On 22/03/2021 21:57, Martin Brown wrote:
On 22/03/2021 18:58, Scott wrote:
How does my car clock work?Â* It cannot be quartz or radio controlled
as it runs slightly fast.Â* It cannot be synchronous because the supply
is DC.Â* Is there any form of adjustment to get it to run to time?

It is almost certainly quartz crystal and not quite trimmed right. Such mistakes were very common. One telescope maker fixed it by adding GPS functionality to reset their RTC to local time. Mine as built from a previous generation lost 15s/month reliably. They didn't install the loading capacitors around the crystal to save money ( about 10c ).

It isn't hard to get a quartz clock good to 1ppm or about 30s/year but only the better manufacturers actually bother to calibrate them.

It is slightly harder for a clock in a car than a watch on someone's wrist which tends to be temperature compensated by the wearer.

I've had quite a number of quartz wrist-watches over the years. The first, bought during a visit to the USA a long time ago, cost under two dollars. None have cost me more than £20. All of them have been accurate to a few seconds a month which is good enough for me.

So it is disappointing to find that car clocks, mine among them, are out by a minute or two a month. I think it's partly the car environment - wide temperature variations make it hard to get a quartz crystal to keep time. The car companies know that it's difficult to get accuracy without a lot of expense, so they don't even try, they buy the cheapest crystals then can get and don't bother to trim them.

On the other hand, most cars now have a sat-nav system so it would be easy to link them to reset the dashboard clock to GPS time at intervals, but that would cost a few pence more per car during manufacture. Since few of us decide against buying a new model of car because it has a poorly designed clock circuit, the problem will persist.



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