Capacitance & Inductance - Defibrillators - (FRCA)

Click to watch the 1 minute concept video on defibrillator capacitors and then the subsequent videos on the physics behind capacitance and inductance.

 Please also refresh your knowledge of basic electrical symbols at the end of the page (also an FRCA competence).

Capacity is the efficiency of storing charge without raising voltage (much). 

The most important component of a defibrillator is a capacitor that stores a large amount of energy in the form of electrical charge, then releases it over a short period of time. A capacitor consists of a pair of conductors (e.g. metal plates) separated by an insulator (called a dielectric). Conductors lose and gain electrons easily, and therefore allow current to flow; whereas insulators do not lose their electrons, and hardly allow any current to flow. 

​Capacity's SI is a FARAD.
One farad is 1 coulomb per volt. A coulomb is a unit of charge and is one ampere-second.

Capacitors can be used to store a lot of energy (start motors etc). Made of multiple materials.
It is really just two pieces of metal. Variable capacitors can tune circuits to change output and used for frequency-tuning.

Charge is the amount of electricity going through a circuit. Symbolized q, is a characteristic of a unit of matter that expresses the extent to which it has more or fewer electrons than protons.

Capacitance is charge over voltage - this is very misleading! Seems to imply if you put more charge, it's capacitance will increase. Capacitance however, is the only thing that cannot change in the equation.  Might it be better to say that Charge is capacitance multiplied by voltage. 

​INDUCTANCE
In electromagnetism and electronics, inductance is the property of an electrical conductor by which a change in current flowing through it induces an electromotive force in both the conductor itself and in any nearby conductors by mutual inductance. It is the change in current as time goes on.

Unit of Henry = Volt-seconds divided by amps.
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Inductor is a solenoid for example. A current through a solenoid creates a magnetic field. 

As the current gets bigger, the magnetic field gets bigger. The loop is creating and changing the field. This is called self-inductance. It doesn't want the current going through it when the current has gone through it... so it basically looks like a battery facing the opposite way to current. So, it will prevent current - a 'bad battery'.  As more and more current begins to flow - eventually the circuit will ignore this bad battery. 

DEFIBRILLATORS
For successful defibrillation, the current delivered must be maintained for several milliseconds. However, the current and charge delivered by a discharging capacitor decay rapidly and exponentially. Inductors are therefore used to prolong the duration of current flow. They are coils of wire that produce a magnetic field when current flows through them. When current passes through an inductor, it generates a flow of electricity in the opposite direction which opposes current flow as predicted by Faraday’s law of electromagnetic induction. This opposition to current flow is called inductance (L) and is measured in henries (H). Inductors typically have values of microhenries (µH).

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Power supply

Step-up transformers are used to convert the mains voltage of 240 V AC to 5000 V AC. This is then converted to 5000 V DC by a rectifier. In practice, a variable voltage step-up transformer is used so that different amounts of charge may be selected by the clinician. The control switch is calibrated in energy delivered to the patient (J), because this determines the clinical effect. If a mains supply is unavailable, most defibrillators have internal rechargeable batteries. These supply DC, which is then converted to AC by means of an inverter, and then amplified to 5000 V DC by a step-up transformer and rectifier as above.

eg) If there were 250v going in, with 20 turns on the primary coil - there would need to be 200 turns on the secondary coil to get the voltage to 2500v.

Patient factors
​Successful defibrillation depends on delivery of the electrical charge to the myocardium. Only part of the total current delivered (about 35 A) flows through the heart. The rest is dissipated through the resistance of the skin and the rest of the body. The impedance of skin and thoracic wall act as resistances in series, and the impedance of other intrathoracic structures act as resistances in parallel with the myocardium. The total impedance is about 50–150 ohms, however, repeated administration of shocks in quick succession reduces impedance.

AC/DC Power Supply
Home and office outlets are almost always AC. This is because generating and transporting AC across long distances is relatively easy. At high voltages (over 110kV), less energy is lost in electrical power transmission. Higher voltages mean lower currents, and lower currents mean less heat generated in the power line due to resistance. AC can be converted to and from high voltages easily using transformers.
AC is also capable of powering electric motors. Motors and generators are the exact same device, but motors convert electrical energy into mechanical energy (if the shaft on a motor is spun, a voltage is generated at the terminals!). This is useful for many large appliances like dishwashers, refrigerators, and so on, which run on AC.

DC is quicker delivery and obviously, DIRECTLY applied - like via a battery. There have been trials which have concluded DC is superior to AC in defibrillation settings, presumably due to the time it takes to deliver a precisely timed shock, that does not oscillate in the delivery of power.