Quick question on capacitor charge/discharge - diyAudio
 Quick question on capacitor charge/discharge
 User Name Stay logged in? Password
 Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read Search

 Solid State Talk all about solid state amplification.

 Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
 5th January 2013, 08:23 AM #1 diyAudio Member   Join Date: Oct 2006 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
 5th January 2013, 10:10 AM #2 diyAudio Member   Join Date: Dec 2010 Location: wigan Ohms law would help lots in this situation... V over I equals R
 5th January 2013, 10:11 AM #3 diyAudio Member   Join Date: Jun 2005 Location: Kent, UK 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.welwyn-tt.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.
 5th January 2013, 10:16 AM #4 diyAudio Member     Join Date: May 2008 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. Last edited by bobodioulasso; 5th January 2013 at 10:21 AM.
 5th January 2013, 10:56 AM #5 diyAudio Member   Join Date: May 2007 You may also need to worry about the effect on the cap. How does your discharge current compare with the ripple current rating?
 5th January 2013, 11:28 AM #6 diyAudio Member     Join Date: May 2008 Indeed, that's why 5 ohms is too small.
 5th January 2013, 04:12 PM #7 diyAudio Member   Join Date: Oct 2006 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.
 5th January 2013, 05:59 PM #8 diyAudio Member     Join Date: May 2008 Curiosity: May we know what for?
 5th January 2013, 06:55 PM #9 diyAudio Member   Join Date: Oct 2006 I'm designing a 150W @ 8 Ohm, MOSFET output, fully complimentary, push-pull Class A amplifier with built in active filtering on the positive and negative power rails. I'm trying to limit in-rush 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. Spice.jpg Last edited by pete_schumacher; 5th January 2013 at 06:59 PM.
 5th January 2013, 07:44 PM #10 diyAudio Member     Join Date: May 2008 The big transformer inrush current is the main issue to deal with. Soft charge is secondary.

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are Off Pingbacks are Off Refbacks are Off Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post rocco gibralter Solid State 2 10th January 2009 02:20 PM epicfabrication Parts 1 25th September 2008 01:12 PM thkking Car Audio 16 11th June 2005 08:26 PM Thomas001 Parts 2 21st November 2004 08:25 PM newbie88 Everything Else 5 11th February 2004 06:57 PM

 New To Site? Need Help?

All times are GMT. The time now is 10:26 PM.