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Tapered transformer and two rectifiers! Need help!

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t1d:

--- Quote from: MarkL on December 16, 2019, 06:56:38 pm ---This bridge and tap arrangement probably doesn't have the output waveforms that you want.  It's not symmetric on the half-cycle on the B+ (assuming the two windings are in phase (end-to-end output of 38VAC)).

Below is a quick simulation.

--- End quote ---
"It's not symmetric on the half-cycle..." Mark, please explain more about this. I would like to learn about it. Thank you.

MarkL:

--- Quote from: t1d on December 17, 2019, 01:20:37 am ---
--- Quote from: MarkL on December 16, 2019, 06:56:38 pm ---This bridge and tap arrangement probably doesn't have the output waveforms that you want.  It's not symmetric on the half-cycle on the B+ (assuming the two windings are in phase (end-to-end output of 38VAC)).

Below is a quick simulation.

--- End quote ---
"It's not symmetric on the half-cycle..." Mark, please explain more about this. I would like to learn about it. Thank you.

--- End quote ---
Take a look at the simulation plot.  What I mean is that the +B output (labeled Bout) does not have its peaks at the same level through a full cycle of the AC input, like you would expect of a single full wave bridge rectifier being fed by a single or isolated transformer winding.

Perhaps it's ok for the rest of 001's circuit, which is not drawn, but we don't know.  Presumably, the next stage is some large filter capacitors.  Because the peaks are unequal for the whole cycle, you would only be charging up a filter capacitor once every cycle at the highest peak (instead of twice per cycle on two peaks).  This could lead to unacceptable ripple under load conditions.

In contrast, the +A output (labeled Aout) *does* have the typical bridge rectifier output waveform.  Note that it is overlaid by the green Bout plot where they are equal.

001 was also trying to derive two different DC voltages at the +A and +B outputs, but the plot shows that it doesn't do that as intended either.  Because the peaks are equal, the outputs after the unmentioned filtering capacitors would be equal too.

Hope this makes it a little clearer.  I'm assuming the simplest intentions in 001's circuit.

t1d:
Great tutorial, Mark. Now, I get it. Those are problems I would not have thought of, being a bit noob-ish. Thanks for the effort to explain it.

gbaddeley:
A full wave bridge rectifier block assumes the AC input is ‘floating’ with respected to the rectified +/- outputs, so that the diodes can effectively ‘swap’ the AC inputs every half cycle. The way you have it, the AC inputs are not isolated for both rectifiers, if you have both - outputs  commoned to earth. A solution would be to split the tap between the 8 and 30 volt windings. Does the transformer allow you to physically do that?

001:

--- Quote from: gbaddeley on December 17, 2019, 07:17:34 am --- Does the transformer allow you to physically do that?

--- End quote ---

no

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