It is the maximum amount of voltage that a capacitor may safely be exposed to and store that is indicated by the voltage rating on the capacitor.
Keep in mind that capacitors are essentially storage devices. You should be aware that capacitors retain X charge at X voltage; that is, they can hold a specific size charge (1F, 100F, 1000F, etc.) at a specific voltage. Capacitors are used in a variety of applications (10V, 25V, 50V, etc.). So, when selecting a capacitor, all you need to know is the magnitude of the charge you desire and the voltage at which it will operate.
What is the reason for the varied voltage ratings on capacitors? Because the voltage requirements for a circuit can vary based on the type of circuit you’re working with. It’s important to remember that capacitors provide voltage to a circuit in the same way that batteries do. The main difference between a capacitor and a battery is that a capacitor discharges its voltage considerably more quickly than a battery, but the logic behind how they both deliver voltage to a circuit is the same. A circuit designer would not simply use any voltage for a circuit, but would instead utilize a specified voltage that was required by the circuit design. It is possible that 12 volts will be required for one circuit. Capacitors with ratings of 12V or above would be appropriate in this situation. In another instance, 50 volts may be required. It would be necessary to utilize a capacitor with a voltage rating of 50V or greater. This is why capacitors are available in a variety of voltage ratings, allowing them to supply circuits with a variety of voltages while matching the power (voltage) requirements of the circuit.
It is important to remember that the voltage rating of a capacitor does not refer to the voltage that the capacitor will charge up to, but rather to the maximum amount of voltage that the capacitor should be exposed to and can safely store. When designing a circuit, the circuit designer must take care to ensure that the capacitor will charge up to the desired voltage. Otherwise, the capacitor will not charge up to the voltage. Although a capacitor may be rated at 50 volts, it will not charge to that voltage unless it is supplied with 50 volts from a direct current power source. The voltage rating of a capacitor refers to the maximum voltage that the capacitor should be exposed to, not the maximum voltage that the capacitor will charge to. It is only when a capacitor is supplied with a specific voltage level from a DC power source that it will charge to that voltage level.
When selecting capacitor voltage ratings, keep in mind that a reasonable rule of thumb is to choose a voltage rating that is slightly higher than the voltage that will be supplied by the power source. When selecting the voltage rating of a capacitor, it is generally advisable to allow for a significant amount of wiggle space. In other words, if you want a capacitor to store 25 volts, don’t choose a capacitor with a rating of 25 volts exactly. Preserve some space for a safety buffer in case the power supply voltage ever rises unexpectedly for whatever reason. Taking the voltage of a 9V battery supply, you would observe that it reads higher than 9 volts when it is new and has plenty of life left in the battery. If you utilized a capacitor with a voltage rating of exactly 9 volts, it would be exposed to a higher voltage than the maximum voltage indicated (the voltage rating). Normally, in a situation like this, there shouldn’t be a problem, but doing so is a nice safety margin and excellent engineering practice to take the extra precaution. You can’t really go wrong by selecting a capacitor with a greater voltage rating than the voltage that will be supplied by the power supply, but you can definitely go wrong by selecting a capacitor with a lower voltage rating than the voltage that it will be exposed to during operation. The risk of an explosion and the capacitor being defective and unusable increases when you charge up a capacitor with a lower voltage rating than the voltage that the power source will deliver it. As a result, do not expose a capacitor to a voltage that is greater than its voltage rating. The voltage rating is the maximum voltage to which a capacitor is intended to be exposed and which it is capable of storing. Some engineers believe that it is best practice to select a capacitor with a voltage rating that is twice as high as the voltage of the power supply that will be used to charge it. As a result, if a capacitor is going to be exposed to 25 volts, it is advisable to use a capacitor with a rating of 50 volts to be on the safe side.
Also, keep in mind that the voltage rating of a capacitor is sometimes referred to as the working voltage or the maximum working voltage in some cases (of the capacitor). This means that a capacitor’s maximum continuous operating voltage specified on a datasheet refers to the highest continuous voltage that the capacitor can withstand without getting damaged.