does anyone make oddball multitap transformers off-the-shelf?

[slburris]

I'm looking to do some power supplies for a couple of
projects.  They need to be linear supplies for
low noise, otherwise this would be easy using switchers.

The first is +12v 1amp, +5v 1amp, -12v small ma.  The
second is +24v 2amp, +5v 1amp.

Yes, I could just do the larger voltages and burn a lot of
power dropping to 5v, but I'd prefer to stay away from
making these things room heaters.

A nice solution would be a  center tapped transformer with an extra winding, i.e.:

12V - 0V - 12V - 6.3V

Or a 5 tap variant like:

12V - 6V - 0V - 6V - 12V

You get the idea.  But other than scouring ebay and the usual surplus
places, does anyone make off-the-shelf multitap transformers like this
that aren't a custom order?

Any other projects I would just design a switcher, but these are for a
GPSDO and a surplus 10Mhz rubidium oscillator, both of which want
really quiet power.  I suppose I could just break down and
put in two transformers, but that seems like it would be just giving up. :-)


Scott

[ejeffrey]

There are a few of them out there, I have found them on both Farnell and Mouser, for instance:

http://uk.farnell.com/myrra/74030/transformer-ee25-30w/dp/1214646

http://eu.mouser.com/ProductDetail/Xicon/41FW300/?qs=iLgTiujIJ5vm5diWgC6v8Q%3d%3d

You don't get to choose what you want, you just get whatever fits some special purpose application.

For the -12 volt low current supply you can consider a charge pump inverter.  You still have to filter the supply pretty heavily, but they inject a lot less noise into the rest of the system than magnetic based switchers.

[slburris]

Hmm, the Farnell Myrra transformer appears to be 70-100Khz, not 50/60Hz.

The Xicon transformer would be perfect if it could supply more than 300ma.
I'll have to go looking through their catalog to see if I can find something
similar but bigger.  Maybe I can find someone who cross references to 41FW300
and see if they have anything.

Thanks for the pointers.

Scott

[NiHaoMike]

The first one is relatively easy. Use a center tap transformer with a bridge rectifier. Supply the +12V directly from the bridge and the +5V from the center tap. Use series inductors to reduce noise caused by peak currents. Use a charge pump run from one side of the transformer for the -12V.

The second one is quite tricky. One possible solution is to use the same kind of transformer as for the first one but use the following circuit to rectify it for the 24V output: http://repairfaq.ece.drexel.edu/sam/samschem.htm#schfwvd . Note that you can still use the center tap for the 5V output.

[grenert]

How 'bout these?
http://www.mcitransformer.com/i_4-06_2.html

Or these toroids (if you are in 120VAC-land):
http://www.toroid.com/standard_transformers/rectifier_transformers/rectifier_transformers_mult.htm

[amspire]

You can always just get 24v CT and 48V CT toriod transformers and add some extra turns for the low voltage windings.

Instead of trying to add enough turns for a 6V winding, you could do a 3V winding with a voltage doubler rectifying stage.

A 20VA toriodal transform has dimensions of about 80mm outside diameter and 25mm inside diameter, so there should be room to add a few extra turns. Get some Kapton tape for extra insulation.

Probably worth an experiment. Just get one, add 10 turns, and measure the output voltage from the new turns. From that, you can work out the number of turns you need.

Richard.

[amspire]

Another thought for the 12V transformer.

Just get a 12V center tapped transformer. Have one end connected to 0V, the other end connected to half wave rectifiers providing the +/- 16 unregulated for the 12V supplies. A half wave rectifier from the transformer center tap can provide the 8V unregulated for the 5V supply.

Richard

[ejeffrey]

One more thing: You are building a rubidium oscillator into your circuit and are worried about a few measly watts? 

[vk6zgo]

Unless you have space limitations,why not use two transformers?
Many of the multiple output power supplies I've seen over the years do this.

VK6ZGO

[Zero999]

Yes voltage doublers, adding extra turns to toroidal transformers are both good ideas.

Another thought for the 12V transformer.

Just get a 12V center tapped transformer. Have one end connected to 0V, the other end connected to half wave rectifiers providing the +/- 16 unregulated for the 12V supplies. A half wave rectifier from the transformer center tap can provide the 8V unregulated for the 5V supply.

Richard

I don't understand why you'd want to use a half wave rectifier?

The filter capacitor will need to be nearly twice the size for the same current rating and the transformer could overheat if the current draw is too high as there will be DC flux in the core leading to saturation.

The good reason I can see for  using a half wave rectifier is to use one positive and another negative to obtain a bipolar supply from a transformer with a single winding and no centre tap. This is good for audio amplfiers which draw a nearly symmetrical current from the positive and negative rails so core saturation isn't a problem.

[slburris]

Unless you have space limitations,why not use two transformers?
Many of the multiple output power supplies I've seen over the years do this.

VK6ZGO

I'll probably end up doing just that.  The suggested multitap transformers
don't quite meet the current requirements.  I'll have to keep in mind the
ones that can do +5 and +-12 for future projects that don't need so much
current on +12.

Although since the current projects are one-off's, maybe hand adding a few
turns to a toroid isn't a bad plan.

Scott