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Biomedical Engineering
Defibrillator Energy & Capacitor Discharge Simulator
Calculates the electrical behaviour of capacitor-discharge defibrillators used in AED and ICD devices. Change stored energy, capacitance, transthoracic impedance and waveform type to see peak voltage, peak current, RC time constant, pulse width and delivered energy update in real time, and check the safety margin against the DFT.
Parameters
Stored energy U
J
Energy stored on the main capacitor (adult standard 150-200 J)
Capacitance C
μF
Main capacitor (AED typically 100-200 μF)
Chest impedance Z
Ω
Transthoracic resistance (adult mean 70-80 Ω)
Charging time t_c
s
Time required to charge the main capacitor from the battery
Waveform type
Waveform shape changes the cardiac efficiency
Results
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Peak voltage V_peak (V)
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Peak current I_peak (A)
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RC time constant τ (ms)
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Pulse width (ms)
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Charging power (W)
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Delivered / DFT ratio
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Chest and paddle discharge schematic
Capacitor discharges from anterior-right and apex paddles through the chest. The current direction through the heart reverses mid-pulse for biphasic waveforms.
U: stored energy [J], C: capacitance [F], V: peak voltage [V], Z: chest impedance [Ω], τ: RC time constant [s]. Biphasic waveforms reverse polarity to balance membrane charge, achieving high success at lower energy.
$$P_{charge} = \frac{U}{t_{c}}, \qquad E_{deliv} = \eta \cdot U, \qquad SM = \frac{E_{deliv}}{DFT}$$
P_charge: charging power [W], η: waveform efficiency (BTE 0.95 / monophasic 0.85 / BCF 0.98), SM: safety margin over the defibrillation threshold. SM ≥ 1.5 is the clinical benchmark for ICD implant.
Defibrillator energy design
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An AED is the device they grab on TV shows, shout "clear!" and shock someone. Does electricity stop the heart, or make it move?
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Good question - it actually stops it, briefly. Ventricular fibrillation is a state where every myocardial cell quivers on its own rhythm, so the heart no longer pumps. The defibrillator pushes a strong current through the chest and forces all the cells to depolarise at once. That resets them, and the sinoatrial node can take over again with a normal rhythm. The word "defibrillation" literally means "remove the fibrillation".
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A reset button! How do they generate that much current though? A normal battery can't do it, right?
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Right - 9 V or 12 V batteries are nowhere near enough. That is where the capacitor comes in. A switching converter steps the battery up to 1500-2000 V and stores the energy on a 150 μF capacitor. From U = (1/2)CV², storing 200 J needs V = √(2·200/150e-6) ≈ 1633 V. Discharging into a 75 Ω chest gives a peak current of V/Z ≈ 22 A flowing for a few milliseconds. The wattage is huge for those few milliseconds even though the average power is low.
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If I push the impedance slider up to 200 Ω the peak current drops a lot. Does that really happen on real patients?
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Yes. Dry skin, large body habitus, badly placed pads - any of those can push Z above 100 Ω. Old monophasic units had no real answer except to crank up the energy. Modern AEDs use impedance-compensated BTE: the first millisecond is used to measure Z, then the pulse amplitude and duration are reshaped in real time. Switching to "BCF (constant-current biphasic)" in this tool emulates a control loop that holds I_peak roughly constant regardless of Z.
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What is the "DFT ratio"? It says 1.9 - is something smaller dangerous?
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DFT is the Defibrillation Threshold - the minimum energy needed to convert VF. Clinical adult BTE averages about 100 J. The ratio of what your device can deliver to that DFT is the safety margin. ICD implants always include a DFT test to make sure the maximum shock is at least 1.5x DFT. Below 1.0 there is a real risk that the shock won't terminate VF, which is why this tool turns the verdict yellow below 1.2.
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So medical devices really are designed with that much headroom. Is there a reason not to just crank the energy up?
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Plenty. More energy means more myocardial damage - necrosis, stunning, proarrhythmia. The clinical curve is "success plateaus above DFT, damage rises linearly". Modern biphasic units convert reliably at 150-200 J, so there is no need to throw 360 J. The 60 A peak-current cutoff in this tool reflects skin-burn and over-stimulation risk. The design philosophy now is "just enough, no more".
Frequently asked questions
The capacitor stores energy U at capacitance C, and U = (1/2)CV² fixes the peak voltage V. For example, 200 J in a 150 μF capacitor gives V = √(2U/C) = √(2·200/150e-6) ≈ 1633 V. Real AEDs reach 1500-2000 V and ICDs 700-800 V in milliseconds, then discharge through a transthoracic impedance of 50-100 Ω to depolarise the myocardium.
The monophasic waveform drives current in one direction only and historically needed 200-360 J. Biphasic waveforms (BTE, BCF) reverse polarity midway, putting less electrical stress on the myocardium and achieving equal or higher defibrillation success at 150-200 J. This tool models efficiencies of 85% (monophasic), 95% (BTE) and 98% (BCF) so you can see the gap between stored and delivered energy.
Adult transthoracic impedance ranges from 25 to 200 Ω with a mean near 70-80 Ω. It varies with pad contact, body habitus, skin moisture and respiratory phase. Higher impedance reduces peak current and hurts defibrillation success, so modern AEDs use impedance-compensated BTE algorithms that measure Z during the first millisecond and reshape the pulse on the fly. Move the Z slider in this tool to feel how strongly it changes I_peak.
The DFT is typically 90-150 J (about 100 J on average for adult BTE). Clinically, a margin of at least 10 J above DFT, or a maximum-output-to-DFT ratio of 1.5-2.0, is regarded as safe. ICD implants always include a DFT test. This tool reports delivered_energy / 100 J as a safety-margin ratio and flags values below 1.2 as a warning zone.
Real-world applications
Public-access AEDs: Devices installed in stations, airports, schools and shopping malls are designed for use by untrained bystanders. ECG analysis, shock advisability and energy selection are fully automated. The biphasic waveforms (BTE, BCF) modelled here are the current standard; modern AEDs deliver up to about 200 J and tune each shock to the individual patient through impedance compensation.
Implantable cardioverter-defibrillators (ICDs): Small devices implanted under the left clavicle in patients with a history of lethal arrhythmia. The internal battery and capacitor detect ventricular fibrillation and deliver 30-35 J within seconds. Because of the small enclosure the capacitor is smaller (80-120 μF), so high-efficiency BTE waveforms are mandatory, and a DFT margin test is routinely performed at implant.
Manual hospital defibrillators: Used by physicians in the emergency department and ICU, these allow manual energy selection between 1 and 360 J and support synchronised cardioversion (QRS-locked discharge). 50-100 J is typical for atrial fibrillation/flutter, while 150-200 J biphasic is standard for ventricular fibrillation. Switching the waveform type in this tool gives a sense of how these regimes differ.
Wearable cardioverter-defibrillators (WCDs): Vest-style devices worn by patients who are not yet implant candidates (post-MI, awaiting transplant). Continuous chest electrodes detect arrhythmias and, after gel deployment, deliver around 75 J. Capacitor design and charging-circuit efficiency drive the wearable form factor, so the trade-off space studied here is directly relevant.
Common misconceptions and caveats
The biggest misconception is that stored energy equals delivered energy. As this tool shows, only a fraction of the U stored on the capacitor reaches the myocardium - waveform-shaping components (inductors, IGBT switches, gel resistance) dissipate 5-15% as heat. An AED that shows "200 J selected" actually delivers about 190 J with a BTE waveform and about 170 J with a monophasic waveform. Clinical guidelines that say "shock at 150 J" usually refer to delivered, not stored, energy.
The second misconception is that more energy always means higher conversion success. In reality, once delivered energy exceeds DFT the success rate plateaus and additional energy only increases myocardial damage (transient stunning, proarrhythmia, skin burns). Monophasic units of the 1980s defaulted to 360 J because their efficiency was poor; today, 150-200 J biphasic is the standard. The 60 A peak-current cutoff used here reflects this over-stimulation risk.
Finally, the assumption that bigger capacitors are always better is wrong. For a fixed stored energy U, increasing C lowers V (U = CV²/2), which eases insulation design. But a larger C also stretches τ = RC and the pulse width, eroding the rapid polarity-flip advantage of biphasic waveforms. Real designs cluster around C = 100-150 μF as the sweet spot for efficiency and DFT reduction; ICDs deliberately use about 80 μF at higher voltage to keep the can small. Capacitance, voltage and pulse width are coupled - you cannot tune one in isolation.
How to Use
Enter the target defibrillation energy in joules (typically 120–200 J for biphasic waveforms in AED devices)
Set the capacitance value in microfarads (standard defibrillator capacitors range 100–200 µF)
Input patient transthoracic impedance in ohms (normal range 50–150 Ω; obesity or electrode placement affects this)
Specify the charge voltage or let the simulator calculate it from energy and capacitance
Review peak voltage V_peak, peak current I_peak, RC time constant τ, and pulse width to verify safety margins
Check the Delivered/DFT ratio to confirm energy meets defibrillation threshold (ratio ≥ 0.95 recommended)
Worked Example
A biphasic AED delivers 150 J with a 120 µF capacitor bank into a patient with 80 Ω transthoracic impedance. Simulator calculates: charge voltage = 1581 V, peak voltage V_peak = 1581 V, peak current I_peak = 19.8 A, RC time constant τ = 9.6 ms, pulse width = 7–8 ms (monophasic equivalent). With a DFT of 155 J for this patient, the Delivered/DFT ratio = 0.97, indicating adequate safety margin. Charging power reaches 2.1 kW during the 3–4 second charge cycle.
Practical Notes
Transthoracic impedance varies significantly with electrode size (8 cm vs 13 cm adult pads), contact pressure, skin moisture, and chest size; high impedance (>140 Ω) reduces effective current and may necessitate escalation to the next energy level
Biphasic waveforms (common in modern ICDs/AEDs) achieve defibrillation with 50–70% less energy than monophasic; simulator accounts for waveform efficiency in DFT calculations
Capacitor aging or leakage reduces actual stored energy; verify calibration annually per IEC 60601-2-4 medical device standards
RC time constant >10 ms risks prolonged chest compression interruption; confirm pulse width compliance with rhythm-dependent protocols (VF vs PEA)