Battery Backup for Clock Radio

[WildBoar]

Before surface mount components, hybrid
modules, and super miniaturization it was
easy to float a battery across a small
gizmo like a clock radio to prevent memory
lose in the event of a power failure.<p>A couple of 1N4001 diodes, a battery, and an
optional resistor depending on whether the
battery was rechargable or not.<p>Now I'm actually a little nervous about
messing with the digital clock radio I just
bought. For some reason, you can buy a
clock radio that has a battery backup with
old fashion analog tuning or a clock radio
with programmable memory presets without
a battery backup! <p>I know this seems very strange. After checking
out several large stores in my area I realized
there was no clock radio available that had
memory presets AND a battery backup!<p>Is it still possible to float a battery across
the rectified AC power supply, (like we did in
the good old days) or have the guts of these
devices become so integrated that the risk of
static damage, cutting component leads, etc.,
make it too spooky to tamper with these
appliances anymore? <p>WildBoar

[josmith]

The Timex nature sounds clock radio has presets and backup. It actually keeps accurate time on backup.

[haklesup]

I have actually had corroded back up batteries reduce the useful life of some products. If you rarely have a power failure it is easy to forget that the battery in the clock may be several years old.<p>As a manufacturer, it may be better to force your customers to reset the clock than it is to get customer complaints or returns. Not sure but this may in part explain the absence of such a once common feature.<p>I don't see why you cannot float a battery across the supply nodes but be aware that the battery will power the display during a blackout further reducing its holding time. A properly designed backup would blank the display to save power. <p>Also, good luck finding a battery of just the right voltage if it was not designed for that. It would need to be just a little lower than the DC supply so the blocking diode does not get forward biased but high enough to power the clock and have enough capacity to outlast a typical blackout. Slip an ammeter in series the supply output. If the current is in the 10's of mA or higher, the battery would not last long anyway.<p>I would expect that technology to back up a clock without a battery now exists in the form of super-caps but I am unsure. A clock of any type would take more power to backup than say an SRAM because it is actively running. In low power CMOS the current is nearly 0A in static conditions but each toggle of a gate inside the device causes a current to flow. <p>Most low power backed up clocks I have seen like the BIOS clock in my PC are coin battreries of some type and usually a high capacity type like Lithuim.<p>Check out the clocks at sharper image, they have a large selection but not necessarily cheap prices.

[WildBoar]

Darn it, josmith!!<p>I saw that Timex Nature Sounds clock, pulled
the box off the shelf and read it's features.
I guess the manufacturer didn't bother to
list the memory preset feature on the box.
At least I don't remember seeing it listed.
I looked at so many clock radios. Everything
starts to blur in your mind when you try
to remember which clock had what features.<p>A very special thanks to haklesup!<p>I never expected anyone to provide such a
complete and thoughtful response to my posting.<p>Selecting a battery (or battery pack) with
the proper voltage and current ratings could
certainly be a daunting task.<p>From the manufacturers point of view, leaving
the backup battery out of the design altogether
is probably a good idea. After all, once
you install the a battery the odds that you
you will ever think about it again are, well,
not to good!<p>I remember experimenting with super-caps when
thye first were available through the hobby
magazines. They are amazing components. I
know they have been incorporated into various
low current devices instead of batteries.
Under ideal conditions they could provide
anywhere from a few seconds to several minutes
of power, depending on how much effort was
put into designing the device.<p>Thanks guys!<p>WildBoar

[tonybackache]

My local WalMart Super Center is closing out a RCA Brand Digital Alarm/Radio. Battery back up & preset stations. $14.00

[Chris Smith]

Things to remember.........<p>All Electronics devices runs off DC at some point, other than the signals, etc. <p>All devices inside somewhere, at some point,.... run off DC<p>All dc, can be substituted by a back up dc supply, if you can figure out where to tap in up stream? <p>Exception these rules,...there are too many to mention!!!!!<p>You get the drift?