Potenially dumb question about power, diodes, caps, and DS1302
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Here's my problem: I have a DS1302 (clock/calendar chip) that I want to keep
going for days, when the power fails. I know the chip has a trickle charger
built in, but I don't want to do it that way (because I don't want to mess
it up and fry the chip; I don't know what values to set in the DS1302 to
charge this mother of all caps I'm using, and mail order takes time.)
So I thought I'd just do something really simple. I have a 5v power supply
running a BS2 and the Quadrovox voice module. It aso runs the DS1302, but
between +5v and the DS1302 pin 1 (5v in) I have a diode, and after that, a
big cap (161,000uf, 10vdc - the thing is the size of a soda can) across the
DS1302's power in and ground. Ascii art below. The idea is that the diode
lets power in normally to run both the DS1302 and charge the cap. When the
power fails, the cap runs the DS1302, but nothing else, because of the
diode.
This frequently works fine... except when it doesn't.
Sometimes when I unplug power, (and then talk to the DS1302 via a separate
BS2, which is still powered and shares a common ground), it reports it's
happily keeping time, running from the cap. But sometimes it reports that it
isn't. It's not the that cap is running out of juice: when it fails, the
test meter says the cap is still at 5v. And when it works, it can keep time
for (at least) minutes this way. When it fails, it seems to fail the instant
the power pull is pulled.
The power supply is trivial: one half of a 12v transformer rated to 0.45A,
through a full bridge rectifier, across a 6.3v 3300uf cap and then into a
7805A to get 5v. No problems there.
Is there something really, really obvious that I'm missing?
Conversely, what's a good value to trickle charge a huge old cap, via the
DS1302? It has mysterious options for single diode/dual diode and
0K/2K/4K/8K; I have no idea what's appropriate (except at 0K I'm gusssing
the inrush current on an empty cap wouldn't be good for the chip).
Clues appreciated.
1N4001
+5v---- |>|
Pin 1 DS1302
|
cap 161000uF 10v
|
Gnd
Pin 4 DS1302
\
Pin 8 DS1302
going for days, when the power fails. I know the chip has a trickle charger
built in, but I don't want to do it that way (because I don't want to mess
it up and fry the chip; I don't know what values to set in the DS1302 to
charge this mother of all caps I'm using, and mail order takes time.)
So I thought I'd just do something really simple. I have a 5v power supply
running a BS2 and the Quadrovox voice module. It aso runs the DS1302, but
between +5v and the DS1302 pin 1 (5v in) I have a diode, and after that, a
big cap (161,000uf, 10vdc - the thing is the size of a soda can) across the
DS1302's power in and ground. Ascii art below. The idea is that the diode
lets power in normally to run both the DS1302 and charge the cap. When the
power fails, the cap runs the DS1302, but nothing else, because of the
diode.
This frequently works fine... except when it doesn't.
Sometimes when I unplug power, (and then talk to the DS1302 via a separate
BS2, which is still powered and shares a common ground), it reports it's
happily keeping time, running from the cap. But sometimes it reports that it
isn't. It's not the that cap is running out of juice: when it fails, the
test meter says the cap is still at 5v. And when it works, it can keep time
for (at least) minutes this way. When it fails, it seems to fail the instant
the power pull is pulled.
The power supply is trivial: one half of a 12v transformer rated to 0.45A,
through a full bridge rectifier, across a 6.3v 3300uf cap and then into a
7805A to get 5v. No problems there.
Is there something really, really obvious that I'm missing?
Conversely, what's a good value to trickle charge a huge old cap, via the
DS1302? It has mysterious options for single diode/dual diode and
0K/2K/4K/8K; I have no idea what's appropriate (except at 0K I'm gusssing
the inrush current on an empty cap wouldn't be good for the chip).
Clues appreciated.
1N4001
+5v---- |>|
Pin 1 DS1302
|
cap 161000uF 10v
|
Gnd
Pin 4 DS1302
\
Pin 8 DS1302