Help with the bridge rectifier connection - paralel or series

[txescientist]

Hi,

Which is the better way to connect bridge rectifier to get one sec winding voltage and two sec windings in series? Left or right configuration on the attached schematic? Transformer has two separate secondary winding, not center tapped.

Thanks!

[Zero999]

Neither. Use a single bridge rectifier and the two transformer windings in series, which is equivalent to a centre tap.

[anishkgt]

You would not need two bridge rectifiers here. Just one would be enough but you'd just one output with higher current in parallel or higher voltage in series.


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[Zero999]

Here's the image I was looking for. If the windings are in series, the part where they join is equivalent to the centre tap. As you don't need regulation, you're using a 9V transformer and no regulators, so ignore that part.

[digsys]

Either is technically ok, but I'd go for #2. It's cleaner. Just note the the bottom 12V bridge will carry BOTH currents.
The CT option suggested has a -12V, or it puts the GND in a bad place. The OP wanted +12V and +24V

[txescientist]

Thank you for the reply. I need full bridge rectification, Vout ~= Vac*1.41
I am using this transformer with two secondaries because I need 20-24V for the charger, and 9V and 5V for control circuitry (OPAMP, PIC micro...) and don't want to burden linear regulator with higher voltage tap. Also, I have another smaller transformer for the -9V rail.

[Ian.M]

The single bridge + series pair of secondaries with center tap is only an option if the secondaries are well matched both in voltage and DC resistance.   If not, its best to connect each secondary direct to the AC terminals of its own bridge rectifier and *ABSOLUTELY* *NOWHERE* *ELSE*, then 'stack' the outputs of the bridge rectifiers.

For the single bridge circuit, the usual configuration is to ground the center tap for equal + and - rails out of the bridge, but if you ground the - side of its output, the center tap of the secondary will supply half the DC voltage of its + terminal. 

[Zero999]

The CT option suggested has a -12V, or it puts the GND in a bad place. The OP wanted +12V and +24V

The ground connection is arbitrary. All voltages are relative. 

[tron9000]

Hi,

Which is the better way to connect bridge rectifier to get one sec winding voltage and two sec windings in series? Left or right configuration on the attached schematic? Transformer has two separate secondary winding, not center tapped.

Thanks!

GAH! neither, yuk!

Thank you for the reply. I need full bridge rectification, Vout ~= Vac*1.41
I am using this transformer with two secondaries because I need 20-24V for the charger, and 9V and 5V for control circuitry (OPAMP, PIC micro...) and don't want to burden linear regulator with higher voltage tap. Also, I have another smaller transformer for the -9V rail.

Hero's first solution is what I would do, then regulate down the supply voltage for the electronics. Providing th electronics weren't pulling a large amount of current the reg would manage. Or you could do this?


but if your charge needs to be referenced to the electronics, then you'll need to isolate the charger -ve from the electronics 0V - we need more context in what your trying to do?

[mariush]

If you don't care about efficiency, you could just make up a capacitive power supply directly from 24v AC or whatever you have on the output of the transformer.

See http://ww1.microchip.com/downloads/en/AppNotes/00954A.pdf , first 2-3 pages.

Just change 110-120v AC to 24v AC and use a ~9-10v zener diode to get your 9v for opamps and then you can use 5v ldo powered from 9v.
You can probably omit R1 from circuit and your X2 capacitor may be rated for lower voltage, just enough to handle transformer's output voltage + maybe around 20-30%

[txescientist]

Sorry I didn't explained what I am making in the first place. I am building variable DC load to test SLA batteries with an integrated charger. All parameters as voltage, current, temperature are sent to PC via USB.

So, charger circuit is ground referenced with the rest of the circuitry. Need a full bridge rectifier circuit. Currents are small, 500mA for charging (20-24V rail) and maybe 100mA for the PIC and the rest...

Found this http://www.hammondmfg.com/pdf/5c007.pdf but it is just for common configurations...

[drussell]

That is certainly the normal way of doing it and I would think it is the simplest....

Like Hero999 says, where you connect the center tap and your "ground" are arbitrary, it just depends on whether you want two "stacked" voltages (positive or negative) or symmetrical +/- around your 0v point.

[David Hess]

If Hero999 had not posted that solution then I would have.

[Zero999]

If Hero999 had not posted that solution then I would have.

There's nothing special about me. I was just the first person to see the thread.

Thank you for the reply. I need full bridge rectification, Vout ~= Vac*1.41
I am using this transformer with two secondaries because I need 20-24V for the charger, and 9V and 5V for control circuitry (OPAMP, PIC micro...) and don't want to burden linear regulator with higher voltage tap. Also, I have another smaller transformer for the -9V rail.

How much current does the -9V supply need to provide? Do you really need another transformer? Can't you use a voltage doubler?

I know that gives -12V but that might not matter, as you could use a regulator (LM78L09) or zener diode to get 9V.

[txescientist]

That could be done, it is a good way to get negative supply, seen it in many power supply designs that allows voltage to be set to 0V. I will use another transformer just to shift some load of the first one. Problem is that I already have the case for the project and the main transformer is limited by the case size. But, with two transformers, I can power up all the circuitry (charger, laod, fans etc.).

Thank you all for the fast and good suggestions and solutions  . Should have used switching PSU, but that is an other story...