If this was wired up, what is the power rating the resistor needs to be? Initially there is 17.5 Watts dissipated by the resistor, but this is for a very short amount of time. So, for instance, would a 10W resistor suffice?

Does the power rating need to be different if the supply is pulsed on/off compared to being on steady? Does the capacitor size matter in choosing the correct power rating of resistor?

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  • \$\begingroup\$ Is 1mF equal to 1000uF? \$\endgroup\$ – Andy aka Sep 12 '13 at 13:25
  • \$\begingroup\$ Yes, 1mF is equivalent to 1000uF \$\endgroup\$ – brett s Sep 12 '13 at 16:21

Depending on its specific design, the resistor will be able to handle short peaks of power dissipation far above its rated capacity.

With regard to a pulsing supply: that depends on the supply and how it behaves when "turned off". If the supply becomes high impedance in the off state, nothing much will happen as the capacitor just retains the charge (presuming the exact circuit as depicted with no loads other than the series resistor). If the supply is shorted when off (or a load is draining the capacitor), it's a different story - the capacitor will charge when the supply is turned on and discharge when the supply is turned off.

Generally, if the charge time of the capacitor isn't very long, calculating the RMS power dissipation over a single cycle and taking a safety margin of 2x should give you plenty of headroom.

  • \$\begingroup\$ Is there an easy way to calculate RMS power dissipation over a single charge cycle? \$\endgroup\$ – brett s Sep 12 '13 at 16:22
  • \$\begingroup\$ And when the supply is off, the capacitor drains. Is the drain time different from the charge time? \$\endgroup\$ – brett s Sep 12 '13 at 16:23

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