Hi all I have an idea for a variable psu 0-300 volts that involves stepping down 240VAC to 12 volts then rectified and regulated with lm317 for variable voltage on the low side and then to inverter stage back to ac and stepped up to higher voltage, rectified and finally connected to load. My issue is it will be used to power audio circuits and must have a smooth accurate voltage control. So I would really appreciate any ideas for an inverter circuit that will operate with output voltage corresponding and consistently proportional to input voltage, and with as close to pure sine output as possible. Undoubtedly there are better designs out there but I would like to stay firmly with the original idea as an experiment and also a working bench supply. This psu design will be built to comfortably handle 100MA load current as it will be used for powering small instrument amplifier test circuits of say, 6n2p dual triode and single ended or pp output stage using u/el84(s). Any help ideas/links will be greatly appreciated. Thank you for taking the time to read my post.

 

with as close to pure sine output as possible

Why, if you are going to rectify it anyway? Pure sine at ~30W is going to be more difficult than other solutions.

 

If you are going through all of that to power a high-performance audio circuit, I doubt that you want an inverter based on a switching topology. That leaves analog <gasp>, basically a sinewave oscillator driving a power amplifier driving a reverse-connected 12V power transformer. Not hard to do, but does it really get you anything. What are your noise and distortion limits/budgets/requirements?

IOW, why?

ak

 

The only reason for EVER building an inverter is that you have no access to an AC source. Doing so, when starting with an AC source, is just about the most foolish proposal I can image outside of overunity schemes.

 

You probably should have started out with why you want to do it this way. Though the word 'experiment' was used.

 

why do you need a pure sine wave to power audio circuits?

 

if you need a pure sine wave, why not feed a sine wave into an audio amp with a stepup trensformer in the output?

 

down 240VAC to 12 volts then rectified and regulated with lm317 for variable voltage on the low side and then to inverter stage back to ac and stepped up to higher voltage

The flaw in this arrangement is this.

OK so you step down your voltage to 12 volts, rectify and use a regulator to generate a 'smooth' DC output.

Then you say you want to input this to an inverter?

What is going to power the inverter if you want a higher voltage out than you put in?

 

What is going to power the inverter if you want a higher voltage out than you put in?

Nothing say’s you HAVE to use the same transformer ratio for the output.

 

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I find that a lot of people don’t realize that a VARIAC is not isolated and that the current rating is only at the 100% level.

 

I find that a lot of people don’t realize that a VARIAC is not isolated and that the current rating is only at the 100% level.

I find the first hard to believe.

And the second is not true.

 

And the second is not true.

A 10 amp VARIAC won't supply 10 amps at 25%. Try it with yours, be sure to take pictures.

 

It absolutely will.

Variacs are rated for current and max VA.

Meaning they will not deliver rated current at MAX voltages.

Here is a typical printed spec.

"

• Max VA: 300VA, 3 Amp. Max (surge), 1 year warranty
• A rating of 300VA means that at 100volt or lower, the unit can sustain at maximum 3amp. If at 120 volt the unit can sustain at 2.5amp"

 

Nothing say’s you HAVE to use the same transformer ratio for the output.

I'll grant you that I did not explain it very well, but that is not what I meant.

Yes the invertor may include a step up (or even a step down) transformer to any desired ratio. That is how electronic ignition systems develop the ignition kilovoltage from the 12v battery.

The switching section in an invertor is designed to run from a particular direct voltage.

It will not run well from any other voltage, and may not even run at all, due to biasing requirements.
So a 12 volt invertor will not run from 6 volts.

However the OP proposes to supply (power) the invertor with a variable direct voltage and expects the output to vary in sympathy.

I find that a lot of people don’t realize that a VARIAC is not isolated and that the current rating is only at the 100% level.

Yes the variac is an autotransformer so in that sense there is not the normal isolation achieved with a transformer having primary and secondary windings.

However neither this statement nor that of inwo are quite correct.

The variac has a VA rating and a maximum current rating.

The load must be such as to satisfy these simultaneously.

So for a 3A variac you must not draw more than 3A at any output voltage setting.

Most are rated so that the max current can be drawn at full output voltage so for a 3A 240 volt variac this means 720VA, although they would usually be rated for 750VA.

Some cheaper ones, especially in the past, might have been rated at 3A 500VA so in that case you could not draw max current at full output voltage.

Here are the safety instructions for some current models

 

whhy waste the circuitry going from ac to dc to ac to dc? if you run your ac input voltage into a switcher with a variable output to begin with, you have the same thing without going back and forth as much. and if you want to start with dc, why not run that into the same inverter, the rectifier will pass it and it will run on dc.

 

whhy waste the circuitry going from ac to dc to ac to dc? if you run your ac input voltage into a switcher with a variable output to begin with, you have the same thing without going back and forth as much. and if you want to start with dc, why not run that into the same inverter, the rectifier will pass it and it will run on dc.

I agree that the proposed block diagram is vastly overcomplicated.

But that is what the OP wants to do.

Running 240 volts AC into a switcher will not generate a higher voltage, it will only generate a lower voltage than the peak at DC.

In fact the ac voltage cannot be run directly into a switcher without rectification first since otherwise every other half cycle would discharge the reservoir capacitor. I suppose this rectification could include a voltage multiplier scheme.

 

and stepping the 250 vac down to around 12 volts controlled by an lm317 will not give much power to supply the sinewave inverter that steps the voltage up and rectifies it back to dc. then you would need a variable output sinewave inverter with varied input around 12 vdc input to whatever your output is.
if you want a variable dc supply, why not use a triac dimmer into an isolation transformer, then a bridge rectifier and filter? can be varied to above 300 volts and down to 0.

 

why not use a triac dimmer into an isolation transformer

You certainly won’t get the full voltage out of a transformer by driving it with a TRIAC because of the phase shift between current and voltage caused by the inductance. The TRIAC will actually turn off early when at 100%.

 

maybe yo cant get full voltage out, but thats the way that really large supplies are made for plating supplies, around here, up to 30KA at 0 to 15 volts and use scr's or triacs to regulate the output. much easier to regulate the input supply than pass the out put current through the regulators. just feed back from the output to control the phase angles.