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Battery Charger Design

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Good morning everyone !

I just overlooked your comments (thanks) and I am going to use a 20Vac transformer (will update schematic when I get home). As for the current limiting circuit, it doesn't matter whether you put it before or after the voltage regulator as long as you keep in mind the 1.25V drop. You can always play around with resistor values for the voltage regulator until you get the right Vout. Anyways, I am off to that blasted Sayal electronics to get the components. I Still need to design the board layout so hopefully everything will be ready tomorrow (unless I burn my house down :D ).

Thank you everyone for your help and interest in this project !
 
Frosty_47 said:
As for the current limiting circuit, it doesn't matter whether you put it before or after the voltage regulator as long as you keep in mind the 1.25V drop.
The LM317 needs to have about 1.7V from input to output to function, in addition to the 1.25V across the current-setting resistor. If the current-regulator is ahead of the voltage regulator then its overhead voltages don't need to be added to the voltage regulator.
 
However it is still important to budget for those voltage drops in the overall voltage regulator design.
 
Design Update

Hello Everyone !

I updatted the design (Jeez, when will this stop?!)

**broken link removed**

I put in 35Ω Rheostat for variable current adjustments. Also, notice the 1.25Ω R1 was put in to limit the current to a maximum of 1 Amp when the rheostat is at zero Ω. I also changed the CHRG/LOAD LED so that it doesn’t mess up the current regulator (a big thanks goes to Tekwiz for this suggestion). R4 was changed to 5KΩ Potentiometer to allow variable voltage regulation. I also added additional capacitance because I plan to use this charger as a regulated power supply. I will put in a rotary switch that will provide the right voltages for battery charging. There will be another switch that will switch between the battery charger and power supply modes (basically the switch will disable the rotary switch and enabling the 5KΩ POT and vise versa…). I even went as far as purchasing 0-1A analog panel ammeter (It’s a beautiful piece of equipment :D :D :D ). I will also purchase analog panel voltmeter. Fuse and over voltage protection diode are not shown in the schematic but will also be included. Basically that’s it for now, I am currently designing a PCB layout in Eagle as I am typing this so, hopefully I can get this project finished by the end of this weekend. I will post the results and possibly pictures when I am done (that is, if any one interested…).

I still have two more questions before I ruin my PCB:

1) Is 3KΩ RL large enough? And do I still need RL?

2) Where should I put my PWR LED so that the current flowing through it will not toggle on the CRHG/LOAD led when there is nothing connected at Vout ?

Thank you !
 
The more expensive LM117 can use a resistor value as high as 240 ohms for R3. The LM317 uses 120 ohms so that its voltage doesn't rise without any other load. Then the R4 pot should be 2.5K.
The 3K resistor won't be enough of a load for an LM317 with 240 ohms as R3 at low output voltages and might waste power at high output voltages.

Connect the power LED and its current limiting resistor (a power resistor) to the unregulated 32VDC supply across the 6000uF capacitor.
 
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