SLA battery charger (OT)

[spongebob]

I'm trying to construct a battery charger for a 12.6V sealed lead acid battery, based on the Texas Instruments UC3906 charger IC. The datasheet for the UC3906 contains a schematic for a simple charger:



The datasheet also shows how to calculate the resistor values for Ra, Rb, etc. depending on battery type. The maximum charging current Imax is set by Rs:

Rs = 0.25V / Imax

So Rs would have to be 0.5 Ohm to achieve a charging current of 500mA, right? I have never seen resistors smaller than 1 Ohm, do I have to put two 1 Ohm resistors in parallel?

The Q1 transistor obviously has to handle the charging current, can anyone name a PNP transistor that can go up to 1A?

My plan is to build a small portable battery powered amp, and a SLA battery should last much longer than a normal 9V battery.

[Peter Snowberg]

0.5 ohms sounds right to me..... but always get a 2nd opinion for everything I say.

Sure.... two 1 ohm resistors in parallel would be a good way to go. Try looking at Mouser for "current sense resistor" or "current sensing resistor" and you'll get hits with values down to 0.005 ohms. They're usually in their own section. Check out the units from Ohmite at the bottom of this page: http://www.mouser.com/catalog/617/363.pdf

For a pass transistor, I would suggest something like a TIP30 (http://www.fairchildsemi.com/pf/TI/TIP30.html) at the minimum. The problem with using a small transistor there is that it's fairly delicate. You might want something a little larger. To go mondo large... you could use a MJ2955 (complement to 2N3055), but that's in a TO-3 package :mrgreen:. Just about any common PNP power transistor should be fine.

Best of luck on yourt supply!

Take care,
-Peter