Polarity of coin cell battery question

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serialrammer

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Hello, I am working on building a small and simple data collector.
Basically, the circuit is going to have a Microchip 8-bit uC and
a serial EEPROM. The circuit is going to be powered using a 3V
coin cell with the positive terminal connected to the VDD of both
chips and the negative terminal connected to the VSS of both chips.

Suppose that I (or somebody using my circuit) screws up and puts
the coin cell in backwards. Do you think that irreversible damage
will occur to either the uC or the serial EEPROM? How should I
put a diode into the circuit if damage will occur? How do I choose
the diode?

Thank you for any help or insight you can provide.
 
Simple diode + Fuse will save your circuit. This solution is better than diode in series because it is not droping voltage on the diode. In this case if you connect the battery wrong way, the diode will be forward biased and will blow the fuse. The diode can be any, like 1Amp (1N4007) and the fuse should be two times more current then your circuit is normaly taking.

Sorry for Image quality, I didn't have Eagle installed
 

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PICs are pretty sturdy devices. I've connected power to them backwards many times without any ill effects. And coin cells can't source a lot of current anyway, so I'd say you're okay.

Mike
 
upand_at_them said:
PICs are pretty sturdy devices. I've connected power to them backwards many times without any ill effects. And coin cells can't source a lot of current anyway, so I'd say you're okay.
Click to expand...

If you connect the supply backwards, the protection diodes will all conduct, effectively putting pairs of series diodes in parallel across the supply. So this will pull the supply down to about 1.4V, with the diodes getting hot as long as the supply lasts.

As Mike suggested, the small cells can't provide much power anyway, and the PIC should survive with no ill effects!. I've accidently fitted PIC's backwards before, and they have got so hot I've got one with my fingerprint burnt in the top :lol: it still works fine!.
 
I've accidently fitted PIC's backwards before, and they have got so hot I've got one with my fingerprint burnt in the top (Laughing) it still works fine!
Click to expand...

The finger or the PIC (grin)?

Regards, Mike
 

The PIC :lol:

The fingers aren't so good :cry:

I inadvertently removed the finger tips of my right hand a number of years back (with an electric plane!), they grew back far better than I thought?, but I don't have 100% feeling in them. Even the fingerprints grew back, although they are somewhat changed by the scar tissue across them :lol:
 
LOL That sounds like you were a Man in Black... :twisted:
 
That must of hurt.It survied that?!?!

I once got my fingerprint in melted solder.(I let it go fast enugh not to burn my finger)

555 timers survive realy well too if the suply is weak enugh.I once put one backwards powerd from an bunch of AA Ni-Cd cells.I smeld that distinctive burning electronics smell.And it surevived too.(An nother one vasent that lucky and got fryed and it also made a slitght mark in the breadboard)
 

The diode is good but a 1A fuse is pointless. The short circuit current of a coin cell is not 1 amp so it can't blow the fuse. And blowing the fuse renders the device useless so unless there's a user-replaceable fuse I don't know what the point is.

The self-resetting polyfuses are great, though they do add some substantial impedance to the source. I think just the diode is sufficient.
 
No, I said the diode can be any (1Amp is very common), and the fuse should be rated for two times the normal device current!
Ofcourse this solution is not needed for "coined cell" sized supply, but it's usefull if your source is much harder.
 
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