# why i can't use capacitor in a battery charger

Discussion in 'The Projects Forum' started by adeel, Feb 9, 2013.

Jun 28, 2008
16
0
hello . friends i am very surprise i just made a battery charger which consists of two 12 volts transformers in series then a bridge diode of 200 amp i used
triac bta 40 in ac supply to controll ac voltage so i can adjust ampare. i used a 10000 uf capacitor of 35 volts in dc volts . And i installed tree fans to cool the things inside of box . i used ic 7812 i supply current to ic with 4.7 ohm resistor of 5 watts . Now problem is when i started to charge battery from this charger at 50 to 80 amp after some time capacitor starts to heat and giving smoke . i turned off the charger and replace the capacitor but other capacitor was again heating . So i removed the capacitor. after some time ic 7812 was dead i am surprise what is happening max voltage of chager goes to 17 volts. Guys can u plz tell me how to handle this situation. And is a battery chager required a capacitor or not. thanks

2. ### crutschow Expert

Mar 14, 2008
20,878
5,930
Post your circuit diagram.

Typically a battery charger doesn't need a capacitor on its output since the battery basically acts like a large capacitor to the AC ripple voltage.

3. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,349
730
In very rough terms: The capacitor will try to "smooth" the full wave rectified output of the supply to keep the voltage constant. The higher the current draw from the output, the more the capacitor has to "work" and is often unable to keep the voltage where it "wants to" when charging the battery. This makes it get hot.

If the peaks of the voltage output by the power supply are >35 Volts, this will cause the capacitor to self destruct, signs of which are heat. The 12V rating is the RMS value, the peak is 1.4x that voltage, so in series at 24VACRMS, the peaks are roughly 32.5V, if the transformers are giving slightly higher output voltage, it will be going over the rated voltage of the capacitor.

If you are running a 7812 from a 24V output, that will get quite hot as well, since it is a linear regulator, it dissipates the extra power as heat.

4. ### Ron H AAC Fanatic!

Apr 14, 2005
7,018
682
Post your schematic, including the triac and its control circuitry.