Power Dissipation Trough Time. - Page 1
Forum section: electronics / beginners

author: fc3000
author_rank: Regular Contributor
timestamp: January 30, 2021, 04:44:24 pm
content: Hi all,I am connecting a power transistor to a +29V.I am connecting the Emitter to a Power resistor and the to ground.because i want to limit the current to 2A: 29V/2A=15.5R.The PD would be 4x15.5=58W. That is hot!So my question is; if the transistor is on for only 0.1ses ( every 2Min) is much less?Can i get away with a much smaller power-resistor?I was thinking PD/60sec would be the right thing to do.RegardsFrancesco C

author: TimFox
author_rank: Super Contributor
timestamp: January 30, 2021, 05:02:38 pm
content: Is there a question?

author: fc3000
author_rank: Regular Contributor
timestamp: January 30, 2021, 05:12:21 pm
content: Sorry Tim, my PC wen wrong.I have finished the question.Francesco C

author: Manul
author_rank: Super Contributor
timestamp: January 30, 2021, 11:29:34 pm
content: 100 milliseconds every 2 minutes would be 1/1200 duty cycle. So average power (over full 2 minute period) 58/1200 = ~0.048W. I would say, 1/4W resistor could possibly survive that, but just barely. You would need to calculate the temperature rise from a single pulse based on resistor mass and specific heat capacity. Not so easy. Better just try and measure peak temperature. For robustness it would be good to take a bigger (like 2W) resistor to minimise temperature slopes. Cyclic variations of temperature may break the resistor mechanically.

author: fc3000
author_rank: Regular Contributor
timestamp: January 31, 2021, 10:02:19 am
content: Thank you Manul,that was very helpful.Francesco C

author: srb1954
author_rank: Super Contributor
timestamp: January 31, 2021, 11:44:45 am
content: Quote from: fc3000 on January 30, 2021, 04:44:24 pmYes, but as well as the average resistor power dissipation you also have to consider the peak power dissipation rating of the resistor which becomes the limiting factor in this case.With such a low duty cycle the average power dissipation will be less than 50mW so it would appear that a 1/4W rated resistor would be more than adequate. However, due to its limited peak power rating, that 1/4W resistor might only be capable of safely dissipating 1W for the 100ms duration pulse before the resistor heats up excessively.To achieve the 58W peak power rating with a 100ms pulse you are probably going to need to use at least a 5W wire-wound power resistor (e.g. Vishay AC05) to avoid damage.You need to check the various resistor manufacturers' datasheets to determine the allowable peak power vs pulse duration and duty cycle. Unfortunately, not many resistor manufacturers publish comprehensive peak power rating graphs these days so you may have to do quite a bit of research to find the required information.

author: Whales
author_rank: Super Contributor
timestamp: January 31, 2021, 11:48:36 am
content: Do some resistors come with Safe Operating Area (SOA) graphs like mosfets do? That would be informative.

author: Ian.M
author_rank: Super Contributor
timestamp: January 31, 2021, 12:02:19 pm
content: Generally you can assume that an ordinary wirewound resistor can handle five times its rated power as a pulse overload, and if its pulse-rated ten times. Other types of resistor tend to be less robust. As srb1954 has pointed out, you need to consult the specific resistor manufacturer's series datasheet if severe overload pulses are expected in normal operation of your circuit. Also its important to note that substitution will not be accepted without a design review.

author: srb1954
author_rank: Super Contributor
timestamp: January 31, 2021, 09:19:01 pm
content: Quote from: Whales on January 31, 2021, 11:48:36 amYes, a few manufacturers provide this information in the form of a graph showing the allowable peak power or pulse energy vs pulse duration and duty cycle. Due to these graphs having to cover a range of resistor values the designer has to calculate the resistor's peak pulse power or energy rather than just the voltage and current ratings that suffice for looking up a SOA graph for a single transistor type.The Vishay AC05 type I specified in response to the OP has some fairly comprehensive graphs of this information but this level of detail is a rarity these days. You won't get this type of information from the average commodity resistor manufacturer as it takes an enormous amount of product testing time to fully characterise the pulse performance, reliability and life of a resistor considering all the parameters involved in pulse operation.

author: srb1954
author_rank: Super Contributor
timestamp: January 31, 2021, 09:34:59 pm
content: Quote from: Ian.M on January 31, 2021, 12:02:19 pmUnfortunately, the design review process usually isn't properly followed in most organisations producing commercial grade products.It used to really annoy me when I spent several hours carefully selecting a specific brand of resistor for its pulse capabilities only to have manufacturing or purchasing later go and substitute a generic resistor based on nothing more than resistance, tolerance and nominal power rating.

author: Ian.M
author_rank: Super Contributor
timestamp: January 31, 2021, 10:45:28 pm
content: I bet that got your goat. However if you've sufficiently documented that Rnnn must be rated to dissipate X watts for Y milliseconds at max. temperature T and provided part numbers for a minimum of two suitable parts in current production, and annotated the BOM accordingly, + emailed the same to your manager, cc yourself (so you can print it with all raw email headers) it will be very difficult to pin the blame on you or even your manager unless they were dumber than rocks.

author: srb1954
author_rank: Super Contributor
timestamp: February 01, 2021, 03:58:24 am
content: Quote from: Ian.M on January 31, 2021, 10:45:28 pmDesign features like that definitely got documented in the engineering notebook or a design report but the trouble is nobody in production or purchasing bothered to look at those documents before instigating a component change.Purchasing and production basically only look at the BOMs so it would be better to be able to add a note to the BOM but I haven't ever seen a BOM system flexible enough to add such free-form notes.