Current limiting

[crkt_tek]

New poster, seasoned technician.
Im designing a dc system in my apartment (half of which is more of a workshop then an apartment) to automate and interface sevral appliances lights, and security features. Mostly for my own amusement but also as some good advertising to get some custom projects. For now i am actually using a 12 volt deep cycle boat/ RV battery to allow for larger currents than i have ben able to draw from a few AC to DC power sources i have built. I figured it was better to make a 500mA "trickle" charger for sevral reasons. so my question is i want to eliminate the possibility of a large draw on the battery over loading my charger, or the 120v power in my apt. If wired directly with only one battery my logic says i need to design a circuit to automaticly disconnect the charger positive wire when current exceeds a certin value. Ive drawn out a schematic where voltage drop is read across a small value "sense" resister in series with the positive charger wire. This value i want to direct to one input of a op comparitor, and a supply voltage with a variable resister for calibration to the other input. The comparitor output would then feed the base of a transister, which would drive a NC relay that would open the charging circuit until the current fell. Anyone seen any similar circuits i could refrence? an other input would be helpfull too, thanks

[pebe]

In your circumstances it would be better to float charge rather than trickle charge (float charging is how a battery is kept topped up in a car). If you charge the battery from a fixed 13.8V source it will take current until that voltage is reached, then the current will reduce to a very low value.

You could use a 7812 regulator with a couple of resistors to increase its output to 13.8V, and fit a suitable resistor in the feed to the battery to limit the maximum current. Typically a 6.8ohm resistor would limit the current to 560mA if the battery voltage fell to 10V.

Another, more efficient, way of obtaining 13.8V would be to use a 5Vwall socket power supply like this
http://www.ebay.co.uk/itm/UK-DC-5V-2A-S ... 2c6899d32a

with a boost converter like this
http://www.ebay.co.uk/itm/XL6009-DC12V- ... 4acea5f030