The Relationship Between Capacitors and Resistors

Electronics software such as Crocodile Technology® is perfect for testing circuits without actually building them from real components.

Both capacitors and resistors are important components in circuits, especially delay or timer circuits. Combining resistors and capacitors in a circuit will increase / decrease a timing sequence. A simple circuit is shown shows four capacitors and resistors in parallel. On the left hand side of the circuit an LED is seen, this is protected by a 300 ohm resistor. As the switch is closed the capacitors can be seen to charge up and the LED lights immediately. When the switch is opened the LED stays on for a short time and then fades slowly. This happens because the each capacitor has a charge of ‘electricity’. This is released slowly when the +9 volts is switched off.


The total capacitance is calculated by simply adding the values of the capacitors together.



Using electronics software such as Crocodile Technology®, change the resistor values to 1K. When the switch is closed the LED lights almost immediately. When the switch is opened the LED stays alight for a longer time and fades more slowly until final it emits no more light. Increasing the value of the resistor causes the capacitors to charge up more slowly. Most important of all, when the switch is opened the capacitors discharge their electricity more slowly. The LED stays alight for a longer time



QUESTIONS: 1. Using simulation software such as Crocodile Technology®, build a capacitor and resistor circuit incorporating a relay. The relay should remain energised for a short time after the circuit switch is opened.

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