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Capacitors - Stored Energy

Potential power and energy stored in capacitors.

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capacitor

Capacitor - Energy Stored

The work done in establishing an electric field in a capacitor, and hence the amount of energy stored - can be expressed as

W = 1/2 C U2(1)

where

W = energy stored - or work done in establishing the electric field (joules, J)

C = capacitance (farad, F, µF )

U = potential difference (voltage, V)

Capacitor - Power Generated

Since power is energy dissipated in time - the potential power generated by a capacitor can be expressed as

P = dW / dt                    (2)

where

P = potential power (watts, W)

dt = dissipation time (s)

Example - Capacitor, energy stored and power generated

The energy stored in a 10 μF capacitor charged to 230 V can be calculated as

W = 1/2 (10 10-6 F) (230 V)2

= 0.26 J

in theory - if this energy is dissipated within 5 μs the potential power generated can be calculated as

P = (0.26 Joules) / (5 10-6 s)

= 52000 W

= 52 kW

Be aware that in any real circuit, discharge starts at a peak value and declines. The energy dissipated is a very rough average power over the discharge pulse.

Capacitor - Time to Discharge at Constant Power Load

The time to discharge a capacitor at constant power load can be expressed as

dt = 1/2 C (U s 2- U f 2) / P               (3)

where

dt = discharge time (s)

U s = start voltage (V)

U f = final voltage (V)

Breakdown (puncture) Voltage

Puncture voltage at 1 MHz (V/mil, V/0.001 inch):

  • Air: 240
  • Alsimag: 240
  • Bakelite: 300
  • Bakelite, mica-filled: 325 - 375
  • Cellulose acetate: 250 - 600
  • Formica: 450
  • Glass, window: 200 - 250
  • Glass, Pyrex: 335
  • Mica, ruby: 3800 - 5600
  • Mycalex: 250
  • Plexiglas: 990
  • Polyethylene: 1200
  • Polystyrene: 500 - 700
  • Porcelain: 40 -100
  • Quartz, fused: 1000
  • Steatite, low-loss: 150 - 315
  • Teflon: 1000 - 2000
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