Timeline for Resistor surge rating
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19 events
| when toggle format | what | by | license | comment | |
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| Apr 27, 2017 at 5:54 | vote | accept | Jamie Lamb | ||
| Apr 25, 2017 at 0:45 | comment | added | jonk | @JamieLamb I'm mostly worried about your chart given in your original post. That places a sharp limit in the early period, regardless of the final duration. And with an RC time constant in the hundreds of microseconds, most of the energy will be delivered early on. They definitely place a hard limit there, which for the first 100 us (I think) is the most stringent part. | |
| Apr 25, 2017 at 0:18 | comment | added | Jamie Lamb | equation 3 Jonk, I can convert the waveform into an equivalent square wave pulse like is done for designing TVS diodes, I believe its a good approximation. The pulse will be in the region of ms not us but obviously that depends on the size of the capacitor. i understand its not realistic to take a periodic signal with infinte period but real world electronics should be designed to allow turning on and off so thats my logic, every half a second or so | |
| Apr 25, 2017 at 0:01 | comment | added | jonk | @JamieLamb Don't you mean equation (4) on page 2? That one is for exponential decays. And by the way, that equation isn't useful for figuring out short bursts. As \$t_p\rightarrow \infty\$ (for a single pulse case) the average power figure goes to zero. And that tells you nothing useful about uncovering single pulse fusing events. | |
| Apr 24, 2017 at 23:45 | comment | added | Jamie Lamb | That datsheet says thin film resistors are preferable over thick film but from what I can gather thick film resistors come in higher wattages | |
| Apr 24, 2017 at 23:41 | comment | added | Jamie Lamb | vishay.com/docs/28870/pulseloadsmdlimit.pdf, equation 3 Jonk. I can use the technique from EMC where they convert an exponential decay into an equivalent square pulse and it works well for designing things like TVS diodes etc. Just to simplify the question | |
| Apr 24, 2017 at 23:18 | comment | added | jonk | @JamieLamb I added a few notes and changes to the answer. Just so you know. | |
| Apr 24, 2017 at 23:17 | history | edited | jonk | CC BY-SA 3.0 |
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| Apr 24, 2017 at 23:03 | history | edited | jonk | CC BY-SA 3.0 |
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| Apr 24, 2017 at 23:00 | comment | added | jonk | @JamieLamb For very short pulses, a resistor is a fuse. Shock it with enough energy in a short enough time and some tiny spot inside of it simply burns out or explodes. (Hmm. I see an error in what I wrote.) That's because there's no time for heat to flow to the spot's surroundings. But if you meter out the energy over a longer time, then it gradually becomes limited instead by its dissipation rating. You can post a link here in the comments, I suppose. | |
| Apr 24, 2017 at 22:39 | comment | added | Jamie Lamb | I couldnt post the Vishay data sheet because not enough rep, I could only name it. Pulse Load on SMD Resistors: At the Limit, theres an equation in there which is just power derated by duty cycle and in my mind that should work for any resistor but it doesnt match the graph from the actual resistor used, very confusing! It was the main reason I asked the question to try and work out why | |
| Apr 24, 2017 at 22:38 | comment | added | Jamie Lamb | I have a DC input of 100V, two 3.3 Ohm resistors in parallel so equivalent 1.65 Ohms. Theres a diode and EMI filter and the ESR of the capacitor all work to decrease my measured current No problem though I can work with what you supplied, its really appreciated | |
| Apr 24, 2017 at 22:36 | comment | added | jonk | @JamieLamb I didn't look at the datasheet much more than to look at the chart and to use the curve there to estimate the short-term energy it could tolerate vs time. It was a simple derivation, since the derating line is "straight" on the log scale. | |
| Apr 24, 2017 at 22:34 | comment | added | jonk | @JamieLamb Yes, the Vo is the 132 Vdc that I estimated based on your 40 A figure. (40 A times 3.3 Ohms.) | |
| Apr 24, 2017 at 20:18 | comment | added | Jamie Lamb | The supply voltage is 100VDC and you are correct I am using two of these resistors in parallel to give a total resistance of 1.65 Ohms. What still doesnt make sense to me is how the equation in the Vishay datasheet doesnt agree with the graph, if you look at it its simple (V^2/R)*(duty cycle), it makes sense to me, if it was a 1 second pulse with a 1 second period then its DC and there wouldnt be any derating so in my mind that Vishay datasheet equation should work for all resistors, it looks like it simply derating for power. Thanks for you answer its appreciated | |
| Apr 24, 2017 at 20:14 | comment | added | Jamie Lamb | Thanks for this answer, the Joules per Ohm is not available from datasheets I could consult the manufacturer, maybe if I get time later it will round it all off. I can see how you have arrived at you equations but I havent checked the details (yet). In your equation for Edecay is Vo the input voltage? | |
| Apr 22, 2017 at 19:30 | history | edited | jonk | CC BY-SA 3.0 |
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| Apr 22, 2017 at 19:19 | history | edited | jonk | CC BY-SA 3.0 |
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| Apr 22, 2017 at 19:12 | history | answered | jonk | CC BY-SA 3.0 |