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For context: I am looking to step up 12V AC at ~25Hz to ~75V. Required output current is no more than 3W.

I'm looking for a small PCB mountable transformer which would do this job. I think that I understand the basic premise of transformers and that the ratio between the primary and secondary coils should be somewhere in the region of 1:6 to achieve this step up.

I've been looking at a variety of Flyback transformers such as this one: https://docs.rs-online.com/40b2/0900766b810a6c6a.pdf. The datasheet has a diagram as follows:

Transformer Pinout

This implies to me that there are 3 primary and 3 secondary coils. I am presuming this is to allow for a variety of ratios to be used, as required. However, it is not at all clear from the datasheet what those ratios are.

The datasheet seemingly gives some example turn ratios:

Turn Rations

Though these seem somewhat incomplete. My questions are:

  • Am I reading this diagram correctly? Are there indeed 3 coils on each side?
  • How do you go about determining the number of turns on each coil?
  • The datasheet says the ratio of 1-3:5-6 is 2.8:1. How is this possible? Looking at the diagram, those coils seem to be both on the same side of the transformer

Edit: It’s feasible this is an X-Y problem and there’s a better solution. I’m looking to make a ring generator for an old analogue phone. I have a prepackaged unit made by PowerDSine but as a learning exercise it seemed interesting to make one. Sample circuit I’m working off is below. I have a 12V input and a 555 + RC filter generating a decent sine wave at the frequency I want - I figured I could feed it into a couple of MOSFETs and then step up to 75v.

enter image description here

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  • \$\begingroup\$ 25 Hz? 12V rms? at what source impedance? Resistive load? load regulation error? \$\endgroup\$ – Tony Stewart EE75 Jan 31 at 19:40
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    \$\begingroup\$ "Required output current is no more than 3W." A Watt is a measure of power, not current. If you want three watts of power, say so. If you want some amount of current, give a number in amperes. \$\endgroup\$ – TimWescott Jan 31 at 19:48
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    \$\begingroup\$ This has the look of an XY problem. A transformer that will step 12V up to 75V and carry 3W of power at 25Hz is going to be much larger than a transformer designed for 125kHz, it will probably not be "small", and it'll be hard to find because 25Hz is such a low frequency. So what are you really trying to do? \$\endgroup\$ – TimWescott Jan 31 at 19:56
  • \$\begingroup\$ If you were absolutely bound and determined to do this with a transformer, you could probably start with this one, take it apart, and rewind it to the correct ratio. Or you could have one custom-made. Transformers have to be designed with the specific purpose in mind, and 25Hz step-up applications are not thick on the ground -- so you aren't going to just buy a suitable one. \$\endgroup\$ – TimWescott Jan 31 at 20:01
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    \$\begingroup\$ Do you need faithful audio reproduction, or do you just need to power something at 25Hz? How faithfully does the output need to match the input? \$\endgroup\$ – TimWescott Jan 31 at 20:02
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A 150 kHz flyback transformer is totally unsuitable. Your best bet would be to use a transformer rated at mains frequency, and run it backwards. As voltage scales with frequency, your 12 V and 75 V windings would have to be rated at a minimum of 24 V and 150 V.

A transformer rated for 120 V would probably not have sufficient headroom to over-run it to 150 V, so you would need a 240 V transformer.

Your 12:75 ratio means you'd appear to want a 240 V transformer with a rated output of nominally 38 V. However, that would be rated for step-down use, which means the voltage would be correct after losses. A higher ratio means a lower rated secondary output voltage. With a small transformer having a relatively poor regulation, you may want a transformer rated at around 30 V, for an apparent 1:8 step up.

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  • \$\begingroup\$ Thanks :) I picked up a 30->230V mains transformer so I'll give it a try and see what output voltage I get \$\endgroup\$ – phil-lavin Feb 2 at 16:47
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That transformer is unsuitable for use at 25 Hz, as it says 150 kHz.

It's a flyback transformer, and no flyback transformer is suitable for use at 25 Hz, as they are meant for frequencies far above the mains frequencies, like tens to few hundred kilohertz.

The auxiliary winding is to power up the mains side switch mode chip that drives the primary coil.

It reallly has two parallel primaries, and three parallel secondaries, and as the datasheet says, they must be paralleled, it is not for adjusting the turns ratio.

And it really has three separate windings on the same core, it does not matter where exactly they are drawn, but the aux is meant to be used on primary side so it is drawn there. The secondary has typically better isolation than aux so it is drawn alone on the other side.

The primary to aux ratio really is 2.8 to 1 and the primary to secondary really is 2.33 to 1.

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