Resistor datasheet: How can I use the pulse graphs? - Page 1
Forum section: electronics / projects

author: 741
author_rank: Frequent Contributor
timestamp: August 24, 2022, 03:51:50 pm
content: WRT to this datasheet (https://docs.rs-online.com/3b75/0900766b80d98b26.pdf) -I'm specifying a series R for an LED that pulses 10ms each second. The Vishay DS mentions 'single' vs 'continuous' pulses, but how do I account for Duty Cycle?In particular, the LED in question pulses 10ms each 320s normally/all the time, rising to 10ms each second when showing an alarm state (a rare condition!), and I'm thinking of allowing 100mA (the thing has to be visible head-on from about 4m in a fairly well lit setting (500 lux)).I'm considering using 2 (or more) series resistors to keep the power (V^2/R) per resistor lower.I've always tended to order 1% resitors by default - but here I see that 5% types have 'better specifications' (higher power rating for a given case size), presumably because they only need to stay inside 5%. For example "CRGH Series High Power" (https://docs.rs-online.com/1490/0900766b812d7379.pdf). The 0805 case types are rated at 0.33W continuous.I see no pulse charts here, but my guess is they would be similar to the Vishay parts of the same case size.

author: srb1954
author_rank: Super Contributor
timestamp: August 25, 2022, 05:49:19 am
content: Quote from: 741 on August 24, 2022, 03:51:50 pmI would use the "ratings from the 'continuous' pulse graph. Although the graph doesn't specify a duty cycle in your application the 1% duty cycle is low enough that you shouldn't get excessive heating.QuoteThe reason that you can get a higher power rating for the 5% types is that there can be a greater range of drift allowed for in the load life test. For example, if a design requires a 1% resistor then you wouldn't want a 2% further drift in the resistor value so the power rating has to be downgraded accordingly to minimise the lifetime drift. Whereas, with a design specified with a 5% resistor, an extra 2% drift over its life would not be a concern and consequently a higher power rating can be tolerated.However, there is another factor to consider with pulse ratings. Closer tolerance resistors are often laser trimmed, which you can sometimes see as an L shaped mark in the resistor element. The trim cut into the resistor element causes current crowding around the cut area and, during pulse operation, higher peak temperatures in a much smaller part of the resistor element. This high peak temperature can cause damage to the resistor element or drift in the value and the peak power rating is limited to avoid damage to the resistor element or excessive drift.On the other hand looser tolerance resistors usually don't require trimming and the energy of a pulse is uniformly dissipated over the whole width of the resistor giving lower peak temperatures and less damage to the resistor element.N.B. The best pulse performance is often obtained with the old carbon composition resistors as the pulse energy is dissipated throughout the resistor volume not just in a surface film.QuoteIt isn't really safe to assume that 2 different brands of resistors would have have similar pulse power ratings as there is a wide variation of pulse power ratings between manufacturers of apparently similar resistors. For safety, I would stick with the resistors explicitly specified for pulse operation.

author: niconiconi
author_rank: Frequent Contributor
timestamp: August 25, 2022, 06:09:37 am
content: Vishay has some application notes on pulsed power calculation.According to Vishay it's pretty simple (at least on paper, although you may want to derate it significantly for practical reliability). Basically, for a continuous pulse, first, multiply peak power and its duty cycle to get the average power, make sure it's below the DC power rating of the resistor. Then, compare the peak pulse power and duration with the continuous pulse rating chart. Also, to use the non-repetitive pulse graph, the DC rating can be ignored, but the repetition rate of the pulse must be very low so that the resistor is allowed to cool down between each cycle.12 things to know about resistors in pulse load applicationshttps://www.vishay.com/docs/48516/_ms9702509-2003-vishaychecklistpulseload.pdfPulse Load Handling for Fixed Linear Resistorshttps://www.vishay.com/docs/28810/pulseloadhandling.pdf

author: exmadscientist
author_rank: Frequent Contributor
timestamp: August 25, 2022, 06:52:11 am
content: Setting aside the fact that an LED drive application requiring a resistor better than 10-20% probably needs a better drive circuit than just a resistor, if you need pulse power handling in a traditional SMT chip resistor package, take a look atStackpole RPC series chip resistors. The 5% and looser options are untrimmed, completely avoiding the failure modes srb1954 discussed. This isn't easy to find, or at least it's not easy to find manufacturers admitting to it. Accordingly, the pulse handling capability is excellent... for a chip resistor. A high-quality through-hole wirewound will still survive where these fail (source: seen the holes blown in 'em, changed toheavy-duty wirewounds, no more resistor vapor). I believe either TE or Vishay (or both) has a nearly identically rated series, but I don't think that one explicitly specifies being untrimmed.Quote from: 741 on August 24, 2022, 03:51:50 pmYou probably want theCRGP series, which have both a high power rating and a pulse spec. Nice resistors! Like all other resistors, though, they seem to still get holes blown in them if mistreated....

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 25, 2022, 07:18:57 am
content: Presumably, the pulse curve can be derated by an amount corresponding to the average power / initial temperature from the last pulse(s). Which in fact, they helpfully give: the Continuous Pulse plot.If this is an emergency indicator as seems to be hinted... you might not want to be pushing the limits of any components, lest they become unable to serve their purpose. The safe play is to use adequate ratings, if not for the full pulse as continuous, then just a less aggressive pulse rating.The dissipation can also be avoided entirely, using an inductor and diode, and a pulse train instead of a flat 10ms pulse. Or using enough LEDs that the dissipation isn't in the dropper resistor, or etc.No hints as to size or voltage range have been given, so it feels worthwhile to add these notes.Tim

author: 741
author_rank: Frequent Contributor
timestamp: August 25, 2022, 09:13:21 am
content: I was hopeful about the type CRGP Series - but the D.S. shows a period of 10 seconds, and a pulse count of just 1000.

author: srb1954
author_rank: Super Contributor
timestamp: August 27, 2022, 01:08:47 am
content: Quote from: exmadscientist on August 25, 2022, 06:52:11 amComparing the graphs in the Stackpole datasheet between the trimmed types (< 5% tolerance) and the untrimmed types very clearly shows the improvement in pulse power rating that can be obtained from untrimmed resistors.The Stackpole datasheet is very unusual in this regard as most manufacturers don't provide this degree of information on their pulse power ratings.