If they are paralleling two 35A diodes in the new builds, they may have repeated the same mistake.
Does this give a 50% derating (a typical factor) for full load forward current? I dunno. One can do the math [1] and figure it out. A 50 A bridge (no need to parallel) is OK by design. That's just a smidge under 50% derating. I used a 75A bridge, IIRC. 73 de AI6KG [1] https://www.st.com/resource/en/application_note/dm00098381-current-sharing-in-parallel-diodes-stmicroelectronics.pdf On Fri, Apr 24, 2020 at 12:11 PM Ray Albers <rayalb...@gmail.com> 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> > <#m_-4856506079513387734_m_-8205502227484171307_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 arch...@mail-archive.com
