Quick question on capacitor charge/discharge
Hey guys, I'm looking for a general feel from the experienced designers here on what size resistor can safely discharge 20,000uF at 70V.
Would a 10W 5 Ohm resistor be able to handle the surge without failure? I realize the instantaneous power is quite a bit over the rated amount, but can something of that size safely handle the occasional discharge/charge duties? The goal is to bring the cap up to at least 50V (~.5 sec) before bypassing it with a relay or MOSFET. Thanks 
Ohms law would help lots in this situation... V over I equals R

Most wirewound power resistors seem to have surge ratings of 5 times their nominal rating for 5 seconds max. Shorter times should allow higher than this but it's usually not specified so you're on your own. I found one range that are meant for short surge usage, a 7 watt resistor seems to be OK for up to 60 joules of energy.
http://www.welwyntt.com/pdf/datasheet/WHS.pdf According to this calculator Capacitor Energy (E) and RC Time Constant Calculator you'll have 49 joules to contend with using your values, but there again your time constant is only 0.1 sec so over to you to play around with numbers. 
T=R*C
0.5/0.02= 25 Ohms 10w should be OK considering the short time. (I do use a soft start with 47 ohms 20w) I would use 25 ohms also for discharging the cap to avoid sparkle. 
You may also need to worry about the effect on the cap. How does your discharge current compare with the ripple current rating?

Indeed, that's why 5 ohms is too small.

Thanks guys (except for the snarky response repeating Ohm's law).
Ripple current rating is a steady state rating, just like the rated power of a resistor. As an example, look at the actual current levels in a 20,000uF cap on a full bridge rectifier providing a steady 4 amp load current to a 70V rail. At the peak of the rectified 60 cycle source, the peak current into the cap will exceed 12A at 120Hz with a short duty cycle, and the ripple voltage on the cap will be a mere 1.5V. That's a pretty common setup for a power amp. The RMS ripple current is by definition, 4A, but the peak currents are repeated spikes of over 12A. Discharging the cap with a 5 Ohm resistor falls easily within such a scenario. I have no doubt the cap can handle that discharge cycle. It's the resistor that concerns me. The 12A initial current is an instantaneous power level that's quite a bit higher than rated, obviously, but it is very short duration. I was curious if the actual resistive element in a typical 10W wire wound resistor could withstand the initial surge. I guess I'll have to look into using a higher wattage resistor just to be safe. 
Curiosity:
May we know what for? 
1 Attachment(s)
I'm designing a 150W @ 8 Ohm, MOSFET output, fully complimentary, pushpull Class A amplifier with built in active filtering on the positive and negative power rails. I'm trying to limit inrush current using a 10W, 5 Ohm resistor that is shunted by a MOSFET after the first stage capacitor reaches 50V. Spice modeling shows that this occurs in roughly .5 sec using a 70V (peak) rectified voltage. Here's a shot of the current in the resistor and the voltage on the 14,100uF cap (4700uF x 3). The filter stage consists of a floating regulator that drops a couple volts and reduces ripple voltage from ~1.5V to ~20mV. An additional pair of 4700uF caps is at the ouput for the 20000uF total.
The rapid drop in current at the end of the plot is where the MOSFET bypass has kicked in. Attachment 321719 
The big transformer inrush current is the main issue to deal with.
Soft charge is secondary. 
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