Fuse question/suggestion

max72

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

That should work. Fuses are important with lead acid and nicad batteries. Fuses would be a few amps?

kwinn

Unless the charger has its own fuse you may want to put a fuse between it and the 12V battery as well.

max72

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

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

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