DarrenF
Well-known member
That's interesting about the QDs. I never looked at QD current ratings... but I tried to find some (for Molex and Amp/Tyco QDs) and they don't seem to publish any?!? They have max voltage ratings (300 or 600V), but nothing about current. I found a few on Jameco's website (off-brands) that did have current ratings of 7-10A. So yeah, I can see how 20 (or 25) Amps might cause a problem. Heck, that's even more than some sources recommend for 18ga wire...
To expand on what I was saying in my earlier post about the peak diode (and thus fuse) current being rather higher than the average current, see:
http://www.geofex.com/Article_Folders/Power-supplies/powersup.htm
http://www.ece.drexel.edu/courses/ECE-E352/lab2.doc
http://answers.yahoo.com/question/index?qid=20100117073529AAjMI3c
The middle one, specifically, says: "Diodes in the bridge conduct only in the time period from T0 to T1. The diode current must replace the charge lost by the capacitor during its discharge. As the magnitude of the filter capacitor increases, the peak current through the diodes must increase to replace the charge in less time. Therefore it is not always best to choose the largest value of C1 available. In a dc power supply, you can rely on the stages following the FFWR to significantly improve the voltage regulation."
That's why removing one of the Big Blue filter caps may improve the situation; allowing more time for the rectifier to charge up the cap means less peak current. And yes, you'd need to connect the wires together that were formerly on a terminal of the cap (only 1 side is really necessary to remove it from circuit).
But going after the voltage drop at the QDs of the fuse block sounds like the best thing to do first. If it's turning the QDs black, there IS too much voltage drop (and heat loss) there, and it should be addressed.
If I owned a PP, I'd seriously look at seeing if there's enough room in it for two transformer assemblies (one for each ARII). Or maybe this design is bad enough that the best thing to do is to replace the +5VDC supply with a switcher. (I know I know, sacreledge... I like original linear power supplies as much as anyone, but in PP it seems to be really pushed further than it should have been.)
To expand on what I was saying in my earlier post about the peak diode (and thus fuse) current being rather higher than the average current, see:
http://www.geofex.com/Article_Folders/Power-supplies/powersup.htm
http://www.ece.drexel.edu/courses/ECE-E352/lab2.doc
http://answers.yahoo.com/question/index?qid=20100117073529AAjMI3c
The middle one, specifically, says: "Diodes in the bridge conduct only in the time period from T0 to T1. The diode current must replace the charge lost by the capacitor during its discharge. As the magnitude of the filter capacitor increases, the peak current through the diodes must increase to replace the charge in less time. Therefore it is not always best to choose the largest value of C1 available. In a dc power supply, you can rely on the stages following the FFWR to significantly improve the voltage regulation."
That's why removing one of the Big Blue filter caps may improve the situation; allowing more time for the rectifier to charge up the cap means less peak current. And yes, you'd need to connect the wires together that were formerly on a terminal of the cap (only 1 side is really necessary to remove it from circuit).
But going after the voltage drop at the QDs of the fuse block sounds like the best thing to do first. If it's turning the QDs black, there IS too much voltage drop (and heat loss) there, and it should be addressed.
If I owned a PP, I'd seriously look at seeing if there's enough room in it for two transformer assemblies (one for each ARII). Or maybe this design is bad enough that the best thing to do is to replace the +5VDC supply with a switcher. (I know I know, sacreledge... I like original linear power supplies as much as anyone, but in PP it seems to be really pushed further than it should have been.)
