Over powering a wire wound resistor Pulse rating Joules - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: April 01, 2019, 04:01:40 pm
content: We need to provide a dummy load of 500to 120 MAINS for 15s, once per hour. So ~29WTypically wire wound resistors have a pulse or short term over-power rating for example 5x or 10x for 5s, longer than that specifications are fuzzy at best.For a reality check I connected up a 5W wire wound resistor and applied 30W for 15 seconds, temperature moved from ~ 20 deg C to 65 deg C after 15 seconds, so temperature riseseemsperfectly acceptable.Looking at this part: HPP5-500RJ8https://riedon.com/media/pdf/HPP.pdfIt has a Pulse rating in Joules of 5.8?Based on 29W*15sec = 435 J, not sure how this relates to the 5.8 specified. Obviously I'm missing something?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 01, 2019, 05:03:13 pm
content: Why does it need to be smallest/cheapest?If it doesn't at all, just get a bigger one, 10-50W is very affordable. Don't even need a heatsink, if you use an aluminum body type.Which, a note: aluminum-body resistors tend to be cheaper and smaller. Well, they are, until you factor in the heatsink and mounting labor required to use them at ratings! Without heatsink, they're rated a tiny fraction of the total rating, and traditional ceramic body resistors are the better overall deal.For low average dissipation (averaged over minutes+), they'd actually be perfect here, though. The extra conductive mass yields lower peak temperatures in operation like this.Don't forget to put a thermal switch or other limiting/lockout/protection on it! All too often I've seen, e.g. in industrial equipment, mains precharge resistors, that didn't get switched out and then had to power the full load by themselves, which needless to say they didn't take kindly to.Regarding energy -- it's a sliding scale with respect to time. Obviously, for peak power on the order of rated power, the maximum pulse width is infinite, and therefore so is the energy. So, as peak power goes up, the total energy is hyperbolic with time.If the resistor body heated up uniformly, didn't dissipate heat during the pulse, and dissipated it only afterwards, then all the energy of the pulse would be stored as heat and the peak temperature would depend only on the total energy of the pulse. That would give peak power inversely proportional to time -- constant energy.But that's a silly case. Real resistors dissipate power when their outside is hot, and real resistors are made of materials which take time for heat to diffuse through them. So we expect the pulse energy to rise (eventually rising very sharply towards infinity) at lower peak power levels. We should expect modest overloads (2x say, but probably not 5x+) can be tolerated for a long time, presumably not forever (material degradation will kick in), but handling quite a lot of energy in the process, in any case. We should also expect that energy goes down at short time scales, where less of the resistor itself has been heated up (except for some wirewounds, and high-energy bulk/composition resistors).As it happens, the response tends to be energy proportional to sqrt(t) or thereabouts (at peak powers much higher than rated, or equivalently: pulse durations much shorter than the overall thermal time constant). Which implies a diffusion mechanism, and indeed heat is diffusive, so that makes sense.Or equivalently, peak power proportional to 1/sqrt(t).Real datasheets show this curve having an exponent of 1/3 (i.e., cube root) to 1/2 (sqrt), but it may be as low as 0 (constant energy).What use is this? For present case: we're in the hyperbolic range, so, it's not very useful. We only have one datapoint (e.g. 5x rating 5s), not enough information to extrapolate the curve.For short pulses: we can infer some things about devices of similar construction. Given a datasheet for, say, a pulse-rated wirewound resistor, we should expect similar parts have similar pulse ratings, even if they aren't rated for it.And, given that we don't require a guarantee of operation for this part.An example might be mains surge arresting, or crowbarring a fuse, or other very infrequent stuff like that: the equipment might be expected to handle ten cycles in its entire lifetime, say. In that time, it's unlikely that the part will fail (and it probably will, due to, say, inferior terminal welding, or coatings that can't handle the thermal shock, or..).Of course, the obvious exception that you'll inevitably encounter, is the one customer with super dirty power that fatigues your protection scheme to failure in a couple of months. Then it's a business decision: so-and-so many unhappy customers, or this-and-that added cost for the larger or pulse-rated part.Whereas when reliability is required (say, test equipment, a surge generator that's literally doing this its entire life!), definitely opt for the pulse rated part.Tim

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 01, 2019, 05:08:17 pm
content: Although it is tempting to think aluminium resistors can be abused due to the thermal mass, you need to factor in the internal thermal resistance between the wire and the case, and the lack of space for anything to expand internally.I have seen one explode, shooting an end terminal across the room, trailing the resistance wire behind it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 01, 2019, 05:23:30 pm
content: ^ This.All the more reason to mind the ratings.Tim

author: Jester
author_rank: Frequent Contributor
timestamp: April 01, 2019, 06:14:36 pm
content: I found this and it's helpfulhttps://www.mikrocontroller.net/attachment/323666/Pulse-Overload_AN.pdfSo with a 7W W22, not great, however with 2x5W W215, probably okay

author: nctnico
author_rank: Super Contributor
timestamp: April 01, 2019, 09:31:41 pm
content: I have used these resistors in a pulsed power application. I don't know if it is in the datasheet but IIRC there is good documentation on the maximum loading versus time.https://nl.mouser.com/datasheet/2/427/acac-at-222859.pdfEdit: couldn't resist and checked. It seems these also top out below 10s. This means you have to use resistors which are rated for 30W if you want to have any reliability. The problem isn't in the outer temperature of the resistor but that of the resistive element. It will become way too hot.

author: Jester
author_rank: Frequent Contributor
timestamp: April 01, 2019, 09:42:56 pm
content: Quote from: nctnico on April 01, 2019, 09:31:41 pmThe only reference I could find for > 10s was the Welwyn application note and it shows what to expect up to 100s.Although not exact, I think you can estimate based on the 5x or 10x ratings for example the 7W Welwyn has a 10x, 5s rating so 7x10x5 or 350J maxI was looking at 29W for 14s or 406J and I would be a bit past the curve for the 7W part (x4.15), however with the 2x5W part; 5x10x5 = 2x250J, and when I look at the graph (x2.88), I'm left of the curve. Not sure how applicable this is to other manufacturer resistors?