does anyone make oddball multitap transformers off-the-shelf? - Page 1
Forum section: electronics / general

author: slburris
author_rank: Frequent Contributor
timestamp: August 16, 2011, 05:23:09 pm
content: I'm looking to do some power supplies for a couple ofprojects. They need to be linear supplies forlow noise, otherwise this would be easy using switchers.The first is +12v 1amp, +5v 1amp, -12v small ma. Thesecond is +24v 2amp, +5v 1amp.Yes, I could just do the larger voltages and burn a lot ofpower dropping to 5v, but I'd prefer to stay away frommaking these things room heaters.A nice solution would be a center tapped transformer with an extra winding, i.e.:12V - 0V - 12V - 6.3VOr a 5 tap variant like:12V - 6V - 0V - 6V - 12VYou get the idea. But other than scouring ebay and the usual surplusplaces, does anyone make off-the-shelf multitap transformers like thisthat aren't a custom order?Any other projects I would just design a switcher, but these are for aGPSDO and a surplus 10Mhz rubidium oscillator, both of which wantreally quiet power. I suppose I could just break down andput in two transformers, but that seems like it would be just giving up. :-)Scott

author: ejeffrey
author_rank: Super Contributor
timestamp: August 16, 2011, 07:31:33 pm
content: 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/1214646http://eu.mouser.com/ProductDetail/Xicon/41FW300/?qs=iLgTiujIJ5vm5diWgC6v8Q%3d%3dYou 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.

author: slburris
author_rank: Frequent Contributor
timestamp: August 16, 2011, 09:59:09 pm
content: 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 somethingsimilar but bigger. Maybe I can find someone who cross references to 41FW300and see if they have anything.Thanks for the pointers.Scott

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 17, 2011, 12:58:36 am
content: 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.

author: grenert
author_rank: Frequent Contributor
timestamp: August 17, 2011, 01:25:39 am
content: How 'bout these?http://www.mcitransformer.com/i_4-06_2.htmlOr these toroids (if you are in 120VAC-land):http://www.toroid.com/standard_transformers/rectifier_transformers/rectifier_transformers_mult.htm

author: amspire
author_rank: Super Contributor
timestamp: August 17, 2011, 01:52:16 am
content: 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.

author: amspire
author_rank: Super Contributor
timestamp: August 17, 2011, 01:59:50 am
content: 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

author: ejeffrey
author_rank: Super Contributor
timestamp: August 17, 2011, 09:07:20 am
content: One more thing: You are building a rubidium oscillator into your circuit and are worried about a few measly watts?

author: vk6zgo
author_rank: Super Contributor
timestamp: August 18, 2011, 03:20:39 pm
content: 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

author: Zero999
author_rank: Super Contributor
timestamp: August 18, 2011, 04:35:49 pm
content: Yes voltage doublers, adding extra turns to toroidal transformers are both good ideas.Quote from: amspire on August 17, 2011, 01:59:50 amI 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.

author: slburris
author_rank: Frequent Contributor
timestamp: August 18, 2011, 05:05:05 pm
content: Quote from: vk6zgo on August 18, 2011, 03:20:39 pmI'll probably end up doing just that. The suggested multitap transformersdon't quite meet the current requirements. I'll have to keep in mind theones that can do +5 and +-12 for future projects that don't need so muchcurrent on +12.Although since the current projects are one-off's, maybe hand adding a fewturns to a toroid isn't a bad plan.Scott