power inverter construction

Just curious because I don't have one around to rip apart, when you buy one of those 75W power inverters that look like a salt shaker and plug into your car cigarette lighter, how are they getting the 120V inside? Is it a 1:10 transformer stepping up some low voltage interpolated sinusoid or is it a DC-DC converter circuit? Or... is it something else?

12V DC -> some astable multivibrator like thingy -> 12V AC -> transformer -> 120V AC (hardly ever even close to a sine waveform)

lg, couka

Gahhhrrrlic:
Just curious because I don't have one around to rip apart, when you buy one of those 75W power inverters that look like a salt shaker and plug into your car cigarette lighter, how are they getting the 120V inside? Is it a 1:10 transformer stepping up some low voltage interpolated sinusoid or is it a DC-DC converter circuit? Or... is it something else?

They are generally very complex circuits.
Purchased devices of ebay are impossible to compete with on economic grounds.

…but despite circuit complexity, a transformer is definitely involved? I would assume this to be the better choice because boost converters are not as efficient. Am I correct in that this is the industry standard? Those inverters never seemed very heavy to me. Don’t the transformers weight a lot?

They all have ferrite transformers being driven by a push pull power oscillator with at least 2 power mosfets running at around 33 Khz, stepping the 12 V up to whatever is needed for the output voltage, a rectifier stage ,and then a H bridge output stage producing the AC.
The cheapo versions simply produce a modified square wave output , whilst the better
versions synthesise an AC sinewave output.
Either way, its not cost effective to make them.

Gahhhrrrlic:
...but despite circuit complexity, a transformer is definitely involved? I would assume this to be the better choice because boost converters are not as efficient. Am I correct in that this is the industry standard? Those inverters never seemed very heavy to me. Don't the transformers weight a lot?

They use high frequency switching, so the transformer can be really small, since for the same rate of flux change a shorter period needs less total flux. Typically a 1000 times mains frequencies allows a transformer 100 times
lighter (the extra factor of ten due to the low maximum flux density in ferrite compared to silicon steel laminations). In practice the heat dissipation in the ferrite means you don't really get that full factor
of a hundred, high frequencies mean these "iron losses" are a big thing and ferrite magnetic properties
are quite temperature sensitive.

You wouldn't happen to know where I can buy small transformer modules that are typical for this application would you? To my surprise, simply using keywords in ebay and google haven't turned up much of anything. I keep getting plug-in transformers or big huge industrial transformers in the results.

Normally you have to design and wind your own.
Ferrite cores can be had from some vendors.

Really? Transformers aren't available off the shelf? Seems like opportunity is calling for a vendor to fill that need. I suppose I can scavenge 1 from a plug in transformer for some random piece of electronics. Surplus stores sometimes carry those, but then you're stuck with whatever's inside.

Transformers aren't available off the shelf?

Of course they are. Example.

I think the op is looking for ferrite for a switchmode supply rather than audio or mains types.

Gahhhrrrlic:
small transformer modules that are typical for this application would

Since the need is being filled by cheap ready made units, there is no appetite to sell the cores separately.

“You wouldn’t happen to know where I can buy small transformer modules that are typical for this application would you?”
Buy a ready made unit, then salvage the parts.

Put your efforts into an area that isn’t already been saturated.

.

Ferrite isn't hard to search for :wink:

http://www.ebay.ca/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR12.TRC2.A0.H0.Xferrite.TRS0&_nkw=ferrite&_sacat=0

.

The link by j remington actually lists a ferrite kit.

Google on RM7 core.
Used to be a very popular size.
There are different sizes and also different permeabilities.

Also gapped and ungapped.

The Epcos site should list the technical specs for most of them.

EDIT

Ferroxcube is another seqarch term.

You may find this useful

http://info.ee.surrey.ac.uk/Workshop/advice/coils/

Nice! I would have never found these by myself.

Not planning to re-invent the wheel btw. That would indeed be a waste of time. However there's plenty of money in "value added" products.