Bad Capacitor

High value Farad capacitors are often used to filter power surges and thus stop oscillations. However, the circuit below is a bad circuit.

Figure 1: Bad circuit.

Because we all seen diode capacitor rectifiers in power supplies some people think they can connect capacitors directly to DC sources. In way those people are not completely wrong and low Farad value decoupling capacitors are sometimes connected in parallel with digital integrated circuits without a series resistor. This is why this circuit is classified as bad rather than wrong. However, connecting capacitor directly to DC source without a resistor might cause the capacitor to explode because the capacitor voltage is zero when the circuit is first turned, thus capacitor will be draining high current from battery.

Where either the capacitor will explode or not will depend on the following specifications:

- capacitor Farad (capacitance) value,

- internal resistance of the battery,

- maximum capacitor power, voltage and current characteristics.

It was a common practice in the past to connect 470 uF capacitors in parallel with AA/AAA and typical 9 V batteries (to filter power surges) because those batteries have a high internal resistance. However, most 12 V batteries have low internal resistance and can supply dangerous high current.

This is the improved circuit.

Figure 2: Improved circuit.

Using the value of 10 ohms for series resistor might not be sufficient for preventing capacitor explosion or failure. You will need to do calculations or simulations.

Assuming that the current entering the voltage regulator is very small we can make the following assumptions:

- the maximum capacitor current will equal to 12 V / 10 ohms = 1.2 A.

- the maximum voltage is approximately 12 V.

- the maximum power is when capacitor is charged to half supply voltage: 6 V * 0.6 A = 3.6 W

If you check the specifications you might notice that you might need to connect additional capacitors at input and output for this particular voltage regulator. However, the this theory is beyond the scope of this article.

You can also research information on decoupling capacitors if you want to know more.