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itsmrdavid 2nd September 2005 07:20 AM

Interesting new (mis)utilization of class D devices
Ok, i've posted this idea a few places, and gotten a few verbal
slaps from snobby engineers for having birthed it... but i still think
it's workable.

Here it is:

A moderate power modular Class D device that can handle
plus and minus 12v (24v rail to rail).

A full H bridge of fets - probably 25 per device, 100 total, of IRF840.
Might need some current drivers for that many fets, I dunno; but
I figure 20khz is fine for switching frequency. Might be ok at the
lower frequency.

A series of 26 x 12v car batteries, center grounded (13 per side),
to power the circuit. Take the plus and minus 12v for the class D
driver chip from the two center batteries.

An isolated and regulated supply section and crystal controlled
oscillator to make a 60hz sine wave.

Negative feedback to lock the voltage into a range similar to
115/230v power lines.

And Voila! - a heavy duty AC power supply for your home!!!
Theoretically it'd be enough to run things like an oven, electric range, water heater, etc...

Then all you need is a metric-assload of cash to buy solar panels.

Any comments?

Jocko Homo 2nd September 2005 09:08 AM I missing something...............?

Shouldn't that include some way of going from +/- 12 VDC to 120/240 VAC? Like maybe a big hunk of iron and copper..........normally called a transformer. Maybe?

So......this thing is going to generate.........let's say 7 kW or so to power a typical electric stove, or hot water heater? Maybe 400 A @ 12 VDC? Something like that?

Ok.................why? Forget the workable part.......I want to know the why part.

Maybe then we will verbally slap you. Especailly if you have a sense of humour.


Kenshin 2nd September 2005 09:26 AM

It's just a so-called "high frequency inverter" (translated from Chinese nickname), many companies are manufacturing these things.

Usually a 12V battery + 40KHz inverter + pulse transformer + rectifier to produce more than 300V DC (instead of 26 batteries -- managing many batteries are nasty work) , then use a class-D bridge amplifier running at 20KHz to get the final 220V 50Hz output.

No heavy 50Hz iron core transformer needed, this is able to output ~200 watts with 2x or 4x IRF3205 + small size ferrite transformer + 4x IRF840. This power is enough for a TV set.

Jocko Homo 2nd September 2005 03:27 PM

Powering a TV set is a long way from powering an electric stove, which is what he proposes.

Yes, doing it your way obvously works. He just left out all of those details, which is probably why he was verbally slapped.


scott wurcer 2nd September 2005 04:29 PM

This is no problem, 8-T105 batteries designed for this and a couple of Xantrex inverters designed to be coupled out of phase for 220V. A good bit of 4/0 cable too. You could probably make dinner and bake a pie before you need a recharge. That's why solar homes cook with gas (or the barbie).

scott wurcer 2nd September 2005 04:39 PM

On a similar point I have looked at using our class D amps to make a smart battery charger.

Kenshin 6th September 2005 03:44 AM

Use a reverse running class-D amp to boost 12V car battery to 35V for car audio use; or use a synchonized one running off a typical toroid transformer as a rectifier without input current harmonics.

Another benefit: the energy send back from the subwoofer to the PSU capacitor could be send back to battery/mains. This eliminate the danger of pumping a capacitor and blow it.

runebivrin 6th September 2005 05:40 AM

Ahem, unless I'm mistaken he's suggesting connecting the 26 12 volt batteries in series to get enough voltage to generate the 50/60 Hz directly, without a transformer.


mzzj 6th September 2005 09:32 AM

basically you are re-inventing the wheel :)

UPS systems in >>kW range use system like you described, for example I have seen couple of largish UPS systems running on around 40pcs of 39Ah or even 75Ah batteries series connected for 400V DC rail. Instead of 25pcs parallei connected tiny mosfets these monsters usually use big IGBT's for switching.

Now, who needs more than 30kW in kitchen? :D

These are just a crappy small ones for most critical operations like emergency cooling pumps, compared to bigger motors in same steel factory like 14MW turbocompressor motors :)

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