We're getting far off the topic of Elecraft radios ... > > > Investigation showed that the steel chassis was not 100% flat and the > contact surface of the rectifier was compromised.
Good lesson. Contact resistances need to be accounted for in the thermal model and analysis.. They are never zero like they are in the textbook. Nothing is ever flat (P-V). TIMs (thermal interface material) can help but need to go into the model. 73 de AI6KG On Fri, Apr 24, 2020 at 8:52 PM Martin Sole <hs0...@gmail.com> wrote: > I can add perhaps another data point on the rectifier matter. Years back > my employer, one of the largest ground to air radio manufacturers was > having failures with their mainstay transmitter, a 50w carrier AM unit. > It had a large toroidal transformer and a packaged rectifier. The > rectifier was bolted to the steel sub chassis and all was fine in > regular intermittent service but in ATIS service the transmitter runs > essentially continuous, so about 160-180 PEP, and rectifiers were failing. > > Investigation showed that the steel chassis was not 100% flat and the > contact surface of the rectifier was compromised. The fix was to insert > a (really) flat aluminium bar under the rectifier and bolt that to the > chassis. The bar did much of the heat sinking and had many more contact > patches with the steel sub chassis. Even paint ridges can be a source of > reduced contact leading to overheating of the rectifier package and > those things really do need to shift some heat. A good way to think of > it is like you would a PA transistor in a 100 watt amplifier, we > appreciate how well they need to be bolted to a heat sink and your > rectifier package wants very similar assessment. > > It's not so much the current that causes failure rather the heat is not > being dissipated properly from the smaller package. > > Martin, HS0ZED > > > On 25/04/2020 02:11, Ray Albers wrote: > > This thread started as my description of the failure of my Astron RS-20A > > power supply, caused by a bad electrolytic capacitor on the regulator > > circuit board. Several hams posted advice that a 20Amp supply was really > > too small for a K3/100, especially if running full power. This led me to > > order an Astron RS-35 supply. > > > > Chris Hoover, AI6KG, posted the below advice, warning about possible > under- > > engineered rectifier diode arrangement in the RS-35A. > > > > Today the big brown truck delivered my RS-35M-AP (the M and AP signify > it > > has meters and Anderson Power Poles on the front panel). Chris > speculated > > that Astron may have made changes more recently. So the first thing I did > > was to open the case to check out the rectifier arrangement. Here's > what I > > found: > > > > My unit bears Serial Number 2019110051. I am speculating that the > leading > > digits 2019 signifies 2019 manufacture. Interestingly, the schematic > that > > shipped with the unit says Rev.1, April 2020. The schematic shows two > > bridge rectifiers, DB3501. Two diodes are used from each bridge, and sure > > enough the diodes are paralleled. The DB3501 is spec'd at 35A, so that > > seems to be an improvement from the 25A diodes Chris mentioned. That's > > what's in the schematic. Inside the power supply, there are indeed two > > rectifier packages bolted to the floor, with heavy (maybe 14ga) solid > wires > > connecting terminals in parallel. I'm unable to confirm what the rating > of > > these diode packages is. They are not labeled DB3501. Instead, they say > > "Astron 5001," and one of them also bears some Chinese characters. So > > custom made for Astron? > > > > Well, I have ordered a 50Amp rated diode package, but am undecided if I > > will replace the diodes in the supply. 35 Amp rated diode bridges is an > > upgrade from the 25 diode bridges that Chris mentioned finding. It does > > puzzle me why, with 50A (and better) diodes being so cheap, would Astron > do > > it this way? I can only think of two reasons: 1) We think having two > > packages bolted to the case will make for better heat dissipation of the > > total heat generated at max current and 2), the ever present, "because > > we've always done it this way!" > > > > Finally, I'll mention that, like my old RS-20A, I found the negative > > terminal bonded to the case. I consider this bad practice so I removed > that > > bond. > > > > 73 to all > > Ray K2HYD > > > > > > On Sun, Apr 19, 2020 at 6:07 PM Christopher Hoover <c...@murgatroid.com> > > wrote: > > > >> You might want to replace the bridge rectifier in that new 35A power > >> supply. > >> > >> Yep, right out of the box. > >> > >> At some point, Astron started shipping RS-35A's with a 25A bridge > >> rectifier. They paralleled two out of four of the 25A diodes twice > over > >> to make, supposedly, a pair of 50A diodes for center-tapped full-wave > >> rectification. That's not good engineering practice as the diodes in > each > >> pair will not share current equally because of differences in Vf and > >> tempco. Once one blow, the other will blow shortly after. > >> > >> After I fixed my own RS-35A with this problem, I've helped several other > >> hams fix this same problem in theirs. Not a random sample, as this is > just > >> folks on my local machine and in local clubs. > >> > >> You can get a 50A bridge in the same package for under $3. > >> > >> Perhaps Astron has fixed the problem since .... > >> > >> 73 de AI6KG -ch > >> > >> > >> > >> > >> On Thu, Apr 16, 2020 at 10:28 AM Ray Albers <rayalb...@gmail.com> > wrote: > >> > >>> Many to all who posted/responded to my recent post about an > electrolytic > >>> capacitor failure in my power supply. Lots of very interesting reading > >>> about peoples' industry experiences - thank you! > >>> > >>> Several have pointed out that using a 20A supply with my K3/100 is > pushing > >>> close to (or over!) the limit. Even though I am measuring just 16A at > the > >>> power level I'm running (and not running anything but the K3 on this > >>> supply) I agree that I'm skirting the edges. So even though I've been > >>> getting away with it for a long time, this morning I ordered a 35A > supply. > >>> I'll probably sleep better. > >>> > >>> 73 > >>> Ray K2HYD > >>> > >>> < > >>> > https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail&utm_term=icon > >>> Virus-free. > >>> www.avast.com > >>> < > >>> > https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail&utm_term=link > >>> <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2> > >>> ______________________________________________________________ > >>> Elecraft mailing list > >>> Home: http://mailman.qth.net/mailman/listinfo/elecraft > >>> Help: http://mailman.qth.net/mmfaq.htm > >>> Post: mailto:Elecraft@mailman.qth.net > >>> > >>> This list hosted by: http://www.qsl.net > >>> Please help support this email list: http://www.qsl.net/donate.html > >>> Message delivered to c...@murgatroid.com > >>> > > < > https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail&utm_term=icon > > > > Virus-free. > > www.avast.com > > < > https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail&utm_term=link > > > > <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2> > > ______________________________________________________________ > > Elecraft mailing list > > Home: http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/mmfaq.htm > > Post: mailto:Elecraft@mailman.qth.net > > > > This list hosted by: http://www.qsl.net > > Please help support this email list: http://www.qsl.net/donate.html > > Message delivered to hs0...@gmail.com > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to c...@murgatroid.com ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
