High current DC with AC on top

# Thread: High current DC with AC on top – 1669 days old

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## High current DC with AC on top

I need a way of "adding" an AC sinusoidal waveform onto a DC voltage at high current. I am sure that I could arrange a transformer's windings such that the DC component cancels out and doesn't saturate the core but I can't seen to be able to do it. I am thinking of a high current DC supply in series with a transformer secondary. The transformer primary is driven with a power amplifier.

The figures are (around) 4V AC, DC is 30V, overall current is 150 amps. So the whole shebang produces a 30V output with a 4 volt sinewave superimposed on it. Oh, the frequency is between 400 Hz and 1200Hz.

The problem, pretty obviously, is that if you simply put a transformer in series with the DC, then the 150 amps will saturate the transformer core and it won't be a transformer any longer. I seem to remember a way of using opposing windings to cancel the DC but I just can't do it (old age probably).

A tough one, which is why I am asking you guys for some help/advice.

TIA
Charlie

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## Re: High current DC with AC on top

If you don't know how to start, me either in this case, make it 'simple' with an 12Volt battery about 5Ah and a Transformer 6.3 Volt about 1Amp, and a Fuse!
Decide where you put the GND/0volt is, and start the test.

Upon result, then go upward in current (and frequency)!

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## Re: High current DC with AC on top

I suppose it's a starting point. I think the transmitter brigade use opposing windings to keep DC out of modulators (John Drew?)

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## Re: High current DC with AC on top

Hi Charlie,
I have no idea how to do it but it sounds very similar to how powerline communication works (although that is on AC) or the old intercoms you used to get (if you are knocking on a bit you might remember them).
I did find something here http://hackaday.com/2014/04/05/open-...communication/ which might help.

Regards,
Bob

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## Re: High current DC with AC on top

Hi Bob

Yes I do remember them and yes, I am knocking on quite a bit. I think the "Ethernet over the mains" units work in a similar way today.

I have almost convinced myself that it can't be done, by the following reasoning:

The transformer must carry the 150 amps of DC. The transformer must add and subtract a sinusoidal current to the standing DC flowing through a winding. Therefore the transformer MUST be subject to a DC flux which is large, and will saturate the core (unless the transformer is very large with an air gap in the core). If a clever flux-cancelling scheme is used then it will cancel the AC modulation as well.

I found a way of capacitively coupling the transformer but then I needed an inductor which would also have to pass 150A, same problem!

Charlie

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## Re: High current DC with AC on top

In RF the coils are often air wound or for low power applications have low current and a ferrite core. Generally though a capacitor is used to keep the DC away.
John

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## Re: High current DC with AC on top

I think a capacitor would do it. Try searching for "diode as a switch". I vaguely remember something to that effect from technical school.

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## Re: High current DC with AC on top

Hi Charlie,
I'm not sure if your 30V DC has to be regulated or what your load looks like but is it better to think in terms of a modulated power supply? Most supplies are only capable of sourcing current but occasionally you come across some that can sink and source - basically a DC power amplifier. At your load current I suspect it would need some hefty heatsinks and the idea of PWM comes to mind but integrating the PWM carrier is a problem unless the load is inductive and can average it out.
You do have some interesting problems.

Cheers,
David

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## Re: High current DC with AC on top

John: I have seen windings in antiphase, like the push-pull of an audio amp and I think that's where I was heading, but if you cancel the effect of the flux then the changes in flux don't have any effect either.

Flosi: Yes, I had a design that isolated the transformer using a big capacitor, then then I had to add an inductor and the same problem appeared - saturation.

David: Yes, the 30V is regulated and yes, a modulated psu would be fine. The manufacturer of the 30V 150A unit does a fast response version with ceramic output caps instead of electrolytics. I will ask them if it is possible to inject a 1Khz sine wave as an error signal to add the small AC waveform. This would be the best solution. But at 4.5 KW it's not trivial to play around with. I like the idea of PWM and will look further into it, thanks for that.

Yes, I do seem to get some interesting things to do. Needless to say, this has nothing to do with Bletchley Park!

Many thanks to all.
Charlie

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## Re: High current DC with AC on top

Hi Charlie, I just googled "how to inject an AC signal into DC" and got this http://masteringelectronicsdesign.co...-power-supply/

Bob

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## Re: High current DC with AC on top

Thanks for that Bob. That is exactly the same circuit as my second attempt but it's the large current (150A) that causes the saturation trouble. The current in the circuit you show is low enough to be OK. I have asked the psu manufacturer if the unit is fast enough to be modulated at 1Khz. I'll report back if I get anywhere.

Thanks again.
Charlie

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