question about dual voltage from a power transformer CT

[slajeune]

Hi all,

I am looking for validation on the following.  Say I have a transformer as such 12V - 0 - 12V.  If I use both ends of the secondary, go through a bridge rectifier and some filter caps, I will get roughly 24V, right?

Now, if I use the center tap and one of the secondary ends, go through a bridge rectifier and some filter caps, I will get roughly 12V, right?

Can I use both of these connections at the same time (i.e. get 12V and 24V from a single CT transformer)?

Thanks,
Stephane.

[R.G.]

"Power Supply Basics", GEOFEX, http://www.geofex.com

If more people would go to GEO and read it, I'd get a lot less typing practice.

[slajeune]

Hi RG,

thanks for the link!

that answers questions 1 and 2.  About question 3, can I rectify and filter the 24v AND the 12v (i.e. to get both voltages at the same time)?

Sorry if this seems very basic, this is the first time I think about using a transformer.

Thanks,
Stephane.

[R.G.]

Say I have a transformer as such 12V - 0 - 12V. If I use both ends of the secondary, go through a bridge rectifier and some filter caps, I will get roughly 24V, right?

The peak voltage of a 12V AC sine wave is 12Vrms*1.414 = 16.98V, and for 24V, 33.9Vdc. You lose two diode drops, so the 12V actual DC is 15.4 and the 24 gives 32.5 - if the AC voltage out is exactly 12Vac or 24Vac. They commonly wind small transformers a little high to make up for winding losses, so you might get a volt or two higher.

Now, if I use the center tap and one of the secondary ends, go through a bridge rectifier and some filter caps, I will get roughly 12V, right?

16.98V, from above.

Can I use both of these connections at the same time (i.e. get 12V and 24V from a single CT transformer)?

If you use a bridge rectifier connected to the outer 24V ends, you get 32Vdc or over, and the CT is always dead in the middle, so if you ground the CT, you get +/- 16-18Vdc.

[slajeune]

Hi RG,

sorry if I am bugging you.  Is this possible (copy and paste the URL in a new web browser):

http://www.geocities.com/slajeunesse/dual_supply.jpg

From what I understand from your explanations:

from both ends of the secondary, I can get over 24V from a 12v - 0v - 12v transformer.  The center is always dead in the middle and therefore from one end to the center tap, I should be able to get over 12v.  The question is can we rectify both an end to end (24V) and an end to a center tap (12V) from the same power supply (as shown in the jpg.  The jpeg goes from 120 to 12v).

Thanks,
Stephane.

[toneman]

only 1 place/point can B gnd.
either--
top
OR
middle
OR
bottom
of xfmr........
NOT 2 places.
your xfmr secondary: 1/2/3, top/mid/bottom.
with 3 being zero volts/gnd...
bridge 1 from 1 to 3
bridge 2 from 2 to 3.
in this example, 3 is common.
hopethishelps
tone

[R.G.]

The question is can we rectify both an end to end (24V) and an end to a center tap (12V) from the same power supply

You can rectify the end to end, and the end to CT. However, you may NOT connect the two outputs at more than one place.  Doing so effectively shorts the tranformer through one or another of the diodes in the bridges. You can trace that out by drawing a wire between the ground symbols in your schematic, and then taking both polarity cases. You'll find a diode or two directly connected across a transformer winding when the diodes turn on. This causes the release of a lot of smoke and occasionally flames.

You can do this. Hook ONE bridge rectifier between the ends of the transformer CT winding. The + output of the bridge rectifier is the most positive, the - output is most negative, and the CT is always in the middle between them. You may now choose which of these three points to call "ground".

If the winding makes 14Vdc per side:
with "ground" being the most positive terminal, we get 0V, -14, and -28Vdc on the outputs. If we make the CT the "ground" terminal, we get +14V on the + terminal and -14V on the - terminal.

If we hook "ground" to the - terminal of the bridge rectifier, the + terminal reads +28Vdc and the CT reads +14V. All these are equivalent, we only changed where we put the "ground" reference.

[toneman]

DOH!!
what was i thinkin(?)
:oops:
what planet is this?
Oh?  earth.
No wonder........
in the Galaxy of Inverted Suns, this works.
But, only on odd numbered power harmonics.
thanx 4 the lesson RG
:?
BTW, what voltages R U trying 2 create??
(from a single, centertapped xfmr)
:?:
tb

[R.G.]

DOH!!
what was i thinkin(?)

Probably the same as I was thinking when I specified two plastic bodied diodes feeding rectified 40Vac into a 50,000uF electrolytic can, but drew them in on the population diagram backwards.

Did you know that plastic diodes can actually explode and throw plastic shrapnel across a room? And that the stuff inside big electros stinks when it's really hot?

...ugh... I try to forget, but it's all really still clear... 8-)

[slajeune]

Thanks R.G. and Toneman,

R.G.  I'll try to digest your last post about using one of the three leads as "ground".

Toneman, I am looking to get ~24V AND ~12V from one transformer.  I know that I could use a simple voltage divider to get the 12V, but, the 12V will need to supply a lot of amps (~1 Amp).  From my understanding, I need more than 1/4 watt resistors for that (if I remember correctly).  Last time I tried to use a voltage divider to feed heaters on a tube, it didn't really fly (let's say it kept stuttering).  That said, it might be the easiest solution (i.e. rectify, filter and regulate the 24V then simply divide it for 12V).

Thanks,
Stephane.

[puretube]

hi Stephane, this is what you want:


isn`t it?
12V/1A for heating; >30V (which you can regulate with a 7824 to 24V)
for plate-supply...

[slajeune]

Hi puretube,

thanks!  That is exactly what I was looking for!!!!

Cheers,
Stephane.

[puretube]

little side-note:
because of the simple half-wave rectifying, there is some more ripple
at the big capacitors, but after the regulator it`ll be OK,
and if you put a 24 regulator at the upper output, that one will be fine, too.
(I drew a smaller 1000uF cap there, compared to the 12V side, since I
suspected you to need the 24V for tube anodes "HiVoltage".
Since that only draws minimal current (mA), you can use a smaller cap.

If it really is for plate-voltage, you can as well leave it at 30V or more. (as long as it doesn`t hum too much - you`ll find out).

[slajeune]

Hi puretube,

yes, the 12V is for heaters and the 24-30V is for plate voltage.  I will probably use a regulator there to ensure proper voltage and very minimal hum.

Thanks for all of your help (RG, toneman and puretube)!

Cheers,
Stephane.

[puretube]

always welcome! (we`ve been thru that, too...);
looking forward to your new adventures,
Ton/puretube.