AVR-Chat

At 10:10 AM 3/15/2004 -0500, Wagner Lipnharski wrote:

>David VanHorn wrote:
>>> That's another point.
>>> If backup usage current is below 500mA, then use plain and cheap
>>> Alkaline ones.
>>> They are safe, shelf life is much longer than anything else.
>>> Of course, lithium current limit is a good idea, perhaps a small
>>> fuse? :)
>>
>>
>> Fuses that small are WAY expensive, and I don't think you can go that
>> low.
>
>
>You mean 250mA and 500mA small fuses?  That's pretty common, radial or axial
>socketed ones.  For sure the fuse will open much sooner than the battery
>detonates into a Radiologic Thermal Event... :)

I think we're talking at cross purposes. I'm thinking of the failed diode case, where the normal supply starts trying to charge the battery. 


>> I once talked to a Panasonic apps guy about a design we had, that put
>> 600nA into a lithium cell. His comment was that it probably wouldn't
>> explode.
>
>
>Are you talking about recharging current?

Yes.


>> If the steering diode on the battery output fails, then you need to
>> still have essentially zero current into the battery, by some means.
>
>
>That's the Alkaline advantage, but a 20 megaohm resistor in parallel will be
>something to think and avoid the battery leaking or contacts corroding.

Then you're pulling current on the backup unnecessarily.
Not much, I'll grant you, but it's still a sloppy solution.
A proper switchover circuit isn't hard, but it's more than a couple diodes. 

A couple diodes works for hobby purposes though.