DS1337 RTC Backup Battery

Safety regs mean that you have to have 2 series diodes from the Lithium battery IIRC. Think what would happen if one went short !

If worried about the volt drop, maybe you could replace one with a mosfet ?

Graham

What's a little fire? UL is a bunch of pessimists!

Can't picture that. What would it look like?

Thanks for the common sense that I appear to be missing.

John

Just looked. Vf at 0.1mA is only 240mV. I think I could live with 0.5V drop?

battery

I know a guy who used to run STC's compliance labs. They set one off intentionally. Talk about brown trousers ! Even though it was in a bell jar.

Gate driven by main power detect.

YW.

Graham

Not surprised. Much easier.

Graham

intentionally.

My day job is airplane/helicopter pilot. We use to use lithium batteries for night vision goggles. Until people started making the mistake of putting them in their pockets and helmet bags unprotected. I know of at least one AH-64 Apache that was brought down by a itty bitty battery that caught fire in a helmet bag in a storage compartment.

Respect lithium batteries....

Presumably you have in mind one of those coin cells - CR2032 etc. I can't see a fire hazard with those.... The 1377 will run for years on one of those....

If it's a lithium battery it can explode/catch fire. Lauda Air lost an airliner it's believed on account of a cargo of defective lithium coin cell batteries !

Graham

resister

I think there should be a resister between Li Bat and the diode. it will be no problem if the two diodes have a short.

A cargo is surely a LOT more than a single coin cell on a PCB. I guess I had better keep an eye on all of those old motherboards we have laying around. One could go up at any moment.

Jim

basically any 2 components in series will be OK, though to avoid charging you'd want one to be a diode in this cct. a 100R will drop less than a diode will IIRC

David

other RTCs in the range have a separate battery pin, and are UL rated without external components... that's why they exist.

David

I understand, but try finding one that has:

SOIC footprint (and not a gazillion pins) Has an alarm INT (not just a time of day alarm either) Is available in prototyping quantities Is I2C (I had to settle for SPI)

etc.

etc.

Un bel giorno snipped-for-privacy@gmail.com digitò:

Several ST RTC have these characteristics, see for example M41T80M6E. Even Digikey sells them.

You can always find an alternative to Maxim parts, and if you don't, it means that your design is flawed.

pcf8583 ???

don't ask me where to buy it though!

ah! RS stock number 296-683

David