Transformers

[poxit]

I want to convert 12VDC to ~120AC (<10mA) to drive a small SmartTint film. Can anyone point me circuits that can be used for this? It's gonna be MCU controlled and needs to be as small as possible.

[uncle_bob]

I want to convert 12VDC to ~120AC (<10mA) to drive a small SmartTint film. Can anyone point me circuits that can be used for this? It's gonna be MCU controlled and needs to be as small as possible.

Hi

Best bet is to just buy one of the multitude of off the shelf converters. They are about as small as what you would practically build. The other alternative is to get the smallest AC transformer you can find with a reasonable center tapped secondary. Hook the center tap to +12VDC and alternately pull each side of the secondary to ground with a MOSFET. Drive the FET off of your MCU. If I remember correctly, you want something like a 22V center tapped secondary and the FET's need to be rated pretty high.

Bob

[poxit]

Thank you Bob, I didn't know about center tap technique.
Now I'll be looking for the smallest transformer I can find on digikey.

[uncle_bob]

Thank you Bob, I didn't know about center tap technique.
Now I'll be looking for the smallest transformer I can find on digikey.

Hi

There are some nasty issues in driving the two FET's. If they both are on at the same time, you short out the transformer. If they both are off at the same time, you get an inductive spike on the secondary. How big a spike you get depends a bit on the load on the primary. Making things even more confusing, you are using the transformer backwards. Most stuff you see about this approach will call the low voltage winding the primary

So: When you fire it up and test it -- put a resistor on the high voltage winding to simulate a load. Yes that's a bit strange. Most circuits get tested with no load and only are loaded once they are known to work ... With a 10 ma 120V load and thus a 1.2W power level, load it ...

Bob

[dvdouden]

afrotechmods recently did a video on this subject and provided some useful warnings  :

[uncle_bob]

afrotechmods recently did a video on this subject and provided some useful warnings  :

Hi

One thing he didn't mention:

The same spike stuff he shows on the output also shows up on the input. That's why early versions of this circuit that used germanium transistors didn't live very long if the load dropped off. It's also why you want some pretty significant voltage ratings on the drivers. It turns out that it's not just voltage. There is also a current spike. A TO-220 device is a good idea ....

Bob

[poxit]

Thanks for the replies bob and dvdouden.. that's the stuff I was looking for.

Will using an H-bridge transformer driver (lMAX13256) help with the spike stuff?

[uncle_bob]

Thanks for the replies bob and dvdouden.. that's the stuff I was looking for.

Will using an H-bridge transformer driver (lMAX13256) help with the spike stuff?

Hi

Not really. You still have the same issue with non-overlapping signals. Essentially it's just something you design to accept or take out with a massive filter.

Bob