Two bridge rectifiers on a centre tapped transformer?

[Commander Kinsey]

I have a centre tapped transformer I wanted to get the most power out of. Thinking a traditional full wave rectifier (just a diode on each side, calling the centre tap 0V at the output) is only running current through one half of the secondary at a time, which is inefficient (think of P=i^2R), I thought about putting a bridge rectifier on each half, then connecting the result in parallel. I made the following diagram by adjusting someone else's, so it may look a bit odd.

I'm a bit confused here because you can trace the current flow and show the output has both 24V (if the current goes through the "wrong" bridge on the way back) and 12V at the same time, which isn't possible. Can someone explain what would happen in this circuit?

Any way to make this work? I want current flowing through both halves of the secondary all the time. But I want half the full secondary's voltage.

[Fredxx]

No, you will get 24V rectification. Only the LHS 2 of the top 4 diodes will be conducting, and the RHS 2 of the bottom 4 diodes. The rest will be entirely superfluous.

You need to get into LT Spice. It's free. A man of your claimed IQ should have no problem in confirming my presumption, or even proving me wrong.

If you want 12V then use:

It doesn't make full use of the copper so you may have to down-rate the transformer a little.

For different topologies:

[Paul]

And whether filtered or unfiltered, the output is *unregulated*.

The output varies as a function of the load current.

Slapping a regulator on the output, makes an output suited to driving a digital logic circuit. The regulator can be an SMPS (efficient) or a linear regulator (inefficient). Linear regulators are easier for hobbyists to assemble.

And while just about every web page shows "1N4007" as The Diode, no, there are other choices you can buy. The reason a lot of 1N4000 series diodes get used, is because hobbyists can buy a bag with a considerable number of them, for not a lot of money. But you can also get wafer-style full wave bridge rectifiers, that will plug into your pegboard. This handles a slight bit more current. The reason I picked this, is I used something similar in my amp for the computer speakers.

Paul

[Brian Gaff]

Yes and the switch mode type can cause a lot of interference if one is not careful but they are efficient by comparison with a series regulator. You can even end up with higher voltages than you started with. There was an early example in a Sony Cassette deck I had. It ran on5 to 6 volts, but the supply rails inside were 24 volts. A device marked Fuji did the work and never broke a sweat.

Brian

[Fredxx]

It would be most helpful to understand what Mr Hucker, AKA Kinsey, is trying to do.

[John Larkin]

He's a civilian, not an actual commander?

[Dave W]

I agree that the OP's circuit gives 24V rectification, but the left-hand pair of diodes in the bridges are the only ones that conduct.

To understand what's going on, I found it easier to imagine the bottom of the winding to be 0V, the centre tap as 12V and the top to be 24V.

[Rod Speed]

He is a middle aged completely unemployable welfare bludger who has never ever had anything to do with any military system.

[John Larkin]

Maybe he has a uniform and marches around and salutes and stuff.

I was in the US Navy for a week. Parts of it were interesting but I didn't decide to stay.

[Animal]

Increase operating frequency. Use low Vf fets to rectify

yep

[Rod Speed]

Nope, he is actually into running around stark naked, quite literally.

He never even got close to that. The best he ever managed was doing the tech in a school for a while and stuffing spam thru letter boxes.

[Max Demian]

Commander is a civilian award. CBE: "Commander of the Order of the British Empire". But you're not allowed to style yourself "Commander <name>". JG Ballard was offered the medal, but turned it down for that reason.

[Tauno Voipio]

You have invented a four-diode bridge fed from the ends of the transformer. The bridge branches connected to the centre tap are in parallel, and they do not serve any useful purpose.

I re-drafted the schematic for LTSpice:

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Version 4 SHEET 1 880 680 WIRE 224 16 112 16 WIRE 384 16 224 16 WIRE 496 16 384 16 WIRE 688 16 496 16 WIRE 112 64 112 16 WIRE 224 64 224 16 WIRE 384 64 384 16 WIRE 496 64 496 16 WIRE 688 96 688 16 WIRE 112 144 112 128 WIRE 112 144 -272 144 WIRE 224 144 224 128 WIRE 304 144 224 144 WIRE 384 144 384 128 WIRE 384 144 304 144 WIRE 496 144 496 128 WIRE 576 144 496 144 WIRE 112 176 112 144 WIRE 224 176 224 144 WIRE 384 176 384 144 WIRE 496 176 496 144 WIRE -272 208 -272 144 WIRE 112 288 112 240 WIRE 224 288 224 240 WIRE 224 288 112 288 WIRE 384 288 384 240 WIRE 384 288 224 288 WIRE 496 288 496 240 WIRE 496 288 384 288 WIRE 688 288 688 176 WIRE 688 288 496 288 WIRE -272 368 -272 288 WIRE -112 368 -272 368 WIRE 304 368 304 144 WIRE 304 368 -112 368 WIRE -112 416 -112 368 WIRE -272 448 -272 368 WIRE -272 576 -272 528 WIRE 576 576 576 144 WIRE 576 576 -272 576 FLAG -112 416 0 SYMBOL diode 128 128 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D1 SYMATTR Value 1N4148 SYMBOL diode 128 240 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D2 SYMATTR Value 1N4148 SYMBOL diode 240 128 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D3 SYMATTR Value 1N4148 SYMBOL diode 240 240 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D4 SYMATTR Value 1N4148 SYMBOL diode 400 128 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D5 SYMATTR Value 1N4148 SYMBOL diode 400 240 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D6 SYMATTR Value 1N4148 SYMBOL diode 512 128 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D7 SYMATTR Value 1N4148 SYMBOL diode 512 240 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D8 SYMATTR Value 1N4148 SYMBOL res 672 80 R0 SYMATTR InstName R1 SYMATTR Value 120 SYMBOL voltage -272 192 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value SINE(0 17 100) SYMBOL voltage -272 432 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value SINE(0 17 100) TEXT 70 454 Left 2 !.tran 100m

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[Rod Speed]

Is ALSO a civilian award. Its also a military rank in both the navy and airforce.

[Commander Kinsey]

Thanks! It just so happens I just downloaded LTSpice, I'd not got round to working out how to use it though. Where do you get the components to insert? I couldn't find the voltage source for example, only buttons for a handful of components.

I have loaded the file you created below. It appears the load gets 24V RMS, as I suspected - I wasn't sure as I could trace both 12V and 24V getting to the load, I guess the 24V wins. I was trying to give it two 12V in parallel, but the power is going forwards through one bridge and back through the other. Is there any way to stop this?

[Paul]

This shows a voltage source symbol.

[Picture]

The .tran statement shows how long the simulation should run, and stands for "transient analysis" or so.

And this isn't a real simulation, in the sense that there's no behavioral model of a transformer in there, to show regulation effects.

*******

And while you're simulating, have a look at the Hammond low-power transformer catalog. The eleventh page has a few equations.

On the thirty-fourth page, you'll find a toroidal transformer. One of the downsides of these (apparently), is a fairly nasty inrush current. If you don't tame the inrush current, it could cause your mains to drop enough, to knock out your computers in the same room. If you use an AC switch to the toroid, it might destroy the switch contacts after some number of cycles. I don't know if Hammond has a recommended circuit for the mains side, or not. The characteristics of laminate transformers might be a bit softer than that.

They also do high power transformers suited to the power company, but this too is likely a bit of a miss, for what you intend. So I don't know if they actually carry the kind of transformer you're looking at. I bought a transformer from them, for my computer speaker amp (drives some leftover bookshelf speakers at a puny power level).

Some of the toroids, have dual secondary windings. You get four wires for output, and if you want to "stack rectifiers without doing LTSpice", you can.

Paul

[Commander Kinsey]

Ah! So the AND gate is actually a symbol meaning "any component". Not very intuitive.

[Fredxx]

Don't worry, it's well beyond your capability.

[John Larkin]

You don't use LT Spice?

[Commander Kinsey]

I used PSpice a long long time ago.