Hardware Insights Forum

Figured I should continue this from the Dell H305P-01 review thread since I don't want to derail it.

Actually they're twice as bad as forward converters (setting aside resonant bridges, which have no mag-amps at all). goodpsusearch (on Badcaps.net) has burned out an ESAD83-004 by putting it on a +12V rail in such a unit. If the voltage pulses from the transformer were not much higher than the output, I expect the rectifier would have been fine. It would also likely have been OK on the +12V rail of a forward converter. I actually think even 60PRV is marginal for the +12V in an old half-bridge design, and would recommend at least an 80PRV rectifier there.

That's what I figured... because you can get away with lower reverse voltages schottkys in forward. So the transformer windings are more like 40V-80V in bridge than 24-40V for the +12V rail, for an example? Is the magamp circuit used as a buck converter there too or does it just smooth the voltages and lower them?

(Yes, you do need a 25V cap before the -12V regulator, if you use one.)

Same for linear regulated +3.3V rails with MOSFETs being used to do so? Also, do 7905 and 7912 regulators have to regulate -24V or -10V all the way down to -12V and -5V?

Also, I have a NPS-250GB which has the 7912 regulator on the transformer end of the heatsink.... there's a 470uF 25V Rubycon YXG on the magamp side. The YXG wouldn't be before the 7912 regulator then, would it, since there's an LTEC TK (22uF, 50V, I guess?) before the regulator but I don't know if such a tiny, general purpose capacitor could realistically do any filtering (and it seems the -12V rectifier is after the regulator for some reason...) besides bulk capacitance? The PSU is full of LTEC LZG (output capacitors) and TK (tiny capacitors) otherwise except for the primaries which are Nichicon LU. I also have a NPS-200PB-73M which has a 10uF, 50V Taicon VT before the 7905 regulator...

EDIT: Okay, I have this figured out. The 10uF 50V Taicon VT or Chemicon KME is after the -5V regulator with the 470uF 25V Rubycon YXG or LTEC LZG before the -12V regulator and a 100uF 16V Taicon VR, VX, or 25V LTEC TK after.

Wester547 wrote:Is the magamp circuit used as a buck converter there too...?

Yes.

Same for linear regulated +3.3V rails with MOSFETs being used to do so? Also, do 7905 and 7912 regulators have to regulate -24V or -10V all the way down to -12V and -5V?

No, the unregulated input isn't double the output (except that the 7905, when used, is usually drawing from -12V). 24V across a 25V cap is not an adequate safety margin. I would guess that the input to the 7912 is somewhere between -16V and -20V.

Also, I have a NPS-250GB which has the 7912 regulator on the transformer end of the heatsink.... there's a 470uF 25V Rubycon YXG on the magamp side. The YXG wouldn't be before the 7912 regulator then, would it, since there's an LTEC TK (22uF, 50V, I guess?) before the regulator but I don't know if such a tiny, general purpose capacitor could realistically do any filtering (and it seems the -12V rectifier is after the regulator for some reason...) besides bulk capacitance? The PSU is full of LTEC LZG (output capacitors) and TK (tiny capacitors) otherwise except for the primaries which are Nichicon LU. I also have a NPS-200PB-73M which has a 10uF, 50V Taicon VT before the 7905 regulator...

I can't answer that one without pictures.

I was wondering, though, if it's possible for the -12V or -5V rectifier to be after the regulator, making it possible for the output capacitor to be after it too... ?

You can't put the rectifier after the regulator. There could be a protection diode connected across the output, though, such that it is reverse biased in normal operation, but conducts if something tries to pull it positive.