12VDC to 110VAC inverter needed

[Paul Zuniga]

I need help to resurrect an inverter that I bought from an e-bay seller. Turns out someone had chopped out the actual inverter circuit board and a small choke coil. I have the actual transformer lump in the case, with the 110V duplex receptacle and 12VDC input cables. It also has a small cooling fan and what appears to be a 12VDC input conditioning circuit board.

I wonder where I might be able to buy a compact inverter circuit board? From my understanding, it just needs to be a couple of transistors and capacitors.

Here's a photo of what I have and what I should have gotten.

 

[KrisBlueNZ]

So you bought an inverter via eBay and you got a unit without its main circuit board? As you can see, the circuitry required is a lot more than "a couple of transistors and capacitors". Why haven't you opened a dispute with eBay?

 

[Paul Zuniga]

The guy gave me a full refund and said I could dispose of the remains.

I figured I'd have a go at making it work on the cheap.

 

[Paul Zuniga]

I found this...

Made it look simple.

 

[duke37]

I do not like the look of that circuit.
The emitter/base junctions will be overvoltaged at 12V input.
The frequency will not be stable, you would be better off driving the output devices from a separate oscillator.
FETs are preferred these days because of the lower loss.

This circuit will give a square wave output, is this what you want?

 

[KrisBlueNZ]

Ah, I see.

I think the circuit you posted in post #4 is not a good design. It's probably only suitable for fairly low power - less than 50W. Its frequency will not be accurate, and it will produce a square wave output, rather than the "modified sine wave" produced by a better drive circuit. Also it requires a transformer with a centre-tapped primary, and I think the one you have isn't centre-tapped.

I think you need a drive circuit that uses an "H-bridge" to drive the primary of the transformer with a "modified sine wave" signal at a fairly high current. This requires four pretty heavy duty MOSFETs (e.g. http://www.digikey.com/product-detail/en/PSMN4R3-30PL,127/568-4897-5-ND/2122715 or http://www.digikey.com/product-detail/en/IRLB8748PBF/IRLB8748PBF-ND/2127672 or http://www.digikey.com/product-detail/en/IRLB8314PBF/IRLB8314PBF-ND/4807868) driven by an H-bridge driver IC (http://www.digikey.com/product-detail/en/ISL83202IPZ/ISL83202IPZ-ND/1650684 or http://www.digikey.com/product-detail/en/HIP4082IPZ/HIP4082IPZ-ND/821447) with a crystal-controlled oscillator and some logic to produce the modified sine wave control signals.

 

[Paul Zuniga]

How does this look?

http://www.ebay.com/itm/MXA059-DC-t...826?pt=LH_DefaultDomain_0&hash=item33724012c2

 

[KrisBlueNZ]

How does this look?

http://www.ebay.com/itm/MXA059-DC-t...826?pt=LH_DefaultDomain_0&hash=item33724012c2

That one needs a transformer with a centre-tapped primary and has a square-wave output instead of modified sinewave.

 

[Paul Zuniga]

I think the wire-wound core in my first photo does indeed have the centre-tap.

Square waveform is okay for everyday use, no? Like running a brick type laptop charger or cordless drill charger, etc. that will only convert the AC back to stored DC...

 

[KrisBlueNZ]

No, the transformer doesn't have a centre-tapped primary. You can see there are only two wires.





Also there are four MOSFETs which probably implies a full H-bridge drive.

There's also a feedback winding - the red and black wires, which disappear into the control board. The drive circuitry is more complicated than I first thought. I suggest you use the transformer as a paperweight. They're nice and heavy and that's the safest thing you can do with it!

 

[Paul Zuniga]

But...but... I wanted to maybe blow stuff up!

 

[Paul Zuniga]

What does that other tap at the upper left do?

 

[KrisBlueNZ]

But...but... I wanted to maybe blow stuff up!

LOL well, you could feed mains in the secondary (black wires) and connect some thick wires to the primary (marked with arrows). That'll give you a nice safe low voltage with lots of current, which is good for making big sparks and carving pits into metal, and blowing up certain types of components... You might blow your switchboard fuse though...

Actually it might be a bad idea to feed mains into the secondary of the transformer. It's not designed to be used that way round. Someone else here may know if it's safe to do that or not. If you try it without waiting for an answer on that, check the transformer for overheating.

What does that other tap at the upper left do?

Which tap? The blue wires? I don't know what they're for. But if the primary had a centre tap it would be another huge fat wire like the arrowed wires.

 

[Paul Zuniga]

This one...

 

[KrisBlueNZ]

Ah, I see it on the photo of how it's supposed to look. Yes, it looks like a centre tap. It goes to the area next to the fan with the big high-current noise suppression choke wound on a blue ring core, which is missing from your unit. It looks like the wires have been cut off pretty close to the transformer too. I can only see the helical plastic guide.

That ring core is probably powdered iron and it's needed to reduce noise back-fed into the 12V supply. Not widely stocked. You could try it without, if you don't care how much noise you inject into your power source.

 

[Paul Zuniga]

I'm going to scrounge around and see IF I still have a couple of UPSs, maybe pull out the inverter portion and try that...

 

[Paul Zuniga]

Dang, as usual, threw out non-working stuff a few months ago when I did a shop cleanup.

HAPPENS EVERY TIME!