How a Transformer-less AC-AC converter works?

Mithun_K_Das · Sep 15, 2014

I've been looking for transformer-less AC-AC converter. How these converters works, what is the working mechanism and everything.

I found a voltage stabilizer that is working very fine, that circuit is using just a big inductor and few FETs. I still don't know what is the technology behind this.

Need help. Thank in advance.

sahu · Sep 19, 2014

Mithun_K_Das said:
I've been looking for transformer-less AC-AC converter. How these converters works, what is the working mechanism and everything.

I found a voltage stabilizer that is working very fine, that circuit is using just a big inductor and few FETs. I still don't know what is the technology behind this.

Need help. Thank in advance.

if posibol draw its sch diagram.
then any body help here.

BradtheRad · Sep 20, 2014

I suppose we're talking about a device that steps up house voltage? It automatically changes the ratio, in order to output constant 220 VAC?

With sufficient power to run appliances?

This topic is of interest when you're getting less-than-normal voltage from the electric power grid.

It's not an easy thing to do, evidently. It takes robust components to convert enough power for it to be useful.

To step up voltage suggests a transformer, or autoformer. That would be an easy way.

Another way would be a capacitor-based voltage doubler. Output is DC. Run it through an H-bridge to get AC.

What is the maximum step-up ratio you want? Does grid voltage drop as far as 110V? Then you want to have a ratio of 2x. You want that to be available from a transformer.

Then at other times, to handle grid voltages in between, you want to limit the action, make it variable. Between 1x and 2x ratio. It would be nice if we could do it smoothly, without cutting up the sinewave. Also without resistive drop, because we desire efficiency.

We can apply triac action, similar to a light dimmer. This cuts off a piece of the sinewave. Nevertheless it is comparatively easy to do. It will need a control circuit in order to get automatic voltage regulation.

Should the triac be on the primary side, or the secondary? I'm not sure.

The above is just my non-professional outlook on the question. It may not be the most practical. Someone with more knowledge will probably have a better idea.

kappa_am · Sep 20, 2014

A voltage stabilizer has to work in both boost and buck mode. a part from simple method to converter AC to AC like that explained BradtheRad and other simple methods, there are some other rather complicated methods like cycloconverter and matrix converter. but all work in buck mode. in my opinion, to add boosting ability you have to add a L-C network to the converter. something like Z-source converter. the schematic would help to recognizing the used method.

Mithun_K_Das · Oct 30, 2014

BradtheRad said:
I suppose we're talking about a device that steps up house voltage? It automatically changes the ratio, in order to output constant 220 VAC?

With sufficient power to run appliances?

This topic is of interest when you're getting less-than-normal voltage from the electric power grid.

It's not an easy thing to do, evidently. It takes robust components to convert enough power for it to be useful.

To step up voltage suggests a transformer, or autoformer. That would be an easy way.

Another way would be a capacitor-based voltage doubler. Output is DC. Run it through an H-bridge to get AC.

What is the maximum step-up ratio you want? Does grid voltage drop as far as 110V? Then you want to have a ratio of 2x. You want that to be available from a transformer.

Then at other times, to handle grid voltages in between, you want to limit the action, make it variable. Between 1x and 2x ratio. It would be nice if we could do it smoothly, without cutting up the sinewave. Also without resistive drop, because we desire efficiency.

We can apply triac action, similar to a light dimmer. This cuts off a piece of the sinewave. Nevertheless it is comparatively easy to do. It will need a control circuit in order to get automatic voltage regulation.

Should the triac be on the primary side, or the secondary? I'm not sure.

The above is just my non-professional outlook on the question. It may not be the most practical. Someone with more knowledge will probably have a better idea.

Thank bro, But I've found a Voltage Stabilizer that is working with it. Input is: 85V-300V and output is 220V +/- 1%. No matter the system is loaded or no loaded the output is fixed in between the range. I tested it many times with various loads. I've some picture of it. I found that they are using a KA3525 (PWM oscillator) and 4 IRF3710, seems like Its converting the voltage into HVDC but also the system have 2 batteries of 12V/7.5Ah in series that means it is converting 24V DC into 300V DC. Then This DC is being converted into AC.

But, interestingly it is true that the system works fine without any battery line. Even it don't need the battery at start up.

The AC is directly connected with a FET, next to that IRF3710 and a Coil is connected next. Then another coil and FET. Finally a cap filter.

Finally the output is taken. Also, the whole system have a micro controller based control unit (blue, top) connected.

See all the images. The output is pure sine wave. Even if I connected this unit with square wave inverter, output is still pure sine wave. That means, it have a sine wave inverter mechanism built-in I hope.

So what do you think about the technology behind this?

- - - Updated - - -

kappa_am said:
A voltage stabilizer has to work in both boost and buck mode. a part from simple method to converter AC to AC like that explained BradtheRad and other simple methods, there are some other rather complicated methods like cycloconverter and matrix converter. but all work in buck mode. in my opinion, to add boosting ability you have to add a L-C network to the converter. something like Z-source converter. the schematic would help to recognizing the used method.

Thanks Man, I think you are right. But to build another one, we need to read about the technology a lot first.

schmitt trigger · Oct 31, 2014

This is very simple.
It is called a online/ double conversion UPS

Wikipedia has a full description, please check it out, it is about 2/3 down the article.

https://en.wikipedia.org/wiki/Uninterruptible_power_supply

BTW; I like your signature...very wise words of advice.

BradtheRad · Nov 1, 2014

Mithun_K_Das said:
Thank bro, But I've found a Voltage Stabilizer that is working with it. Input is: 85V-300V and output is 220V +/- 1%.
...
So what do you think about the technology behind this?

That unit has a lot required of it. It is a major accomplishment. I'm sure many people would love to have one of those.

I see it contains inductors (transformers?).

Since this thread is about transformer-less AC-AC converters, here is my step-up AC converter, based on capacitors.

It does not contain inductors. The energy is stored in capacitors.

The transistors must be precisely biased, if you want the output waveform to match the input waveform.

This topology is just a concept.
It is liable to overheat the transistors.
It draws large current pulses from the grid.
It needs a way to regulate the output voltage.

It may be impractical in the form you see above. However it could be worked up into something that will do the job better.

How a Transformer-less AC-AC converter works?

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