Parallel bridge rectifiers

[JohnRoberts]

Moamps is right, there is a paralleling action going on for the negative half diodes in the two bridges.  Both loads would need to be isolated (pos and neg rails) for diode currents to be separated.

I stand corrected for 1/2 the diodes.  :-[

Symmetric wiring and traces and bridges would be needed.

Any wire resistance in series with these parallel diodes will degenerate temp related current hogging. Diodes have the bad characteristic of lower junction voltage as they heat up, so do not automatically share nicely (all else equal the hotter diodes will draw more current), but any resistance in series (like wires or PCB traces) will counter that current hogging.  (I would not add series resistance to force sharing, if anything I'd be tempted to oversize the diodes.)

JR 

I'd be more  concerned about the stress on the regulator at start up if those are valve heaters - and the regulator heatsinking will be a concern just for continuous current if there is a fair bit of transformer winding headroom to cover mains variation.

 

[Rob Flinn]

Yes, but the original diagram has 2 full bridges not one full bridge & 1 half bridge  ..............

 

[buildafriend]

I'd be more  concerned about the stress on the regulator at start up if those are valve heaters - and the regulator heatsinking will be a concern just for continuous current if there is a fair bit of transformer winding headroom to cover mains variation.

I haven't had the greatest experiences with regulators on tube heaters or filaments. You might want to set an ammeter at peak hold and see what you can catch going into the regulators while they are attached to the load during start up. Like I said, I got great results with a dual secondary transformer into two isolated bridges and a much smaller cap. The annoying part was getting the right sized transformer to make up for the voltage sag without regulators. I don't think I've experienced AC heater sagging like unregulated DC heaters do as you add more tubes to the load.. 

 

[CJ]

note that the transformer VA rating will change for the worse when converting a normally AC output to a DC output,

the math is at home in a book somewhere,

 

[bluebird]

Ding ding ding... CJ you win. I was measuring wrong. I was measuring the current draw at the regulators, which would be just the heater load and high voltage load, then applying that to the transformer alone with resistors. Measuring the current draw BEFORE the regulators revealed a much larger current draw and the transformer was not speced to that load. So I'm getting a bigger transformer.

The regulators are over rated and have plenty of heat sink. I have no problem with cold heater  inrush current...It was just the transformer being too small and I was tricking myself by loading it wrong while testing on the bench.

Example: B+ at the regulator 200V was 160mA.  So in my test I was loading the AC secondary with a resistor to draw 160mA. Transformer had no problem with this as it was specd for 180mA.
Current measured at the transformer secondary with the regulator inline showed 300mA. So almost twice the current draw than I thought.  Loading the transformer with a resistor to draw 300mA makes it buzzzzzzz.

 

[trobbins]

Need to be carefull in stating current, and in identifying the location of that current in the circuit.

DC current into load is no issue.  AC current in transformer winding will have high crest factor, which is difficult to measure as an rms value, and imposes a low PF to the winding.

Current between filter and regulator will be DC plus AC.

 

[PRR]

The AC rating should be TWO TIMES the DC current.

A more exact analysis gives factor of 1.6 or 1.8 depending if heat or sag is the problem. If you order custom windings, then figure this yourself, or show the winder your plan with all details. But in DIY we always buy a stock part, and "2X" gets you a big-enough part with some margin.