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How to Choose a Commutating Capacitor

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 film 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 actualVA 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:

and for a squarewave the current is:

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