TIP: DS323x RTC + SUPERCAP

[SL4P]

A tip for those making DS323x based clocks... amid the current frenzy for using supercaps!
I've used a coin sized 5V supercap as the battery backup on several clock based projects.

Charging through a schottky diode from the primary 5V supply - and it keeps the clock up during brown-outs/ black-outs, and moving the device around the house reliably.
I've seen it hold the clock up much longer (1-hour plus) - not sure why without exploring in detail.  Of course add more caps to extend your viewing time!

[johnnyfp]

How much is the supercap, schokky diode, vs a coin cell?

[SL4P]

A little more - but can be sealed into the enclosure - 'never' needs to be replaced.

[johnnyfp]

But if you expect the circuit to be constantly powered for long periods of time and only have to backup the RTC every now and then when moved, would not the Coin cell last 10years?

[SL4P]

Yes - potentially - but no potting etc.

[johnnyfp]

True. I do like your idea, as I too use the DS323x but need it to last a power outage, not an hour, as from what your saying the only way to get longer backup is putting more caps, vs a coin cell that could last years, and maybe can be potted, and do what all the other manufactures do, and make it a throwaway product (yuk!).

[SL4P]

I  must admit my bedroom clock - I just couldn't buy one that was reliable and readable across the room - so of course - made my own.

I think I've had to re-set maybe twice since 2010 - and one of those was my own fault for dicking around!  Accuracy is great, and no 9V battery to buy - used in retail bedside clocks!

Cheers

[David Hess]

Some schottky diodes will have enough reverse leakage even at 5 volts to significantly cut the backup time compared to the DS323x alone which is only suppose to draw like 0.8 microamps.  This will become much worse at higher temperatures.