how does a electronic device hold a charge,once the batteries are taken out?

[north]

just out of curiosity


for instance we use a palm-pilot in the store I work in for ordering merchandice and inventory.

when the battery runs low we of course change the batteries but the thing that has always interested me is that you don't lose the info while changing the batteries( although you only have a certain amount of time in which to do so). since the power source is OFF.

now I've just started course Electro-Mechanical Tech. in which they have talked about capacitors resistors diodes etc.

so I was just wondering is this how it is possible, that they use capacitors,with resistors and diodes in there as well, it seems logical enough!

 

[Nigel Goodwin]

Capacitors store charge, this may be enough to keep the memory going for a short time while you change batteries

In fact the definition of a capacitor is:

"Capacitance in the property of a circuit of storing energy in the form of an electro-static field"

 

[djtaylor]

or they just have a backup battery

 

[dknguyen]

It could just have non volatile memory...unless you are saying the device stays on while you change the batteries or that it uses volatile memory and the data doesn't get wiped out when you change the batteries- Caps or backup battery.

 

[pljack]

I've always assumed that it was a large cap. If I remember you only a few minutes to replace the main battery. So most likely not a back up battery.

 

[Hero999]

Or they use flash memory.

 

[Nigel Goodwin]

Or they use flash memory.

Except he said "although you only have a certain amount of time in which to do so" - and I'm presuming he doesn't mean 30 years or whatever?.

 

[ArtemisGoldfish]

If I remember correctly, the XBOX used a supercap to keep it's RTC running, so yeah, the capacitor is the best bet.

 

[Sceadwian]

I've used 5 volt super caps to keep a micro controller running for 30 seconds with a blinking LED before it reset, and that's relativly power hungry compared to keeping SRAM or simple RTC. Could probably keep one running for hours or days given the right charge voltage.

 

[dknguyen]

That's something Ive been wondering about. It seems you almost always have to derate caps when you use them (especially tants), and supercaps are rated for like 5V or so- do you need to derate super caps at all? BEcause if you did they'd be useless!

 

[Sceadwian]

The rated voltage for the PowerStor Aerogel supercaps is for it's entire temperature range. And most of the really big ones (50F) are actually rated at 2.5 volts. What the limits of pushing those voltages are I don't know, but because the materials they're made out of is basically a nano-material it's pretty fragile to over voltage.

 

[dknguyen]

Ah, yeah I remember now why I've never used them...they are all rated for 2.5V rather than 5V and I hate using series caps. I'm just curious if they ever have to be derated (because no one ever seems to). Everyone seems to series two 2.5V supercaps and then run them right at the 5V limit. I'm not sure what the tolerances are on that voltage rating. Like with tants it's a 50% derating a lot of the time.

 

[Nigel Goodwin]

It's only tants which seem to over rate themselves, most other devices give honest ratings - a 5V super cap is designed for a 5V rail.

 

[ArtemisGoldfish]

And most of the really big ones (50F) are actually rated at 2.5 volts.

You might want to rethink what really big is, DigiKey sells 5000F 2.5v Ultracaps.

 

[dknguyen]

lol, wow. the largest ones I have seen are 120F...except these were supercaps, not ultra caps.

 

[Sceadwian]

Yeah, only 4,000 dollars, I don't even want to know how big it is.
Super and Ultra caps are the same thing really, both of them are just buzz words though.

 

[bananasiong]

What!? 4000 USD?

 

[Sceadwian]

Yeah, acording to the RC time constant equation that would be 1 and a half hour charge/discharge rate to 1.58 volts through a 1 ohm load. It's basically an electrostatic battery.

 

[ArtemisGoldfish]

Accctually, the 5000F ultracap I saw was only about...$400.

 

[Sceadwian]

The only one I found on Digikey was 4 grand, I'm not sure if that's for multiple units or not, was just a quick view.