Center tapped transformer rectifier question

[Dannyx]

Good day folks. I'm going to use a center tapped UPS transformer for a project and would like some advice on how to rectify its output. The two windings are 18v each and I already use those on the AC legs of a bridge rectifier to get around 40v out which is fine (the center tap is floating at this point), however a separate part of my project requires a lower voltage of just 18v, so how can I achieve that ? Obviously I could just place another bridge rectifier between the center tap and one of the legs, but that's not very space-efficient, so I was thinking of using 2 diodes on the center tap and other 2 already inside the rectifier....complicated I know, so I put together the rather horrible schematic to try and explain how I see it. Is this correct ?

[stj]

Re: Center tapped transformer rectifier question

follow your current paths - boom.
your shorting the windings with the diodes!!

[budm]

Re: Center tapped transformer rectifier question

Build it and find out for yourself if your theory is correct.
Look real close at your drawing.

[fester]

Re: Center tapped transformer rectifier question

you cant achieve that,only if you have separate grounds for every voltage and you dont plan to connect it together. Maybe the simplest thing will be using of some voltage regulator with higher allowed max. voltage on input

[Dannyx]

Re: Center tapped transformer rectifier question

It's obviously wrong given that two people so far have sarcastically suggested trying it out, so I'm not gonna do it - can't really afford to scrap components like that Let's take it step by step then, since my brain hurts when working with center tappers like these....step one: rectify the 40v output - no problems here, I'm positive....pun intended

[fester]

Re: Center tapped transformer rectifier question

Or if you accept more ripple on output,you can make two separeted half-wave rectifiers with lower winding as a ground

[Dannyx]

Re: Center tapped transformer rectifier question

Then I guess the only option is to have 4 diodes (a full bridge rectifier) for the other winding as well (can't mix them together): AC side of said rectifier goes to the center winding and one of the remaining ones and the output will be 18v DC.

[fester]

Re: Center tapped transformer rectifier question

something like this

[Dannyx]

Re: Center tapped transformer rectifier question

something like this

I have seen that before, but I believe that's a half wave rectifier.

[fester]

Re: Center tapped transformer rectifier question

yes it is but if you dont need big current,it is also the solution..

[mariushm]

Re: Center tapped transformer rectifier question

Something like this would probably work :



Note that with two 18v windings you get 36v AC and that's rectified to DC voltage by the bridge rectifier and you get 36 x 1.414 - 2 x voltage drop so actually you'll have a DC voltage with a peak of ~ 50v
Also note that it's probably a 230v to 36v transformer, but here at around 3-4 am where there's small load on the network, i get up to 245-248v at the mains socket which is still legal (it's 230v +/- 10% in Romania and most of Europe, so up to 253v is legal)

The point is with 245v at the input, you're not gonna have 36v you may have 48-40v ac , and you may have 50-55v peak dc voltage. So for example, use capacitors rated for 63v or with higher voltage rating.

As for your 18v requirement ... you'll have 18v rms , if you rectify it to dc you'll have much more. what voltage you actually need? And more to the point, how much current you expect you're gonna need?

If it's just let's say 50-100mA for some opamps or something, you could just use a LM317 HV (which works up to 60v input voltage) adjustable linear regulator to and get it over with .. 40-50v in (since i doubt you're gonna install 100000 uF) , 18v DC out at 100mA means it would only waste (~40v-18v) x 0.1A = 2.2 watts of heat, which can be doable with a heatsink. Or, you can use a 3-5w resistor to drop some voltage before the regulator and then some power would be dissipated on the resistor and some on the regulator ... the regulator would only need around 2v above the output voltage to work.

For lower currents you could even use a zener diode... see App Note AN954 page 6/14 Resistive Transformerless Power Supply : https://cdn.badcaps-static.com/pdfs/...5df14516be.pdf
... change the zener diode to 15v one or 18v or whatever you're happy with, change the resistor to the value matching your peak current (keeping in mind that you'll have ~ 36v AC and not 110v/230v AC) and everyone should be happy.

[Dannyx]

Re: Center tapped transformer rectifier question

The "high" output of the transformer powers one of those ebay power supply modules and I also need a lower output voltage to power a DC-DC converter which runs a cooling fan for the thing...won't go into too many details on how I built this, but I tested it extensively with a variable laptop power brick which goes up to 24v and it works as it should, regulating the output to the fan based on temperature (nothing WOW there), so the voltage, while not too critical, I wouldn't dare let it go beyond 24v either. Current draw ? I'd say around 1A absolute maximum, if that (perhaps if the fan stalls or jams, worst case scenario).

[stj]

Re: Center tapped transformer rectifier question

big clive would run the fan from mains with a capacitive dropper!!

[Dannyx]

Re: Center tapped transformer rectifier question

big clive would run the fan from mains with a capacitive dropper!!

That would work too, but the amount of components required to do so outweighs the benefits of using the unused winding like I want to do, plus it's isolated and "to standard" so to say

[goontron]

Re: Center tapped transformer rectifier question

Keep it simple?

[petehall347]

Re: Center tapped transformer rectifier question

Keep it simple?

is the 9v plus or minus ?

[goontron]

Re: Center tapped transformer rectifier question

^ Positive.

[goontron]

Re: Center tapped transformer rectifier question

It seems i forgot C3 between the 18v rail and 0v rail.... Ooops.

[Dannyx]

Re: Center tapped transformer rectifier question

How does THIS looks ? It's actually the original idea I had and I then thought I could simplify by omitting two diodes. One of you guys posted something similar, but the values were off, so THIS is what I believe it should look like.

[eccerr0r]

Re: Center tapped transformer rectifier question

mariushm has the only workable solution as far as I can tell - only 4 diodes needed and though his drawing only has 7.5V windings, you can use full 12V (24V/CT). A lot of these other "solutions" don't quite work because the two windings are not isolated from each other. If there is a need for isolation of the two GNDs (i.e., 0V potentials), then you'll need two isolating switching converters to get this done or another transformer that has isolated secondaries...

Also note that the capacitance on the output legs will need to be fairly large to filter the higher voltage output...