Power supply design - split rail

[casiomax]

will this design work? since i have 2 x 3Amp transformer but no center tap on the output voltage. anyone can help?

thx.

[n9voc]

 :tu:

Looks pretty good to me!

By putting the two secondaries in series, you have effectively created a 36V center-tappped secondary.


There will probably be some very minor difference in the absolute voltage level of the + to the  - side, as no two transformers are exactly alike.  However, it should be relatively insignificant in your applications!  With 10k uF on each side, it ought to work nicely!

[nashvillebill]

How does one check to make sure the two transformers are properly phased? Or does it matter?

[darwindeathcat]

Just a quick question. Why do you have the 10uf caps bypassing each diode on your bridge rectifier? I'm only asking becasue I'm about to build the exact same type of power supply (2 x non center tapped transformers in the same configuration you have there) for my lm3886 amp, and I haven't come across a PS schematic that indicates you should do that. I guess I mainly wnat to know what are the advantages of having those bypass caps?

[nashvillebill]

Those are 10 nF not 10 mfd...they are there to "smooth out" the diodes, at least that's the theory.  I've done power supplies with and without them, to my ears I can't hear any difference.

[n9voc]

Re- Transformer phasing

Since the primaries are on separate cores, (two transformers) the phasing is irrelevant except that if both transformers are the same phase (start of primary winding to "hot" for example), then the secondaries will be "in phase" for series work - i.e. the "0" winding will be the same secondary wire on both units.

If one transformer is hooked up primary out of phase to the other (start of primary winding of each transformer on different wires), then likewise the secondary windings will be out of phase with one another by approximately 180 degrees.

For practical purposes, hook both transformers up with no load on the secondaries, connect one transformer wire secondary wire to the other, check no load voltage.  If no load is in the vicinity of 36 volts or better, you are good.  If not, hook together one of the same leads of the connection with the OTHER lead of the second secondary, should now read as above. 

Because the units are on separate cores, this will not damage the transformers, if they are on the same core - potential heating of the primary winding can occur with primaries are hooked up out of phase. Also, because the units are on separate cores, the secondary out of phase condition may not be exactly 180 degrees, and thus you would have a small but measurable voltage when the secondaries are hooked up "out of phase"

To verify phasing on a multiple primary transformer, you need a variable voltage source.
leaving secondaries under "NO LOAD" conditions, connect one end of a primary to one end of the second primary.  Apply a SMALL voltage (around 10-20 volts) to the combination and note the no load AC voltage on a secondary. Then, disconnecting the primaries, hook the first "one end" to the other end of the second primary.  Repeat voltage application.
Odds are that with one configuration you will have a small but repeatable voltage on the secondary (where the two primaries are in phase) and in the other configuration you will have virtually 0 volts on the secondary, (where the two primaries are out of phase).

Finally, and of MOST importance:
BE EXTREMELY CAREFUL when dealing with mains voltages!!!  Electrical current moves almost at the speed of light and Mains voltages have sufficient voltage and current capability to be LETHAL!!

ALSO:  when dealing with transformer leads, use a method such as brass brads in a board that will allow you to solder the leads into place on your bench, keeping them from flying around to contact you, your equipment or each other!  (old transformer engineering trick!)


Hope this helps!

[n9voc]

Couple other safety tips from transformer engineering:

1) ALWAYS do any connecting/disconnecting or soldering on transformer leads with mains input unplugged.

2) If possible, get a variable AC source to "ramp up" the mains voltage to full voltage from zero, and if you use such a source - be certain IT is disconnected from the mains BEFORE working on your transformer and any time you reach into it with anything other than the probes of your voltmeter.

Respect electricity, but don't FEAR it.  Respect inspires care, fear causes mistakes.
:tu:

[J M Fahey]

It will work. To check phasing, build a series-lamp fixture , and connect  there both primaries in parallel (label them to be able to identify the primary phase) and the secondaries in series (no load). If you measure 36Vac end to end, fine, label them accordingly; if reading 0 or very close to 0 volts, invert one of them. Good luck.

[casiomax]

thanks for the advice guys! appreciate it.