Electronics › Electronics › Auto Cut Off Battery Charger with LM317 & BD139 › Hi Irfan, I am actually
I am actually building a few SLA chargers at the moment, both with LM317 and also a 5 amp version with LM338T which works in a very similar way. I have adjusted the schematic to show more clearly what it should look like in order to work correctly.
For clarity, the green LED connection is shown not connected to the base of T1 as some people have mistaken that in the original.
R4 has been made 10 Ohms and ZD is now connected via a link to the battery side of R4 where it will sense the voltage on the battery terminal as is required. I notice that R2 is maybe a little low compared to the datasheet and I would increase it to the 240 Ohms suggested as original spec.
Setting up is now a matter of having the whole circuit powered up with no load and adjusting VR1 to give 14.2 volts (or what is suggested on the datasheet for your particular battery) at the positive battery terminal. To check the cut-off works, simply connect the LINK and if a fuly charged battery is now connected, TR1 will pull the adjustment pin low and the output from the regulator will fall.
With a battery under charge, the current flowing through R4 will give a voltage drop, as the battery becomes charged, so the voltage across the resistor is less and ZD will eventually conduct, switching off the charge current. The red LED will stay lit because it is being fed by the current from the adjustment pin but the drain on the battery is really tiny.
I have not actually built this circuit yet but have a few very similar that work well.
As Savio has noticed, the 8.2 volt value for ZD does seem a little low to get a switch-off at 14.2 as shown here. You may need to increase that and a quick guess would be 1.2 volt across the LED then another volt base-emmitter on T1 is only going to give you a little more than 10 volts at best before the zener kicks in and shuts the thing down. You may find it needs to be 12 volts to work to a full charge but as I say, I haven’t tested this yet.
Let me know how you get on with these changes.