Why are capacitors used in defibrillators?


A defibrillator is a machine that delivers an electric shock for a short period of time to restore a stable heart rhythm. Capacitors are usually used in defibrillators instead of batteries to provide such an immediate shock. However, in portable defibrillators, the battery is used as the power source to charge the capacitors.

Capacitors are storage devices that absorb any excess energy transient in a circuit or in a system and then release the charge stored in them when needed. In addition, they also act as short-term batteries that charge and discharge quite rapidly, unlike conventional batteries, although they cannot hold a charge for long periods of time.


How does a defibrillator work?

To understand the reason why capacitors are used in defibrillators, it is important to know the functioning of the defibrillator. So, the defibrillator consists of two circuits, one responsible for charging the capacitor and the other for discharging the capacitor. The process of charging and discharging is done by a small computer inside the defibrillator, but here is a simple circuit of a defibrillator shown:

In the above circuit switches, A and B are responsible for charging the capacitor while switches 1,2,3,4 are responsible for discharging the capacitor. When the power is turned on switches A and B are in the ON position, and charging of the capacitor begins:

Once the capacitor and the charging system are at the same potential, switches A and B move to the OFF position which means the capacitor is fully charged.

Now when the probe of the defibrillator is attached to the specified area of ​​the body the capacitors start discharging resulting in an instant shock to the heart. First switches 1 and 4 are closed and current starts flowing and this direction of current is known as forward direction.

After some time the direction of the current will change and it will start flowing in the opposite direction, the wave form it shows is named bipolar wave form.

Now once the graph reaches zero permanently it means that the capacitor is completely discharged and here is the waveform of the defibrillator:

Here the switching interval is the time when the current changes its direction and all the four switches of the discharge circuit move to off position to avoid short circuit.

Why are capacitors used in defibrillators?

Compared to batteries, capacitors can store charge faster due to their smaller size and advanced technology. Furthermore, defibrillators require a significant amount of voltage at the output, which batteries cannot provide due to size constraints.

Batteries typically use chemical reactions to store and release energy and this puts a limit on how fast it can be charged, and the same is the case with its discharge. Furthermore, batteries tend to degrade faster than capacitors, and thus their charging capacity also reduces. This leads to the conclusion that batteries cannot maintain high voltage levels for long periods of time.

On the other hand, capacitors can easily store high voltage in short period of time due to their structure. Furthermore, the life span of capacitors is quite high in relation to their ability to store charge, especially when it comes to supercapacitors. With a capacitor, an instant shock can be easily given due to rapid discharge with continuous current flow without any spark.


A defibrillator is an electrical device that produces a shock that helps the heart regain a stable rhythm or provides treatment for ventricular fibrillation. Typically, a capacitor is used to deliver a high voltage shock to the heart which is charged either by a power supply or a battery. The use of capacitors is preferred because of their fast charging and discharging, their ability to store high voltage, and their stable output.

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By Ranjan