As promised, I was working on an automatic battery charger alongside my solder reflow oven shield. The idea came from needing a battery backup for our Wendy house alarm system so that if the power fails that the alarm is not wrongly activated (Different partition connected to the house alarm system, i.e. different zones). I found most of the information that I needed over here but didn’t have all the right knowledge so I had to consult online a bit to get all the theory correct. So I’ve designed the circuit board during the months of November and December and finally built it on the 3rd of Jan 2014. This is what I came up with thus far:
I have noticed now that it is better to first drill your holes before edging since you won’t be ripping off a pad by accident. If you do rip of a printed section then you can just transfer the circuit again after all the holes have been drilled and line it up by means of the drilled holes or just use a sharpie to correct it. So I’m going to be creating my PCBs like this in the future, works quite well. 🙂
So after the printed part have been removed from the board, it was all nice and shiny 🙂
Me being who I am, over excited, couldn’t wait to begin populating the board and so I started building the circuit immediately. The transformer, the heat sinks, the relay, the BIG capacitor and the fuse + fuse holder I’ve salvaged off old boards. So I had a 21 Vac winding on the transformer and a 12 Vac winding on the transformer so I opted to work with the 12 Vac winding since then I won’t be placing to much stress on the LM317 variable voltage regulator (12Vac translates to 17 Vdc but with the volt drops of the components leading to the LM317 I was actually getting only 15.2 Vdc out which was too low to be able to provide 13.8 Vdc at the output of the circuit. So I had to hack my circuit apart a bit to change over to the 21 Vac winding which yielded a dc voltage of around 25 Vdc. Using this winding I was able to get the 13.8 Vdc at the output which I needed to charge a battery. So after I was done the circuit looked like this:
The circuit was working but the potentiometer used to set the voltage at which the battery should be charged was too small to allow me to set the voltage to 12.6 V so I had to swap out potentiometer for a different higher rated one. When I finished I could set it to the required voltage. The circuit diagram from which I worked specified a 500 Ohm potentiometer but I would recommend using a 2.5 kOhm potentiometer rather to be able to set you “charge kick in voltage” to a higher voltage. I replaced mine with a 5 kOhm potentiometer since this was what I had laying around.
So after this was done I started testing the circuit and just got a general feel for it. The circuit starts charging when the battery drops below 12.6V and then the voltage set by the LM317 voltage regulator is applied to the battery which in my cas was 14V. When the battery voltage reaches 14.3V the charging process stops and then the battery voltage is monitored again until it drops below 12.6V at which stage the whole process is started again. I was simulating the “battery” by using my desktop power supply. Obviously a 12V battery will never reach 14.3V when fully charged so this value will have to be changed. I just need to figure out which resistor value in the TLC271 circuit I will need to change to be able to set that voltage as well since I’m thinking of replacing the specific resistor that influences this cutout voltage with a potentiometer as well so that you have more control with the circuit for when your battery starts to age for example, you won’t be able to get to the 13.6V that is needed as well.