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# looking for 200VA transformer with many taps Answered

For awhile, I have worked on a linear power supply design, and I think I have a circuit which is pretty stable with 47uF output capacitance, capable of 100mV to 15V and at least 5A (or more) on a single rail supply, and I can of course also mirror the circuit to generate a negative voltage rail fairly easily.

However, to ensure my supply can regulate power efficiently without dissipating too much power (say 50W max TDP) I need a good few voltage tabs, maybe about 5V increments. -20V, -15V, -10V, -5V, 0V, 5V, 10V, 15V, 20V. (unless I do some sort of voodoo with the rectification and inverting voltages, IDK. Ideas?)

I added my design for something that generates voltages between -25V and 25V with only 5 tabs, but due to half-wave rectification, I do not think this design will be suitable. Drawing even 5A from the +25V rail causes current spikes reaching 90A and beyond. Of course series parasitic inductance and resistance would greatly reduce that, but I don't like the looks of it.

I checked sources like Mouser, and Ebay but it appears that transformers with such a number of taps are uncommon to say the least. Where could I source such a transformer?

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## Discussions

These output capacitors seems small, for smoothing mains frequency, 50 or 60 s^-1, ripple.

https://en.wikipedia.org/wiki/Ripple_(electrical)#...

Unless the current you want it to supply is near zero. I mean you said 5A, in your first paragraph there. Try that number, in the formula for ripple voltage, I've linked to there, and see what you get.

Regarding the question of obtaining a transformer with specifically those windings, I think you might have to wind it yourself.

Or maybe buy five small transformers, and connect their output windings in series, taking care to make sure you get them all in phase.

Last time I checked, Youtube had a large number of bad tutorials on the subject of salvaging old microwave oven transformers (MOTs), and approximately one good tutorial, by author MattsAwesomeStuff. Actually that tutorial was three videos, named "Easiest MOT Salvage Tutorial" Parts 1, 2, and 3, here:

I have been in the field of repairing these things for a while back in the day.
From that I can tell you right away that you are lost or need to spent too much money on the transformer.
Prime example from my times was a very angry customer:
Designed a machine with a fixed space for the power supply.
If I remember correctly all up 7 quite different voltages were required ranging from 2A to over 8A max for the main one on 24V.
Switch mode supplies were unknown at that time and using standard circuits created far too much waste heat as you already know.
As we could only fit a single transformer into the space with about 15x15cm to spare for the electronics a multi tap one had to be sourced.
Up to here the power supply was quoted at just under 200 bucks if I transfer the currency.
The changeover to a custom made transformer added another 300 to the bill....

Long story short:
Get a transformer that has a seperate or "on top" secondary providing the max voltage and max amps you need.
Open the core and remove the secondary.
Note the measured output voltage (no load is fine) before you start ;)
Now unwind the secondary and count the number of turns.
Do some math on paper or the computer to link the required voltages to the number of turns.
These are the ratios you need for your voltages.
If you need negative outputs as well it isbest to select a primary that offers the combine voltage, e.g. for a max of +30 and -15 you would need one that gives 45 or 50V so you can make the corresponding centre tap from "common ground".
I found two ways that work fine for these taps:
a) simply exit the wire during winding.
This is good as long as you don't need too many voltages and the prefered option.
You might loose one or two turns all up but that should not affect performance too much.
b) for quite a few voltages or if the wire is really thick.
Remove some insulation where the next tap should be and solder an additional wire onto it that you use to break out for your tap.
Kapton tape makes a nice insulator if you want to avoid the loss of space heatshrink will cause and have no means to properly paint the joint for insulation.

IMHO winding your own multi tap transformer is a prefered option as such transformers can go far over the \$100 mark even for smaller wattages.
If you go modern and use one of these 60kHz chopper chips for simple switch mode supplies you can get away with a salvalged ferrite core of a decent size, like found in old TV's and amps.
This way you not only save space but also a lot of copper wire as you only need a "few turns" compared to 50 or 60Hz at these frequencies.

"Switch mode supplies were unknown at that time and using standard circuits created far too much waste heat as you already know."

What I find fascinating is that the concept of switch mode supplies have been around forever! RAM pumps for instance are perfectly analogous to boost converters, when treating water pressure like voltage, flow like current, water hammer effect like inductance, accumulators like capacitors, and check valves as rectifiers. The only slight difference is the switching element, which is also a check valve operating in a different mode of operation (a latching device). Not sure if there is an equivalent for hydraulic regulators or pressure reduceing device.

Sorry, I guess I should be more precise:
Instead of a normal transformer use a 60kHz driver and a transformer on a ferrite core.
Sure you need to rectify the mains voltage first and use a rated electrolytic cap to filter it but after that the size is tiny compared to the traditional design.
And even if you need a quite pure DC the additional filtering won't take much space either due to the high frequency.
If you want to go safe you can even opt for a circuit with opto-couplers.
Makes it very easy to add status LED's for the outputs as well.
Guess it all depends on how much you are willing to change the current design and how good and clean the DC needs to be.
If you prefer a normal transformer go like above ;)

Yup, a switching preregulator. I am fairly farmiliar with that concept and if I decide to go that approch, I may use a off the shelf PCB module from amazon. Some are rated up to 10A and I have got one to power a couple large LEDs. My main grip I have with those is how to vary the voltage via some signal. A digital pot is a nieve, ugly, and hacky solution.

I might consider rewinding a MOT as Jack a Lopez suggested. What I want to avoid is high frequency noise on the output. Will that be an issue with a switching preregulator?

It depends on the filtering.
But in general they are dirty if not properly filtered, especially under high loads.

Why vintage old stock capacitors and HV 400V film ones? I think I'll need something about an order of magnitude higher in value for the lowest ripple. I see the use of a variac, but the problem is that I would need to have a servo motor vary it, as I want my PSU to be programmable.

I wound a transformer in HS and wanted 2 volts per tap. Did the math, But mine came out 0.3 volts short of 2v by neglecting the magnetizing losses. The primary was thermally cool..

You can consider a Variac as a multi-tap transformer and the Stancor is available in lower current and less expensive versions also provides the safety isolation.

In my experience transformers tend to be either specially made for a job or pretty much standard items with 1 or 2 tappings.

I don't think your going to get what you want off the shelf.

You could try a transformer manufacturer to see what the cost of a one off might be (a lot I guess)