8 ratios in one transformer alternatives - Page 1
Forum section: electronics / projects

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 14, 2019, 09:45:20 pm
content: I am trying to build a project and one of the transformers output 8 different values. It will be pretty hard to buy something like that.I am looking for idea and alternatives to manipulate the output of just one transformer and produce the attached output values.

author: mariush
author_rank: Super Contributor
timestamp: May 14, 2019, 10:11:43 pm
content: You can get a toroidal transformer with one secondary winding or two secondary windings and UNWIND them , then cut them to the lengths required.For example, get an 115v/230v -> 115v AC unwind the secondary and cut at 10v or 20v steps ... if you have 200 meters of wire, you have 200m / 115v = ~1.75m per voltHere's a starting point:https://www.digikey.com/short/p4338m

author: Psi
author_rank: Super Contributor
timestamp: May 14, 2019, 11:53:29 pm
content: Yep, unwinding and rewinding with extra taps is a pretty easy thing to do.As long as you have access to the secondary winding without having to remove the primary.I wouldn't bother cutting it into lengths, wind it in one length but with taps that come out and then go back in again.Keep in mind that you will use some wire length in the taps.So if you start with a 100V transformer and add new taps you will have lost some wire length and therefore lost some turns.So the output wont be 100V any more.Ya just need to add some extra wire or start with a higher voltage than you need.I have a small toroidal transformer you could have, its 70mm diameter and 33mm high.I dunno what the wattage is, you'd have to guess based on physical sizeInput is 230VHowever current output windings are all around ~15-30v.So you would need to replace all of the secondary with new wire to get the 100V + taps that you need.

author: Zero999
author_rank: Super Contributor
timestamp: May 15, 2019, 09:34:35 am
content: If you have a toroidal transformer, don't bother removing any windings, just wind extra ones on top, which can be connected in phase or anti-phase with the existing secondary, to get other voltages.

author: soldar
author_rank: Super Contributor
timestamp: May 15, 2019, 04:48:22 pm
content: You know I have wound many transformers back in my day but now I think I would rather find a pro shop who can make it to order in no time and better than I ever could. They have the supplies and the expertise and can save you a lot of time and grief.

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 15, 2019, 07:56:34 pm
content: Thanks for all the ideas! I really like the toroidal idea.I want to give it a go and try winding it myself but will do it with a traditional ETD ferrite core.Some of the calculations based on the core I bought are bellow.What would be the best way to define the wire gauge? I know there is a relation between cooper area an AMPs.

author: soldar
author_rank: Super Contributor
timestamp: May 15, 2019, 11:38:59 pm
content: Wait a minute! Your OP shows a mains transformer and now you are showing a ferrite core! What are we talking about now?

author: Circlotron
author_rank: Super Contributor
timestamp: May 16, 2019, 12:08:31 am
content: Quote from: gkmaia on May 15, 2019, 07:56:34 pmFerrite won't make enough inductance to work with any decent voltage at 50/60Hz. You would need a ridiculous amount of turns. It's the area under a half-cycle thing. Iron is the only practical way to go. I made an audio transformer using an ETD49 3F3 ferrite once and with 625 turns at 30Hz it would saturate with only 20VRMS. Other than that it worked really well.

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 16, 2019, 12:11:33 am
content: That is correct. Ferrite was my mistake. Will use iron. Just has been hard to find an iron coil former I could use.How hard it is to separate the iron blades of a working iron core transformer? They are glued... any way to release them?

author: BravoV
author_rank: Super Contributor
timestamp: May 16, 2019, 12:12:38 am
content: How about buying working but used isolation transformer and a variac ? Will these get you same result ? And with variac it can output finer adjustment.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 16, 2019, 12:14:38 am
content: To cut down on the number of taps required, you can use Golomb spacing.https://en.wikipedia.org/wiki/Golomb_rulerExample: if you tap a winding so that it's 10V-10V-20V, the possible output voltages are 10V, 20V, 30V, and 40V, selectable using only 2 relays.

author: radix
author_rank: Contributor
timestamp: May 16, 2019, 12:22:39 am
content: If you're getting a custom transformer wound, it would be better to use a balanced ternary type of ratios. So with 10 V and 30 V windings you could get output voltages of 0, 10 V, 20 V (30 V - 10 V), 30 V, 40 V (30 V + 10 V).

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 16, 2019, 12:42:17 am
content: The problem with windings in opposing phase is that the resistance still adds, increasing voltage drop over load.

author: Circlotron
author_rank: Super Contributor
timestamp: May 16, 2019, 01:36:31 am
content: Quote from: NiHaoMike on May 16, 2019, 12:14:38 amWould be interesting to see what you could do with both Golomb spacing and reversing windings to subtract voltage.

author: wraper
author_rank: Unknown
timestamp: May 16, 2019, 01:46:57 am
content: This circuit seems outdated by half a century at least. Most likely there are much better and cheaper modern ways how to do the same thing without using any exotic transformers.

author: Psi
author_rank: Super Contributor
timestamp: May 16, 2019, 02:39:57 am
content: Quote from: gkmaia on May 16, 2019, 12:11:33 amThey're not to bad to get apart.Putting them back together correctly is the hard bit.If you DIY it they will always end up with tiny gaps and hum loudly when energized.So i don't recommend rewinding EI cores unless you have the proper gear.Toroids on the other hand are awesome but have the disadvantage of taking longer to wind by hand since you have to keep threading wire through the center hole for each turn.Are you in Auckland, you can have that 70x33mm toroid if you want.I dunno if its the right wattage for what you want though.

author: soldar
author_rank: Super Contributor
timestamp: May 16, 2019, 11:15:20 am
content: Quote from: NiHaoMike on May 16, 2019, 12:14:38 amI would much rather have a transformer with the required taps than trying to work it out with switches because mechanical switches are much more prone to failure and should be avoided as much as possible.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 16, 2019, 02:25:22 pm
content: Quote from: soldar on May 16, 2019, 11:15:20 amHow else would you switch between the taps? I do think a solid state alternative such as a switching regulator would be the better choice but there's not enough information on why so many output voltages are required.

author: soldar
author_rank: Super Contributor
timestamp: May 16, 2019, 06:50:25 pm
content: Quote from: NiHaoMike on May 16, 2019, 02:25:22 pmWhat I mean is I would rather only switch one side than both ends. Having a double switch is more expensive, complicated and prone to problems than just getting the right taps in the first place.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 16, 2019, 07:56:48 pm
content: Quote from: soldar on May 16, 2019, 06:50:25 pmIn the 10V-10V-20V example, you'll note that only two SPDT relays are needed to achieve all 4 possible output voltages. Start with three separate 10V, 10V, and 20V windings, with one 10V winding permanently in circuit and the other two switched in and out with relays. You'll find that with the correct arrangement, you'll be able to common two connection pairs and reduce the secondary lead count from 6 to 4.

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 17, 2019, 04:10:55 am
content: That is where I am at. Bought a new 500ma iron core transformer.Put it in hot water to remove the iron plates. Build a little 3D winding shaft and attached it to my mini lathe and counter. It will be easy now to wind it.The transformer secondary had 114 turns at 9v. At 240v primary this would give me 3040 turns. Which seems right as it is a rounded ratio.I had a bit of a go and winded 130 turns on the secondary to see how that pans out.Any advice on an easy setup to how to test the new winding? Signal generator or mains on primary? Secondary with load resistor or not?

author: Zero999
author_rank: Super Contributor
timestamp: May 17, 2019, 09:29:51 am
content: The output voltage will drop under load, so there should always be more secondary turns, than the ratio would predict. If a 9V transformer has 114 turns on the secondary, then it will have about 10% less than 3040 on the primary.

author: tkamiya
author_rank: Super Contributor
timestamp: May 17, 2019, 04:31:15 pm
content: What about using multiple transformers? Depending on current requirement, it wouldn't be that bulky.

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 18, 2019, 07:16:28 pm
content: Yes, two is a good idea. That is the outcome.