# Super capacitor charging circuit

#### jobycm

Joined Nov 17, 2011
22
Hi,
I want to charge a super capacitor of 2F(8v) from a 12 volt battery(or from a regulated 10v),
within a time period of around 5 seconds(in worst case below 10s),
without using a charging IC.
Without the current limiting setup the initial current will be a large value which is not desirable,but by using a current limiting resistor i can't attain may desired charging time.
Please any one help me with this.

#### kubeek

Joined Sep 20, 2005
5,748
First, don´t use 10 or 12V to charge a 8Vmax capacitor. You should stay at 7V max if you don´t want to blow it up in a spectacular fashion.

You should use current souce of 1.42A voltage limited to 7V, which will charge said cap in 5sec. Check if this current isn´t larger than what the cap allows.

#### SgtWookie

Joined Jul 17, 2007
22,221
You might also use an automotive lamp as a current limiter. Lamps start off with low resistance, and it gets much higher when they are up to normal temp.

You will need to use some kind of voltage threshold detector to disconnect the source of current before you overcharge the capacitor.

#### THE_RB

Joined Feb 11, 2008
5,438
You could use an LM317 and a resistor (just two parts) as a constant current source. The LM317 will drop about 3v too, which will help stop the cap being charged to too high a voltage.

As it is a constant current you could add another resistor to add a fixed voltage drop and further limit the max voltage the cap will charge to.

#### SgtWookie

Joined Jul 17, 2007
22,221
Actually...
use a couple of 7805 regulators w/heat sinks connected in parallel, and a 200 Ohm resistor from the junction of the two regulators' GND pins to actual circuit ground (the cap ground side)

Charging current should be in excess of 3A, and the cap should be charged to 7v within a few percent in < 5 seconds. The regulators themselves will limit the max charging current. Heat sinks are mandatory.