Thanks to everyone for pointing out my brain lapse -grin- apparently I got confused in my calculations.
I wrote: >To calculate the value of resistor, use Ohms law R=E/I and P=IxE where R is in ohms, >E is the desired drop in volts, and I is the current in amps, and P in watts >For example, lets say your B+ is 50 volts too high. The load draws 0.250 amps. >R= 50/.25 = 200 ohms. >P= 0.25x200=50 watts. The last line should be P= 0.25x50=12.5 watts. When I came up with 50 watts, I thought to myself at the time, "that seems like a big resistor, oh well... la-dee-dah..." and I went about my business instead of checking my work. I should trust the Force when feel an error coming on like that. Steve WD8DAS [email protected] http://www.wd8das.net/ --------------------------------------------------------- Radio is your best entertainment value. --------------------------------------------------------- -----Original Message----- From: [email protected] To: [email protected] Sent: Sun, Oct 4, 2009 3:17 pm Subject: Re: [AMRadio] Solid State 575 mercury vapor RectifierReplacementsneeded Jim W5JO wrote: >What kind of resistor do you use in this type of rectifier build to >simulate the voltage drop? I prefer cylindrical wirewound power resistors for this job (they seem to be able to take more surge20current than the sandy rectangular ones in case of a fault downstream). To calculate the value of resistor, use Ohms law R=E/I and P=IxEwhere R is in ohms, E is the desired drop in volts, and I is the current in amps, and P in watts For example, lets say your B+ is 50 volts too high. The load draws 0.250 amps. R= 50/.25 = 200 ohms. P= 0.25x200=50 watts. If the resistor ends up to be hard to find or expensive, it might be easier to eliminate the reasons why the higher B+ is a problem. And more efficient too. Upgrading some capacitors could be a reasonable approach. Steve WD8DAS [email protected] http://www.wd8das.net/ --------------------------------------------------------- Radio is your best entertainment value. --------------------------------------------------------- -----Original Message----- From: Jim Wilhite <[email protected]> To: Discussion of AM Radio in the Amateur Service <[email protected]> Sent: Sun, Oct 4, 2009 10:40 am Subject: Re: [AMRadio] Solid State 575 mercury vapor RectifierReplacementsneeded Steve, What kind of resistor do you use in this type of rectifier build to simulate the voltage drop? Jim/W 5JO ----- Original Message ----- > I've made my own solid-state plug-in replacement rectifiers for a > variety of tube types. I look up the tube specs and select > solid-state > diodes that stack up appropriately for current and forward and reverse > voltages. I usually end up using either 1N4007s or 1N5408s. From > on-lline sources like Digikey or Mouser or All Electronics the diodes > are very inexpensive, ten cents to maybe 30 cents each. > > I put the diodes on a rectangular piece of perfboard sized to fit > upright with its bottom end tucked within the sides of the base of the > defunct tube it is replacing. A small L bracket holds it in place. > > If you use diodes from the same "run" (which is easy if they are sold > on the paper tapes used by manufacturers today) then I find no > equalizing components are needed. I add a few more diodes than the > calculated voltage required for an extra measure of safety and have > never had a failure, even in broadcast rigs on the air 24/7 with lots > of lightning and power bumps to deal with. > > You can expect somewhat higher voltage from the new stack than the > original tube rectifier - if that is a problem for the rig, then add aC2 > series power resistor to simulate the internal voltage drop of the > earlier tube. So far I've only needed to do that once when the new > higher B+ exceeded the rating of the by-pass caps in the other > sections > of the rig. Otherwise the rigs seemed to thrive on the higher > voltage. > > Steve WD8DAS ______________________________________________________________ Our Main Website: http://www.amfone.net AMRadio mailing list Searchable Archives: http://www.mail-archive.com/[email protected]/ List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Post: [email protected] To unsubscribe, send an email to [email protected] with the word unsubscribe in the message body. This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Our Main Website: http://www.amfone.net AMRadio mailing list Searchable Archives: http://www.mail-archive.com/[email protected]/ List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Post: [email protected] To unsubscribe, send an email to [email protected] with the word unsubscribe in the message body. This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
