Confusion about rectifier maximum voltages- datasheet versus calculations

[imaradiostar]

After confusing some things about FWCT versus FWB rectifiers I thought I'd better order some higher voltage rectifiers.

I was looking at the BY127 and the GP10y- both pretty cheap, not a huge issue.

Here's where it gets confusing. On THIS datasheet for the BY127 it says that the thing is rated for 875 volts RMS which is contrary to my understanding of the rating of a typical rectifier.

If the BY127 is a 1250 volt rectifier it should be good for:

1250/1.4/2=446 volts in a FWCT supply.

The number above would seem to indicate it's good for about twice that. Is it possible they've overspec'd the device just to avoid potential confusion and failures?

(note- the datasheet doesn't indicate what type of circuit the RMS value is for- perhaps that's the catch?)

jamie

[loudthud]

1250V is roughly the peak of 875V RMS. I think that RMS number applies to a full wave bridge configuration. In the FWCT circuit, the diode sees twice the inverse voltage during the half cycle when it's not conducting. Remember that the filter cap holds the peak voltage while the other diode is conducting so it sees the peak to peak voltage of the whole winding.

[R.G.]

LT is dead correct. In a FWCT, the first cap holds the peak from half the winding, and the winding swings to the peak in the other direction, so the diode has to block two times Vpeak, which is 1.414 times Vrms (for sine waves).

[imaradiostar]

makes sense- it's as I originally thought.

jamie

[nevetslab]

LT is dead correct. In a FWCT, the first cap holds the peak from half the winding, and the winding swings to the peak in the other direction, so the diode has to block two times Vpeak, which is 1.414 times Vrms (for sine waves).

This would seem to justify why we usually see two or three rectifier diodes in series in FWCT circuits on high voltage tube amp HT supplies. In the case of using UltraFast rectifiers, a single UF4007 rectifier doesn't have a high enough Vrrm nor Vdc rating for 800VCT secondaries. A pair of them should be sufficient in each leg, though, if I understand this correctly.

[nashvillebill]

I sure wish Merlin could find some time to update his power supply book and include a lot more info on stuff like this! (I've got the first edition)

[tedmich]

I sure wish Merlin could find some time to update his power supply book and include a lot more info on stuff like this! (I've got the first edition)

he's taken it off the market, apparently unhappy with it in general! He's quite the perfectionist...