True. A *good* (and I stress good) switched mode power supply will become a constant current power supply when you exceed its
capacity. So if you short a 12 volt, 2 amp power supply, you will get no more than 2ish amps. If the UL or CE listing is accurate,
the power supply may get warm, but that should not be a problem. So to be safe, buy a switched mode power supply that can deliver
no more than the amperage that you need. That way, if something shorts in the clock, that is all the power it is going to get, which
is hopefully not enough to start a fire. To be sure, you may want to test this.
Unfortunately, there are many power supplies on the market that are unsafe for various reasons. Make sure to get one from a
reputable source, or have the power supply properly examined. Here are some articles and videos about unsafe power supplies:
http://www.righto.com/2012/10/a-dozen-usb-chargers-in-lab-apple-is.html
https://www.youtube.com/watch?v=wi-b9k-0KfE
https://www.youtube.com/watch?v=_exfmbsPqEI
http://hackaday.com/2012/10/10/raspberry-pi-foundation-looks-a-counterfeit-apple-power-supplies/
https://www.youtube.com/watch?v=q9wKjZXDPWI&t=9m39s (USB power supply begins
9:39 in, ends at 17:52)
Also, poorly designed power supplies can add noise to your A/C line or RF interference for nearby devices. Here is one power supply
that mostly works, but poor power filtering prevents a specific device from working (while other devices work fine):
https://www.youtube.com/watch?v=8wkoxZw53Sk
This video explains how a simple switched mode power supply works:
https://www.youtube.com/watch?v=bmtW_oA1fRo
On 02/21/2015 09:54 PM, Jeff Walton wrote:
Regarding your concern about the wooden housing:
I had an old nixie design that I did in college in the 1970 timeframe. It used voltage doubler circuitry for the nixies and
housed a filament transformer for the main power supply, which were all mounted inside the housing. My old design with point to
point wiring and discrete devices is dark ages compared to current designs. That clock ran perfectly for over thirty years with a
wooden and plexiglass case and was no problem - until 2005 when the voltage doubler shorted and caught fire. It had far more
power available inside the housing when things went wrong.
I think that the designs from present day with the external power supply (and limited energy) are quite safe to run with wooden
enclosures. The power supplies are quite safe and if there was a problem with something, it would most likely not be with the
clock itself.
On Saturday, January 25, 2014 at 12:10:21 AM UTC-6, Michel wrote:
I would feel a bit anxious to leave a wooden clock switched on for say 24
hrs/day, especially if it is not my own design. Just
worried it would catch fire at a moment I am asleep or not there. Is that
just me or do other people have that same worry?
I quite like the design though, not an immediate "wow" factor as some other
clocks, but I wouldn't mind having a clock like
this one.
Michel
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