Re: How to convert the AC to AC from high to low without using transformers! Sure you *Could* do it.  But it would be comparatively costly and complex.  The way to get from AC to AC without a transformer is to first convert the AC into DC using a bridge rectifier, then smooth it into a DC Bus using electrolytic capacitors.  Then you would have a high voltage DC bus (in your case 420-925VDC), then this would be used to feed a full H-Bridge inverter circuit which would synthesize a sine wave using a microcontroller (or PWM implemented in hardware) of the appropriate amplitude which would then get you back to AC true sine wave.  Depending on the voltage drop, you will probably need inductors to reduce switching losses and for filtering purposes, but this would be a transformer-less solution.  The advantage is you could synthesize any output voltage or frequency sine wave you wanted up to the DC bus voltage.  However, it would be much less expensive, more reliable, and less complex to just use a transformer.

Re: How to convert the AC to AC from high to low without using transformers! Depending on the current and load requirements, you could either use a simple dropper resistor or may a powerless series capacitive dropper. What is the problem with a transformer.?How is your 300-660 volts arranged, as most 400 v systems are between phases, with it still being 230v to neutral. Unless of course its a delta Yes true delta doesn't have a neutralbut each phase is still only 230v above the neutral (well here in NZ).It's only 400v between the phases ....So unless our OP is trying to get 120-277 volts 3 phase, he probably already has a lower voltage. Yes, his initial email stated that he was trying to convert 300-660V to 120-277, which to me said that most likely OP is trying to run single phase equipment from a 3 phase source.  If the source is a WYE, you are correct in stating that you can go from phase to neutral, and in the case of 480V 3phase input, you would get 277V single phase. 

If his source is a delta, then phase to phase is only option, and I think everyone here agrees, a transformer is the best solution (most other solutions would be equally as large, costly, and in most cases more complex).  ie a rotary phase converter will do the job too (and it isnt a transformer) As Mark pointed out using a Transformer you still get a sine wave.Electronically you can go from AC-DC using a bridge rectifier and smoothing then using some BIG AR$E transistors such as IGBTs under microprocessor control you can create AC or what ever arbitrary waveform you like at the load end.air handling unit fundamentalsThis is how many large AC motor speed controllers work and is well known, considering the energies involved are you  actually going to implement this?where is the reset button on my central ac unit

Henri, What is your target application ? Thank you all for your replies and suggestions.  Actually using or not using transformer is not a first priority. The major challenge is design of a smart circuit to take care of low and high voltage at the same time. Meaning that from 90 volt to 660 volt AC should be passed through the board. @ John ==> It is an electricity meter.We have an AC/DC converter of 12 volt output and 90-305 volt AC input .... using a AC/AC converter for the input VS remove the AC/DC and build a new AC/DC with a wider voltage range is aimed in this project.@ qm ==> as I mentioned before, now using or not using it is not a major concern. indoor garden ac unitsI need to use a component like a comparator to differentiate between 90 and 660 volt maybe! For an electricity meter (space constrained, low current application), the best solution would be to redesign the AC/DC converter section for the wider input range. 

Typically the lowest cost topology is a non isolated buck converter for powering these.  /lit/an/slua721/slua721.pdfThere are plenty of reference designs designed for 3phase that extend to 660vac. As a matter of the fact, I bumped to the following article that explains using Buck or step down converters without using transformers.http://dspace.bracu.ac.bd/bitstream/handle/10361/1840/Design%20of%20a%20single%20phases%20AC-AC.pdf?sequence=1As you said, I need to see how I can use an AC/DC with a wide voltage input. As for the TI 's AC/DC, it seems the VDC is a way far from what we need (12volt). But it might give me some clue! No actually it is said that the maximum output is 10 vDC. And max of 500 vAC. um... this is close but not the exact what I need! Also the frequency is not 60 HZ A Buck converter is a DC-DC converter.  The AC input gets rectified into DC at the beginning ( in the example I referenced, to handle high voltage inputs only 1/4 wave rectification is used, this keeps power component costs lower as ultimate DC voltage will be lower than if it was rectified full wave), the first inductor in the example circuit I referenced blocks the rest of the AC component. 

So AC line frequency is not terribly relevant except for the inductor design.  Since your final application (electric meter) really only needs DC input, there is no advantage to AC-AC conversion.  The first step is to get the input AC voltage to high voltage DC, and use a buck converter (DC-DC) to step down the voltage.  Output voltage and current capability will depend on buck converter design.  As long as isolation is not needed, this is the simplest/lowest cost solution to get high voltage AC to low voltage DC.  I did not design the circuit for you, I merely provided an example of a reference design that you could modify to suit your needs. As the following picture depicts, I might change the voltage to 660 volt and can get 12 vdc. It is worth it to investigate on such design. (June 2014 --- very fresh!) Elements of a Power Supply Why Use Alternating Current? When dealing with electronic circuits, we have to meet the basic requirement of providing electrical power for them to work.

power, your circuit is no more useful or meaningful than a single raindrop The basic purpose of a power supply is to provide one or more fixed voltages to the working circuit, with sufficient current-handling capacity to maintain the operating conditions of the circuit. doesn't have to be fancy; the typical hand-held transistor radio uses a 9-volt battery as its power source. A flashlight uses cells that are physically much larger, but provide a lower voltage. appliances such as television sets, VCRs, and microwave ovens have electronic circuits built in that take power from a wall socket and convert it to the form and voltages required by the other internal circuits of the appliance. Although each power supply has its own individual specifications and characteristics, all power supplies have certain characteristics inWe'll look at the main parts of a power supply on this page and see how they work together. Then, on subsequent pages, we'll take a more

detailed look at each of the parts we haven't seen before, and explore the major variations that are commonly used in modern power supplies. A basic power supply consists of three main sections, as shown in the block diagram below and to the right. Depending on the requirements for a given power supply, the sections can be very simple or extremely complex, or even left out altogether in certain circumstances. Each of the sections serves one or more specific purposes, as follows: Each of the three sections identified above can have a number of variations — even the transformer, which we covered in an earlier Regardless of these variations, each section performs its specific task. However, some circuits do the job more effectively than others, or pick different trade-offs between possible alternatives. effectiveness of each circuit, we compare the magnitude of the remaining ac component, or ripple, with the dc component of the total voltage