-light timers was of sufficiently high power to spoof the zero-crossing detectors in the inexpensive clocks... this would occur if the amplitude of the interfering signal exceeded the hysteresis curve of the Schmidt trigger. This would cause the clocks to advance more rapidly than they should.

"As to why the "green" (plasma-display) clocks are less sensitive to the interference... I believe that the plasma-display technology is inherently more expensive than LEDs, and thus is used only in more up-scale clocks. These more-expensive clocks would have a better chance of having been designed with a more reliable zero-crossing detector... e.g. one which runs the low-voltage AC signal through a low-pass filter designed to prevent interference from spoofing the zero-crossing detector."

See also http://www.onefamilysblog.com/2007/09/cfl-light-bulb-humming-or-digital-clock.html
posted by Dave 9 at 3:02 PM on November 8, 2007

Actually, counting the wall frequency is extremely stable.

While this is likely true for most of the places MeFi members are from it isn't true worldwide. I used to live in Doha, Qatar, and plug-in electric clocks there were useless due to drift, at least at that time. Perhaps the country has stabilized the mains frequency since then. I suspect most third world and many second world countries don't keep the frequency very exact.

But that's not the OP's problem if only one of two identical clocks drifts. As noted, something has gone bad in the clock. It's probably not worth fixing unless it's expensive or still under warranty (90 days for most small electric goods in the US, I think).
posted by 6550 at 6:31 PM on November 8, 2007

Yes, something has indeed gone bad. As noted by many, some clocks derive their "clock" from the AC. Some detect zero crossing, others square up the sine wave and turn it into a pulse, etc.

A damaged crystal is more likely to simply fail than go crazy in frequency.

The most likely culprit in the circuit is a bad capacitor. Caps are used in the timing circuit and if one is acting significantly out of range, it'll mess up the frequency.

Replace the clock.
posted by jdfan at 12:18 PM on November 9, 2007

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