Determine maximum resistor pulse load? - Page 1
Forum section: electronics / projects

author: nctnico
author_rank: Super Contributor
timestamp: December 02, 2015, 11:36:26 pm
content: I'm having some trouble to determine which resistor is suitable to handle the inrush current of a capacitor. My biggest problem is to translate the amount of energy I need to get rid of (6J in 10ms once per minute) to a graph in a resistor datasheet. So far Vishay seems to be the only manufacturer with detailed pulse handling graphs but they all show power versus time and their application notes don't really connect the dots in a way I am confident I calculate a properly sized resistor.Is there more documentation out there?

author: sarepairman2
author_rank: Frequent Contributor
timestamp: December 02, 2015, 11:42:20 pm
content: is this something that can only really be determined statistically? Are there any kind of experiments that can help determine this (i.e. thermal imaging, looking at how hot spots form?)

author: mkissin
author_rank: Regular Contributor
timestamp: December 03, 2015, 12:30:26 am
content: The documentation on this varies wildly between manufacturers and part families. Generally in a case like this you want to stay under the average power handling limitations over the long term and you also need to stay under the instantaneous fusing energy.This datasheet from Ohmite is an interesting (but brief) read:http://www.ohmite.com/cat/res_30.pdfEdit: Actually, I knew someone specced it, this is the type of data you're looking for:http://www.ttelectronicsresistors.com/pdf/application_notes/Pulse-Overload_AN.pdf

author: sarepairman2
author_rank: Frequent Contributor
timestamp: December 03, 2015, 08:43:33 am
content: how about non WW for RF ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 03, 2015, 09:28:14 am
content: Resistor ratings tend to follow a diffusion mechanism. That is, for pulse durations shorter than the thermal time constant, peak power goes as P = P_0 * sqrt(t_0 / t), where P_0 and t_0 represent the power at a given duration within this region.If it were constant energy, it would be straight inverse (P = E / t), no sqrt. This should be more typical of high-pulse, bulk resistors (which are available in rod, tube, disc and other shapes, often intended to be clamped into a stack between heatsinks!), but tends not to be typical of conventional film or wire types.(Energy should still be typical of film or wire types, but, the energy that the actual conductive material can withstand is not nearly as much as the rest of the body, and it takes time for the heat to diffuse out. Hence the effect. Point being, though, for sufficiently short pulses, it should tick up to proportional (constant energy) again. But, this might occur over such short time scales (microseconds?) thatelectrical diffusioneffects take over, i.e., skin and proximity effect! So, better to stay on the safe side. And anyway, even "noninductive" wirewound resistors have enough inductance that they aren't really resistors for pulses this short!)So, if you take that assumption, you can cheat a little more power out of your resistors for arbitrary pulse widths. You won't have the manufacturer's guarantee, but you can at least twist your noise a little, saying it's not completely outrageous. Versus a constant-energy limit, which is probably too aggressive and therefore unreliable, or a constant-power limit, which might be too inconvenient (needing much more volume than actually required).Anyway, as for pulses (such as discharging capacitors), you would be able to apply this weighting rule to a known waveform, and compute the integral. No need for statistics or microscopic analysis, just a little calculus.Tim

author: nctnico
author_rank: Super Contributor
timestamp: December 03, 2015, 09:48:16 am
content: I like the inductance suggestion. I'm going to add that to my pspice model of the circuit.I have also given the peak power versus time graphs some thought. Since power is Joule per second the amount of energy in Joule is power divided by the time interval.

author: dmills
author_rank: Super Contributor
timestamp: December 03, 2015, 10:01:43 am
content: Some of the thick film parts are pulse rated and not overly inductive, Vishay often specify an energy/time curve for pulse applications.The LTO-30 series for example seems to be specified to 10J in a 10ms.Or for something smaller and more SMT, try the RCA-IF AT series again a Vishay part.Ohmite have some serious pulse parts as wellhttp://www.ohmite.com/search.php?appl=Surge/High%20energy&function=results.The pulse thing does however very much depend on the specifics, so datasheets are a must read for this stuff (And always specify that substitutions are not permitted for these parts when sending a board to be fabbed).Regards, Dan.

author: Pjotr
author_rank: Frequent Contributor
timestamp: December 03, 2015, 10:48:09 am
content: To be pragmatic stay on the safe side of the instant amount of energy the resistor can handle. If that is not given I spend a 5 minutes with PSpice for the power over time graph and see how far it matches the pulse data on the datasheet. If you want to avoid al the hassle, 6J is a fair amount of energy for a small resistor, look for the ceramic composition solid body ones, i.e. Tyco/CGS CCR series. The datasheet gives a good example how to determine the actual pulse load. B.t.w. inrush NTC's are often used in this case because of their high energy absorption capability, even when they never get warm/hot in normal use.

author: nctnico
author_rank: Super Contributor
timestamp: December 03, 2015, 12:17:16 pm
content: @Pjotr: Those Tyco CCR resistors look impressive! It seems the 0.5W one can already handle 6J in a 10ms timeframe. Unfortunately they don't go low enough in value. The ideal value for my circuit seems to be around 0.68 Ohm. I think I'll settle for the Vishay AC03 (3W) series which seem to be able to handle the worst case scenario and are also cheap. BTW self inductance seems not to matter on this time scale.

author: Pjotr
author_rank: Frequent Contributor
timestamp: December 03, 2015, 01:37:25 pm
content: Hi ntcnico,Is that 10ms that important then regarding the 1 minute repetition rate?

author: nctnico
author_rank: Super Contributor
timestamp: December 03, 2015, 04:06:48 pm
content: The purpose of the resistors is to limit the rush-in current in order to keep a bunch of MOSFETs within their safe operating area. Unfortunately I have not been able to find larger MOSFETs with a very low leakage current. The 1 minute repetition rate a just there to show the pulses are not (very) repetitive. Typically the device is turned on for at least a couple of minutes. Unfortunately I also have no control over the capacitance of the load.