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How powerful capacitor do u need to power up the whole motherboard so that it executes a instruction and changes a flag o so.

LindusSystem wrote:How powerful capacitor do u need to power up the whole motherboard so that it executes a instruction and changes a flag o so.

Depends. In our solution the capacitor is connected to 24v, and the PC/104 board can handle voltages down to about 10v before it shuts down. The usable range of voltages is the most important factor. If the motherboard has a very narrow range of operational voltages even a very large capacitor will not do much good. For such cases, you need an extra (switching) regulator that can stabilize the voltage even when the voltage on the capacitor changes widely.

We use 22000uF, and it can keep the PC/104 board alive for a few seconds, which is enough to flush disk buffers.

gerryg400 wrote:How do you switch from mains power to battery power quickly enough to not lose your Vcc ?

By connecting the battery directly to the power supply. The solution also must provide some means to keep the battery charged, which means the operational voltage must be suitable for charging the battery.

What a strange discussion!???
Energy stored in electrolytic capacitor of 2000uF at 6.3V is c*u^2/2 = 40mJ.
Energy stored in one NiMH AA accumulator of 2700mAh capacity is 1.2V*2700mAh*3600s = 11.6kJ, which is almost 300000 times more than for capacitor of roughly the same size!

It is indeed strange. A bunch of software guys designing hardware. It is possible to use a capacitor to hold up a power supply for a few seconds in the application that rdos mentioned. Capacitors have several advantages over batteries in these situations. Among these are that capacitors are easier to charge, you don't need full-charge detection (they can't be over-charged), generally have a longer life in terms of charge/discharge cycles, don't require extra regulation in their output and probably won't require a switching circuit to bring them on-line when the power fails.

If you only want a second or 2 of power and there is some tolerance in your input voltage then a capacitor is often part of the best solution.

The best thing is to keep a battery inside the CPU and connect it to ACPI.

gerryg400 wrote:It is indeed strange. A bunch of software guys designing hardware.

Uhm. I consider myself a hardware guy as much as a software guy. After all, that's why I prefer to write OSes over typical programming tasks.

rdos wrote:

gerryg400 wrote:It is indeed strange. A bunch of software guys designing hardware.

Uhm. I consider myself a hardware guy as much as a software guy. After all, that's why I prefer to write OSes over typical programming tasks.

Yes I figured that rdos. I was referring to the guys who didn't know what a capacitor could do.

Yoda wrote:What a strange discussion!???
Energy stored in electrolytic capacitor of 2000uF at 6.3V is c*u^2/2 = 40mJ.
Energy stored in one NiMH AA accumulator of 2700mAh capacity is 1.2V*2700mAh*3600s = 11.6kJ, which is almost 300000 times more than for capacitor of roughly the same size!

Energy stored in electrolytic capacitor of 22000uF at 24v is = 6.3J

1. I'm primarily a hardware guy. I have a lot of commercial projects and here I mentioned that first my PC was developed by myself.
2. Capacitor needs voltage regulation since at discharge with constant current it's voltage drops linearly and with constant power drops even more sharp and the end of discharge.
3. Yes, capacitor may be OK if you need just a couple of seconds.
4. If you consider double-layer capacitor you should keep in mind it's rapid aging when it is under constant potential.
@rdos:
5. J is thousand times less than kJ.
6. 12.7V*8.5Ah*3600s ~= 0.4MJ.