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How to Choose a Commutating Capacitor
1. From circuit analysis or measurement, determine application values
for these six parameters:
Nominal capacitance in µF
Current pulse width in µs
Current pulse period in µs
Maximum peak voltage
Continuous AC voltage in Vrms
Maximum volt-amps (VA)
2. Choose a capacitor from the ratings table of the desired nominal
capacitance with a peak voltage rating no less than your maximum
peak voltage.
3. Check that your application’s rms current is no more than the capacitor’s
Max. Amps RMS. You can calculate the current from your Vrms using the
equations in the following section.
4. Check that your application’s volt-amperes is not more than the
capacitor’s VA capability. The VA capability is the max VA rating times
the Volt-Ampere multiplier from Figure 2 (Current Pulse Width) and that
times the Volt-Ampere Multiplier from Figure 3 (Ambient Temperature).
See the following section for more on using volt-ampere multipliers.
If you need a greater VA capability, repeat these steps for a higher peak
voltage capacitor or consider connecting units in parallel to divide the
VA required. For up to peak voltage of 600 V, you may also consider
polypropylene lm dielectric units, Catalog Numbers SCRN262R through
SCRN266R, with higher VA capability.
Using Volt-Ampere Ratings
The capacitors maximum VA rating is the maximum product of the
sine wave voltage and current that may be applied at 65 ºC without
overheating the capacitor and reducing its expected life. For other
temperatures and pulsed current, use the multipliers of Figures 2 and 3
to derate the Max VA rating.
The Max Amps RMS rating is set by the capability of the capacitor
terminals. Exceeding this limit can damage the terminals and cause
capacitor failure.
Calculate the capacitor’s actual VA load as the product of the rms voltage
across the capacitor and the rms current through the capacitor. To
calculate rms current for an applied sine wave or squarewave voltage,
use these equations.
For a sinewave voltage the current is:
Irms = 2π fCVrmsX10
-6
and for a squarewave the current is:
Irms = C∆V/[0.64(tT)
0.5
] = Ipeak(t/T)
0.5
where (f) is repetition frequency in Hz, C is nominal capacitance in µF, ∆V
the peak-to-peak squarewave amplitude in volts, (t) is the pulse width in
µs and T is the pulse period in µs.
The peak current for the square wave voltage is:
Ipeak = C∆V/0.64t
Figure 1
Pulse Wave Applications
V peak
ΔV
Voltage
I peak
Current
Application Guide
Type SCR, Commutating Capacitors
Type SCRN Film-Paper/Extended Foil Commutating Capacitor
T
