Transformer question

[AkisTzortzis]

Hello, this may be completely silly question but I will try:

I would like to make a transformer with a primary winding of 5 Volts RMS and current of 40 Ampere RMS. Its secondary winding ought to be 35 Volts RMS and 4.5 Amps RMS.

I presume that means the turns ratio should be 1:7 (from 5 Volts to 35 Volts) ?

I am not sure how to select the core , would a toroidal be best? And how about the wire? How thick a wire I need to handle 40 Amps on the primary? And how many turns do I need? For example what is the difference between having 10 primary turns and 70 secondary, or 100 primary turns and 700 secondary?

As I need to have 40 Amps flowing through the primary, at 5 Volts RMS, do I take it that the primary winding would need to have a resistance of 0.125 Ohm? How do I go about choosing the right wire and right number of turns to achieve 0.125 Ohm? And how on earth do I measure such a small resistance?

Any help appreciated.

[Enzo]

The transformer works on turns ratios, not resistance. You want the winding resistance to be as low as possible so as not to cause resistive heating. 40 amps will only flow through the primary when the secondary is loaded enough. Tale off the load and no current flows through the primary to speak of other than from the wire resistance.

If the load is drawing 40 amps from the primary, then the secondary at 35v would be conducting more like 5.7 amps. or put the other way, 4.5 amps at 35v would draw 31.5 amps from the primary at 5 v.

Where is this 40 amps of 5VAC coming from in the first place?

[AkisTzortzis]

OK my (naive) calcs are as follows:

The primary will be driven by the emitter of a transistor. If its resistance is zero then it will be like a short cicruit, no good. The transistor will produce 4.87 V RMS. In order to produce 187.5 Watts of power I'd need a current of 38.42 Amperes. That is on the primary. The transformer will then be 80% efficient passing only 150 Watts to the secondary. The secondary will feed a 8 Ohm 150+ Watt speaker. The frequencies we are talking about are audio, ie from 0 to 20KHz. The load (the audio speaker) will draw 150 Watts of power from the transformer. I calculate this to be 35 Volt RMS and 4.33 A RMS.

So the secondary winding will need to match, ie produce 35 Volts out of 4.87 Volts and 4.33 Amperes out of 38.42 Amperes.

[Enzo]

So there will be a steady DC current through the primary? You might want to think about that.

Transformers pass AC, not DC. Every bit of resistive voltage drop in that winding will become heat. Seriously you do not want the transformer trying to dissipate 150 watts.

In an AC circuit, the transformer reflects the load on the secondary back to the primary. The resistance of the wire only gets in the way. Sorta what you touch on though, in something like a power transformer, the winding resistance can serve to limit fault current, and to protect diode rectifiers from excess surge. But that is not what you are doing.

Why not drive the speaker directly? Unless you want to add some sort of "transformeriness" to the output, it seems unecessary.

[AkisTzortzis]

I cannot drive the speaker directly because I have a DC flowing through. A speaker is a coil wound around a magnet and it can swing backwards and forwards, its "idle" is resting in the middle. If I drive it with DC current it hit the ends of its travel and will only be able to travel in one direction from then on. So I need a transformer so that the secondary winding always have a 0 DC component.

I appreciate the transformer would need to pass 150 Watts of power keeping approx 20% for itself transformed as heat, so if I feed it 187.5 Watts I might get 150 Watts out of it.

The "idle" DC current would be approx whatever gets created by 6 V DC voltage applied on the primary.

Am I making any sense?

[Steve Conner]

Sort of. But does the machine you're trying to make make any sense? That is the real question here.

If you want to pass DC through your primary, you'll need a core with an air gap. So an E-I core or a set of C cores. Toroidal will be useless because of saturation.

Then, the design procedure is essentially the same as for a single-ended valve amp OPT, except that it steps up, not down, but that doesn't affect the math for choosing core size. So for 150 watts of audio down to 20Hz, the transformer will be BIG. You're probably looking at about 30, maybe 40lbs of iron.

You actually specified a low-frequency cutoff of "0" but I assumed you don't really want to lug an infinitely large transformer around.

As Enzo said, you got it backwards: you can't stick a voltage across the transformer and let it define idle current. If you gave it enough DC resistance to let it do that, it wouldn't meet your efficiency spec.

[AkisTzortzis]

Sort of. But does the machine you're trying to make make any sense? That is the real question here.

If you want to pass DC through your primary, you'll need a core with an air gap. So an E-I core or a set of C cores. Toroidal will be useless because of saturation.

Then, the design procedure is essentially the same as for a single-ended valve amp OPT, except that it steps up, not down, but that doesn't affect the math for choosing core size. So for 150 watts of audio down to 20Hz, the transformer will be BIG. You're probably looking at about 30, maybe 40lbs of iron.

You actually specified a low-frequency cutoff of "0" but I assumed you don't really want to lug an infinitely large transformer around.

As Enzo said, you got it backwards: you can't stick a voltage across the transformer and let it define idle current. If you gave it enough DC resistance to let it do that, it wouldn't meet your efficiency spec.

Isn't that how speakers work? They do have 8 Ohm at DC after all. Yes they are heavy, I presume I am looking at 40lb of iron as you said. How about a speaker assembled in such a way that it is trimmed towards the end of its movement range and then *expects* a DC current to "bias" it and bring the cone to the middle of the range? Do these things exist? If they did I'd simply plug the speaker straight onto the output transistor.

[Steve Conner]

Yes, that is how speakers work, but they're extremely inefficient. A PA horn tweeter might be 10% and the average hi-fi speaker is about 0.1%.

I don't know of any speaker designed for a DC bias. Makes you wonder why the push-pull amplifier got so popular. (Hint: because the DC current cancels and the output transformer core/speaker voice coil only see the signal current)

Also, why use a transistor that can only take 6 volts? There are any number of transistors designed to drive speakers directly, like our three-legged friends the MJ15024 and MJ15025. You could use a pair of these running off +/-50V and plug the speaker straight into them. I guess that's not very exciting or high-end, though.

[AkisTzortzis]

6 volts because I want it to be powered from a 12 Volt power supply (OK 13.8 Volts if the engine is running). Then, I do not want it to swing peak-to-peak; I only want to swing half of those 12 Volts, leaving the rest as "headroom" for good measure.

Oh well, I may have to do a push pull circuit in which case the transformer becomes unnecessary. I need to work it out first.

[Steve Conner]

Oh, you're building a car amp. They use a DC-DC converter to step the battery voltage up to +/-50V and then an ordinary push-pull transistor power amp running off that. No giant lumps of iron needed.

[AkisTzortzis]

Not a car amp, I just thought 12 V is a common and convenient supply voltage (and can be used in a car at the same time). I know I can use an inverter and get any voltage I want, but I am aiming for simplicity and class A. I do not want an op-amp with 10K transistors in it, I want just a few discreet components. So my output stage would be a simple class A common collector circuit. I may need to have a bunch of transistors to cope with the power dissipation and they would most likely need a bunch of transistors to drive them (at those currents) but there would be nothing that swings end to end and no class B push-pull at the end. A class A push-pull output stage is what I am looking at now, but how can you ever pair the output transistors to provide perfect symmetry. Still if we assume that the total theoretical swing could be from 1V to 11V (10 volts peak to peak) and we only use say 5 volts peak to peak, it would be linear within those ranges even though it would be consuming as much power at idle as that it would be at full power output swing.

[Steve Conner]

Have you seen Nelson Pass's Zen amp? Might be good for some inspiration.

Trust me, you don't want to use the transformer, and you don't want to try and get 150w from a single-ended Class-A amp, since their efficiency is theoretically limited to 16%.

[Bruce / Mission Amps]

Oh, you're building a car amp. They use a DC-DC converter to step the battery voltage up to +/-50V and then an ordinary push-pull transistor power amp running off that. No giant lumps of iron needed.

You probably aren't old enough but some old farts, like myself are... I can remember hearing the squeal of the DC-DC oscillator spooling up when the 12v was applied to the power supply in vacuum tube 12v radios.

[AkisTzortzis]

The other reason is safety, having 100 volts on the PCB or across the collectors of the output (push/pull) transistors (usually mounted on the same cooler) is an accident waiting to happen. I remember once I put my finger on the transistor to check how hot it was, then I put my other finger on the other one, ouch...

It would be much better to work with smaller voltage and higher currents, but then I'd need a transformer. Back to my original question :-)

So I was looking for a safe voltage and decided 12 Volts would make sense as you're more likely to find a 12 volt power supply than some other.

[Enzo]

OK, so you now have a 50 amp 12v power supply...

Let me ask you this, now that you have touched live circuits with your fingers, got shocked, and discovered they can be dangerous, are you likely to lay your fingers on one again?

There is a reason thousands of models of solid state amplifiers are made the way they are: it works well. What you are proposing is not a very efficient way to proceed.

WHile there would be about 100v between + and - supply rails in a conventional amp, in your high current scenario, failures would be hard pressed to blow fuses, and fire is a very real danger.


But if you must, look at some of the older single supply amp designs. Instead of trying to come up with some ersatz transformer, why not a large electrolytic cap from the output point to your speaker?


Let us say you set the output of your proposed system at 6VDC out of 12v. That means the signal can swing potentially 6v either way - so the output signal is 6v peak. We are talking ideal here, in real life it would fall short of that somewhat. 6v peak is about 4.2v RMS.