Hi,

For a secret project, I am trying to convert 5 V DC from USB to a 250 V AC. I am fairly new to electronics, but I have considered op-amps, and transformers. Transformers would require me to switch DC to AC and then step-up. This seems like the easiest option out there, but I would like to know if there are any other methods to do so.

Thanks

 

And how much current do you need at 250Vac? Since USB is limited to ~2.5W, even if a step-converter was 100% efficient, the max current you could expect would be I=P/E = 2.5/250 = 0.01A or 10mA.

 

And how much current do you need at 250Vac? Since USB is limited to ~2.5W, even if a step-converter was 100% efficient, the max current you could expect would be I=P/E = 2.5/250 = 0.01A or 10mA.

I will need around 30 mA of current. I am sure I will be able to find a different power source seeing that USB won't provide the current required.

 

In my books it says most USB ports are good for 1 amp so you have about 5 watts to work with.

Given that for higher loads paralleling ports together is a second option if the device has two or more ports to work with.

 

If your power supply identifies itself as a telephone charger (only requires a resistor as I recall), you can legitimately get more current that 500 ma. Some hubs only monitor aggregate power to all connectors, so you have a good chance of getting more than 500 ma.

For the last several months I have been working on an project that involves generating 240 VAC/50 Hz using audio amplifier chips. This circuit does not require power but it needs the high voltage sine waves.

A couple of tips about the transformer:

Magnetizing current is going to a factor. The first transformer I tried drew about three amps P-P of magnetizing current. Those amps have to come through the USB, then through your power supply if you have on on the board, then through the audio amplifier chip(s). The amplifier chips(s) need to handle those peak currents.

In my case, I was driving a five volt winding to get 240 VAC on a transformer being driven "backwards". The amplifier had to supply 2.82 x 5V x [something to make up for the losses in the tranformer]. In this case it took 20 volts P-P out of the amplifier. For that reason, you would probably need to have a boost converter to power the amplifier from 5 volt USB, and of course efficiency will be a concern.

The degree of coupling is important; I tried some pcb mount isolation transformers because I suspected they would have low magnetizing current; I was right about that, but I and had not checked the regulation specification the small amount of magnetizing current combined with high resistances and very large leakage inductances meant that they could not put out more than 180 volts unloaded without severe harmonic distortion appearing.

Manufacturers don't talk much about distortion in the datasheets for power transformers but I eventually found one that works nicely.

By the way through all this, the load on the transformer's output is only a few hundred microamps average.

If waveform fidelity is not important, you might do better with one of those multivibrator arrangements similar to this this one:

It is not high fidelity, but it will probably be more efficient than using an audio amplifier because the transistor will saturate. Here's the Google Search http://bit.ly/1Ida8e1