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max72
01-10-2012, 06:02 PM
I'm doing a small box with GSM and the propeller, to be used on the go.
It would use 12V from car battery, and have a 6V sealed lead battery.
The 6V will be charged via a L200 chip.
After the diodes (schottky) there will be a 4V switching regulator to drive the GSM, and a propeller board.
Considering the batteries I would like to avoid troubles with shorts, so I'm planning to place 2 fuses.
The attached image shows a block diagram of what I have in mind, and in red the fuses.
Any suggestion is welcome.

Thanks in advance,
Massimo

Dr_Acula
01-11-2012, 01:04 AM
That should work. Fuses are important with lead acid and nicad batteries. Fuses would be a few amps?

kwinn
01-11-2012, 01:11 AM
Unless the charger has its own fuse you may want to put a fuse between it and the 12V battery as well.

max72
01-11-2012, 08:23 AM
The fuses would be a few amps, the idea is to be short safe. I have a cigar lighter adapter with a 5A fuse inside the tip, and I had the (unplanned) opportunity to verify the value is ok.
kwinn, thanks, I'll move the upper fuse left, before the L200 charger.

Massimo

bill190
01-11-2012, 05:10 PM
In general fuses are used to protect the wiring from heating up and causing a fire due to overload or short.

And the amperage load on wiring should not exceed 80% of the wire's amperage capacity.

Then there is "voltage drop" with distance. And more so with DC. So if running wires many feet, a larger wire gauge would be needed. Although the fuse could be smaller to protect the device(s) at the end of that wiring.

Anyway here is a handy dandy wire gauge chart for 12 volts, amperage, and distance of wire run...
http://www.offroaders.com/tech/12-volt-wire-gauge-amps.htm

max72
01-11-2012, 05:43 PM
Thanks.
The current draw is in the 100mA range, with short peaks during GSM communication.
The idea is to avoid fire in case of a short.

Massimo