Choosing a suitable power transformer - Page 1
Forum section: electronics / beginners

author: Electrolitty
author_rank: Contributor
timestamp: November 15, 2023, 08:22:59 pm
content: Hi,I am looking for a suitable power transformer (230V primary) to build a small regulated linear power supply that will deliver +15V, -15V and +5V (max. 300mA per rail). The schematics should closely follow those of a vintage Tektronix 1101A power supply.Seehttps://w140.com/tekwiki/images/a/a9/070-8276-02.pdfpage 48 of 61.I have been searching for quite a while on the electronic components distributors web pages (specifically the one with the rodent's name) but couldn't find a transformer with suitable values. I am now thinking of simply using two transformers wired in parallel, one with dual (more or less) 18V (with center tap) for plus and minus 15V and a second transformer for the +5V rail.That is not really elegant but I can't think of a better solution. Perhaps I simply didn't find the right one in the transformer list with 5500+ entries.I thank you in advance for your input.

author: coppice
author_rank: Super Contributor
timestamp: November 15, 2023, 08:42:23 pm
content: That market for transformers is not what it used to be, now the majority of appliances are using switch mode supplies. So, the range of off the shelf transformers has shrunk. However, most of the linear transformers in products which need more than a single rail have always been custom wound for that product. It is economic to wind quite low volumes of customised transformers, as long as they use a standard platform. Choose a core and primaries to suit your needs, and they wind the appropriate secondaries on top of that. If you are close to the vendor they used to be happy to make one offs that were not crazily expensive, suitable for prototyping.

author: jwet
author_rank: Frequent Contributor
timestamp: November 16, 2023, 02:32:05 am
content: I think the two transformer design is where you'll end up. One of biggest advantages that manufacturer have over home builders is custom magnetics. Most PS's for a lab PS use a main winding of 70V or so center tapped for the main rails, often these will have taps too to minimize power dissipation at lower voltage. They also have an aux winding for say 15v that they can rectify and tie anywhere and then maybe a small 5v winding to make a negative rail.Here are a few ideas-1. Rewind transformers yourself. This can be done but is pretty hard core and is art as much as science. I've never done it but I've watched videos (instant expert!). I think the easiest way might be toroidal transformers. I have "modified" off the shelf toroids type (taken widings off).2. Find a starving custom transformer guy. These are rare these days. There is one near me in Sanford, NC. He will do big HV transformers for HAM transmitters or big welding transformers. Its labor intensive and he needs to make a living. He comes to the Raleigh, NC Hamfest each year and probably sells a few. He has regular customers that keep him afloat.3. Buy old broken power supplies. I have a few around here that I bought for this purpose because they had a nice XFMR. Mostly getting very dusty but one the list. They have a three tap winding on a +-60v main that had a bunch of relays taking them in an out originally.4. Stack up some standard stuff and remember Torioidal transformers- Mouser, Digikey, etc sell these as well as a lot of semi-custom toroids around +-70V or so at 500 VA for reasonable. Audio amp builders use these and its an active hobby. Then hack together what you can out of a couple of off the shelf transformers for your control and aux windings.I really don't like swtichers for lab supplies but its getting harder and harder to avoid them especially with the prices.Good Luck.

author: srb1954
author_rank: Super Contributor
timestamp: November 16, 2023, 03:54:19 am
content: Quote from: Electrolitty on November 15, 2023, 08:22:59 pmA Bel-Signal 14A-56-515 would do the job. It is available ex stock from Mouser but is probably not the cheapest around.It is only rated for 250mA from the +/- 15V outputs but you could easily get the 300mA you need if you substantially reduce the load on the winding for the +5V supply, which is nominally rated at 3A.

author: MathWizard
author_rank: Super Contributor
timestamp: November 16, 2023, 06:46:47 am
content: So Tektronic made a PSU based around LM317's ? Wow I expected some big discrete BJT circuits.For such low power, I'd look at salvaging transformers from stereo's, CD players, etc. A lot of them have center taps. I've made 2 copies/versions of the same vintage 30V-1A PSU. I've used different combinations of salvaged stereo transformers for it. Now I'm using a big multi winding 1, capable of a lot more more than I'm using.

author: Electrolitty
author_rank: Contributor
timestamp: November 16, 2023, 01:11:34 pm
content: Thank you very much for the answers, they took me a good step further.As I would like to realise the project without too much delay I prefer to either use two transformers or the solution suggested bysrb1954.As the power supply connector providing power output has 4 pins (common, +5V, +15V, -15V) one would need to have a combined common lead for the 3 voltages. Considering the schematics of the BEL transformer, is it feasable to tie common of 5V and 15V together? I'm somewhat sceptical. Never did something like this.

author: jwet
author_rank: Frequent Contributor
timestamp: November 16, 2023, 05:29:13 pm
content: Yes- the great thing about transformer secondaries is that that they're totally isolated, you can stack them as AC or DC. Be careful pulling a load that would put tow dissimilar windings in series but tying things together is fine.