Using same winding on separate rails?

[John123]

Hi, I've got an old salvaged mains transformer with multiple isolated windings and would like to know if I can configure it like this? Or am I having a brainfart.

The 17vac winding is used twice, both for the low voltage control rail and as part of the main B+ rail. The transformer is probably rated for about 80VA going by weight so it would be better to utilize all that extra voltage to help get the power available to the flyback primary up.

Am I going to experience problems doing this? The control circuit doesn't need much current but the B+ obviously is going to need all it can, I'll configure the current sensing to limit it to sensible levels and put fuses on both rails.

Or maybe there's another configuration with discrete diodes that I could use? I recall coming across some very weird and wacky multirail configurations in the past with other equipment but can't remember how it was set up.

Thanks.

Edit: Just realized that most of the components don't need to be there, here it is simplified.


Edit 2: Just realized this thread belongs in the "electronic circuits" board (sorry).

[klugesmith]

Nice pictures. Thank you.

Point 1: 
As you know, the transformer VA rating goes with its size and weight (and frequency, and tolerable temperature).
The designer had to partition the VA rating among the four secondary windings.
They all have different voltages and, I bet, different current and VA ratings. 
You can make some good guesses based on DC resistances.

When wired in series, just for BR2, all secondary sections carry the same current.  It should not exceed the design limit for any section.

Point 2, about tying BR1 and BR2's negative DC terminals together, when BR1 and BR2 also have one AC terminal in common.

As an educational exercise:

Part 1.  Consider just BR1 and the 17 vac winding.
Draw the voltage waveforms of both AC terminals, with respect to DC negative (which you have defined as Ground).
You will find that they are both well-defined when some rectifier diodes are conducting.
At instants when no rectifier diode is conducting, you still have a well-defined voltage between the AC terminals.  But the AC nodes collectively have room to float up or down, within limits which you can tell us about, with respect to Ground.

Part 2. Now repeat the exercise with just BR2 and its AC inputs.  If the answer to main question is still not clear, sketch in the voltage waveform for top of 17V winding (when BR2 is the only load).

Is there a solution where the AC terminal voltages are the same according to both Part 1 and Part 2?

[John123]

Nice pictures. Thank you.

Point 1: 
As you know, the transformer VA rating goes with its size and weight (and frequency, and tolerable temperature).
The designer had to partition the VA rating among the four secondary windings.
They all have different voltages and, I bet, different current and VA ratings. 
You can make some good guesses based on DC resistances.

When wired in series, just for BR2, all secondary sections carry the same current.  It should not exceed the design limit for any section.

Point 2, about tying BR1 and BR2's negative DC terminals together, when BR1 and BR2 also have one AC terminal in common.

As an educational exercise:

Part 1.  Consider just BR1 and the 17 vac winding.
Draw the voltage waveforms of both AC terminals, with respect to DC negative (which you have defined as Ground).
You will find that they are both well-defined when some rectifier diodes are conducting.
At instants when no rectifier diode is conducting, you still have a well-defined voltage between the AC terminals.  But the AC nodes collectively have room to float up or down, within limits which you can tell us about, with respect to Ground.

Part 2. Now repeat the exercise with just BR2 and its AC inputs.  If the answer to main question is still not clear, sketch in the voltage waveform for top of 17V winding (when BR2 is the only load).

Is there a solution where the AC terminal voltages are the same according to both Part 1 and Part 2?

Hi, yeah I'm really struggling to visualize all this. Spent all evening trying to get my head around it before making the thread. Are you trying to tell me that it'll go bang?

[klugesmith]

If you want to try it, instead of figure it out, start with a variac.

Then _if_ the concept is fundamentally flawed,
and you don't check temperatures by burning your fingers,
any smoke will be released quietly.

If the concept is sound, the variac test won't set your schedule back too much.

[John123]

If you want to try it, instead of figure it out, start with a variac.

Then _if_ the concept is fundamentally flawed,
and you don't check temperatures by burning your fingers,
any smoke will be released quietly.

If the concept is sound, the variac test won't set your schedule back too much.

I don't have a variac sorry, and I'm not smart enough to figure it out myself. Would you pretty please grace me with the answer? 

Edit: Will this make the 25v capacitor go bang? @klugesmith

[Twospoons]

Yes, it will go bang. 
You can check this very easily with a SPICE simulation.