Dave -- How did you come up with the 0.5J number ? The energy to be dissipated in a load resistor is going to depend on the resistor value, generator source impedance, stored voltage and the size of the energy storage cap – and even then it’s difficult.
If you have a 500 ohm source and 500 ohm load, half the energy stored in a charged capacitor will be dissipated by the source impedance (if it’s resistive) and half in the load. Of course, this is an oversimplification because commercial 1.2/50us generators use more complex wave forming networks where much of the stored energy goes into shunt resistors to get the waveforms right. In any case, Energy = Power x Time and maximum power occurs when the load impedance matches the generator source impedance. For load impedances NOT equal to the source impedance of the generator, the delivered energy will be less than ½ the available stored energy. Just to make it more complicated, you’ll also need the current waveform…. The reason I say all this is that I’ve come across a number of specs calling for testing with “x” joules, or using a generator of “x” joules. One can easily calculate stored energy but the available energy to a load is usually much less due to the waveform network and for most generators this is unknown. Pt is difficult because you need the current and voltage waveforms, which depend on the load impedance of the test object so the generator may need to be constructed differently for different loads. Good luck BTW – something like a 10 w carbon comp resistor will work fine and won’t blow up – you could probably get away with a lower wattage, but 10W should be safe.( Let me know if you can find a supply of carbon comp resistors) – ceramic will work fine as well. Best Regards, Mike Hopkins From: Dave Wilson [mailto:[email protected]] Sent: Thursday, October 29, 2009 12:59 PM To: [email protected] Subject: [PSES] Converting energy to power rating for surge transient Hi, I need to determine what power rating a resistor has to be to adequately dissipate 0.5J (a 1.2/50us surge of 500V from a 500 ohm source). Anyone have a conversion formula? Thanks in advance, Dave - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher <[email protected]> David Heald <[email protected]> - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher <[email protected]> David Heald <[email protected]>
