40V + 40V Mains transformer about 25–40 VA?Hi - Page 1
Forum section: electronics / projects

author: Chris56000
author_rank: Frequent Contributor
timestamp: August 28, 2020, 05:07:30 pm
content: Hi!Can anyone recommend a mains transformer suitable for 115V or 230V supplies, with two 40V secondaries, each secondary current rating of 0.3A to 0.5A per winding, suitable for series or parallel connection of secondaries?It's for use in a Curve Tracer Project!Chris Williams

author: nali
author_rank: Frequent Contributor
timestamp: August 28, 2020, 09:25:16 pm
content: Sounds like the sort of thing you can salvage from an old consumer audio amplifier. I salvaged one recently from my old Technics SUV-650 which has 42+42+9.7V. It's quite a hefty lump of iron!

author: Chris56000
author_rank: Frequent Contributor
timestamp: August 29, 2020, 10:02:06 am
content: Hi!I'm looking for a quotable source of a new component, rather than a salvaged one, as I want to feature the Curve Tracer as a suggested project in my forthcoming Test & Measurement Theory & Servicing Book:–https://www.vintage-radio.net/forum/showthread.php?t=166405This three year project, to be completed by July 2023, will write up many years of my personal and other Members' experience in repairing T & M and accessible in one free place!The book will be CC Licenced and a GitHub Repository will be opened once content is ready to be placed in it!Chris Williams

author: nali
author_rank: Frequent Contributor
timestamp: August 29, 2020, 10:49:28 am
content: Ah OK that makes a difference, I read it as a hobby project although I did wonder why you wanted 115/230v.The only txformer people I've worked with is Vigortronix who are UK based, they've been pretty helpful fielding inquiries before, PM me if you want a contact.

author: Chris56000
author_rank: Frequent Contributor
timestamp: August 29, 2020, 12:55:49 pm
content: Hi!The book will feature dual 115/230V operation in all the mains–operated circuits I propose to feature as buildable projects as that is the world standard for items of T & M – they're either universal operation, or have a readily accessible tapping switch – there's a very useful rotary "Bulgin" type that is fully touchpoof, mounts in a single round hole and consists of 2 ganged DPDT switches, compatible with tapped–primary transformers, dual primary transformers, or the 115/230V switched bridge–rectifier arrangements in mains–transformerless circuits!The book will be written with UK terminology and symbols as it was the UK Vintage Radio Forum I am originally writing it for!Chris Williams

author: andy3055
author_rank: Super Contributor
timestamp: August 29, 2020, 08:48:41 pm
content: How about this?https://www.newark.com/c/transformers/toroidal-power-transformers?secondary-voltages=2-x-40vUK Farnell should have these also.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 30, 2020, 01:03:01 am
content: I would not bother too much with transformer selection.The transformer is a bit on the big side for PCB mounting, so I would just use scew terminals for AC input, and leave the transformer selection to whomever wants to build the kit.Transformers are heavy. which is a burden on shipping costs.We're living in a global world. there are many local producers of transformers.When I buy a transformer here in europe, I can hardly be bothered if it can also be run on some other mains voltage on the other side of this dirtball.Transformers are expensive. Many will want to adopt a transformer they have lying around.If possible, use a 4 terminal screw connector, so builders can use a transformer with a center tap, or a transformer with 2 completely separate secondary windings.Write in the article that you can easily determine the phase of secondary windings by connecting a random end of each winding together, and apply mains voltage.If the voltage of the open ends are then twice the winding voltage, they are in series with a center tap. If the voltage is very low, you've put them parallel and should swap one winding.Then, in the description give some range of transformers that are suitable. For example: a voltage range of 36V to 45 Vac and of 500mA secondary. (This seems a lot for a signal tracer).For kit's it's also handy to accommodate for different models and sizes of heatsink. For example if you have a 48V transformer, but the voltage regulator gets a bit hot.Some may argue this is not very "eco responsible" but for a device you use occasionally it hardly matters.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 30, 2020, 01:12:32 am
content: If it's a book with multiple sections / projects, I would handle the power supplies separately. Mains wiring is very depdent on the housing. location of plugs and switches etc, and unless it's a full kit with included enclosure there will be many variants.