Confused about dual primary transformer.

[RDV]

I've got the biggest one here:
http://www.hammondmfg.com/183.htm

I don't quite get how to wire this thing up to get the maximum current.
The part about the dual primary is messing with my pea brain.
If somebody could tell me which wiring(series? parallel?) you use to put 115v(rather than 230) in, I'd certainly appreciate the help. I'm building a dual supply for a SS amp(using a LM3886). The wiring diagrams are here:
http://www.hammondmfg.com/5CHook.htm

TIA

RDV

[RDV]

I'm guessing it's the 1st and 3rd diagrams for 115v in and 36vct out. One thing I'm wondering is would I get more power by just wiring the secondaries individually? Would I then get 18v at each secondary with more amperage?

Thanks

RDV

[R.G.]

First of all, tranformers gots polarities. See those dots on the transformer schematic? All the leads with dots go positive (and negative, in turn) together.

Each winding used as a primary needs to be driven with its rated voltage. In the case of the dual 115V ones, you connect dot to dot, non-dot to non-dot, effectively paralleling the primaries.

If you get one winding reversed, you will see major smoke when you power it.

The transformer's rating is based on having enough amperes flowing into the core so the secondaries can take their designed current out. Dual primaries are made half the otherwise-required wire size each, so to get full power out, you must have the designed current flowing in each primary winding. You do this by paralleling for 115, or series for 230. Both situations give you equal current in each winding, and contribute each primary's fair share of ampere-turns to the M-field inside.

If you series two windings, you must connect a dot to a non-dot, and then put voltage across the series setup. again, if you connect it backwards, major smoke pours out, and the transformer is itself tranformed into a Darkness Emitting Diode (also called a DED).

You cannot get more power out of a tranformer than it's designed for by any means except running it at a higher frequency. So, no, hooking each secondary up individually will not help. What the spec is telling you is that each 18V secondary will do 1/2 of the specified 18V current when they are paralleled; this is the same current they will supply in series, which is shown as 36Vct. Same voltages, same currents, hooked in series instead of parallel.

You want connections 2 and 3 (primaries parallel, secondaries in series) for your amp, since the LM3886 is designed for +/- power supplies.

That 56VA transformer will likely power the 3886 to about 35W, as you lose some in rectification and voltage sag. If you're trying to pull more than that out of it, you need a beefier transformer. I prefer a 42Vct 2A transformer for the 3886. Parts express used to sell one of these for $15, but I can't find it at their site now. Even better is a 115/115/22/22 toroid. I got a few of these on ebay.

[toneman]

Hi RDV,
mayB i can help here.
dual primaries are for 120VAC OR 240VAC.
But U knew this(?)
240VAC in requires twice in resistance of 120VAC.
so.....
for the primary side,
U want the Parallel(in phase) diagram(#2).
[for 240VAC in, U want the windings in series, diagram #1]

now the secondary.....
2 get 18VAC out , @ 3.12A, U want the windings
of the secondary in series(in phase) diagram #4.

got it?
PM me if U need more help.....
stay(trans)formed
tone

[RDV]

Thanks fellas. I'm pretty dumb about all this, but when I do anything new I ask questions, read, and remember my lessons. I wish I'd gotten a bigger transformer but maybe there won't be too much clipping like this. I'll probably use it at home mostly anyway.

RDV

[toneman]

OOPS!
:oops:
i should have said "parallel" in phase for the secondary.
the xfmr is 36V CT(centertapped)
so...18V each side.
paralleling them gets twice the current of one.
B sure to wire em as the diagram.
keep the phasing correct---
otherwise, U get zero....  
afn
Tone

[RDV]

Okay, I'm trying very hard to come up with the parts for this dual supply. It seems 10,000uF caps at a 50v rating are very hard to come by. Would 4700uF 50v be sufficient for the divider network, or would 10,000uF 25v be enough? I'm looking to make this thing quiet and useful.

TIA

R.D. Vance

[R.G.]

From "power supplies basics" at GEO, the voltage you'll get from each 18V secondary is 18V * 1.414 = 25.42V minus the diode drops, so just under 25Vdc... **if** your AC line is correct, and if they didn't precompensate for sag by winding the transformer to maybe 20V to take sag into account.

So no, you can't safely use 25V caps. 35V is OK. 4700 is fine for cap values. The Vox Super Beatle at 120Wrms used 4700uF caps in the power supply. Somewhere over 4700uF you get diminishing returns. Build it, and if ripple is too bad, buy another pair of 4700's and parallel the originals.

[RDV]

Thanks R.G., I went ahead and purchased 4ea 4700uF 50volt caps. The selection at Mouser left a bit to be desired.

RDV

[RDV]

Finding "Power Supply Basics" at Geofex was not easy, so to save others some trouble finding this helpful document, here it is: http://www.geofex.com/Article_Folders/Power-supplies/powersup.htm

RDV

[Mark Hammer]

I lucked into a store clearing out a bunch of these dual primary Hammond transformers for a buck or two each, although I suspect a different series.  I bought a handful of lower current (300ma, I think) 34vct ones and some larger 12 or 18v ones.

Many thanks to you folks and Hammond for clearing things up and giving me the thumbs up on how to use them.