Hi, i hava a little problem: i need a costant voltage drop between a source (a battery charger) and a load (battery to be charged). Voltage drop should be 4.8V; maximum current=3,7A. I thought about a zener diode, but nothing i've found with these features. Any suggestion? Thanks
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You can use a bjt on the low side of the battery(so the battery floats with - connected to the collector).
Linearize the bjt's base and program it so that when the V_CE > 4.8V it acts to close the base off and when V_CE < 4.8V it acts to drive the base(this will require a 4.8 zener or some combination of diodes or voltage reference).
Once you do that the bjt can easily take the 3.7A(on a heat sink as it is about 20W).
You could bias the bjt easier using a pot if your willing to monitor it. If you want to get fancy you could use some sort of feedback method such as a uC or what I described a little above.
The point is that a bjt can act like a constant voltage drop by driving it's base appropriately(the base current will generally follow inversely with the change in V_CE so it holds V_CE constant).
Another method is to use a switcher which is less wasteful but more complex.