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Sulaiman

Mon Nov 17 2014, 06:22PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Could you build something like this

for less.

Given the real and legal risks, cost, time and effort, is it worth DIY ?

Dr. Slack

Mon Nov 17 2014, 07:43PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

If you are going to DIY something, then pushing power into the grid sounds like exactly the inverse problem to that which a PFC controller solves, pulling power out of the grid in a close enough approximation to a sinewave, with current waveform synced to the voltage. Could you hack one of those, perhaps inverting a reference somewhere?

Don't forget that a gird is, or at least should be, a short circuit. To feed it you need a constant current source.

furnace

Tue Nov 18 2014, 04:17AM

Registered Member #4992 Joined: Wed May 23 2012, 03:57PM
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Posts: 108

Ash small wrote....

Obviously you still need a sufficiently large amplitude to be able to measure the crossing points, etc, but as Shrad pointed out, if you amplify the output from the step down transformer, you get a signal that corresponds to the mains frequency in your area.

Can you be a little more specific as to how this would go about??

Shrad

Tue Nov 18 2014, 08:13AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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an inverter, for outputting a sinus wave, will need to switch the DC bus at a duty cycle proportional to the 50/60 hertz signal so that the output, when filtered out by the parasitic inductance and filter of your system, will be a sinus wave

this is a switching system which behaves like a class D amplifier designed to work at mains frequency

if you want to design an inverter yourself the best thing to do would be to design a small class D amp and make it work with an input signal which will be the output of a small mains transformer

that done, you should be able to scale the design

Ash Small

Tue Nov 18 2014, 09:29AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

furnace wrote ...

Ash small wrote....

Can you be a little more specific as to how this would go about??

Here's a link to the Wikipedia page on class D amplifiers.

You don't actually need to bother with the bit that says

"After amplification, the output pulse train can be converted back to an analog signal by passing through a passive low pass filter consisting of inductors and capacitors."

EDIT: You can then use the 'chopper' output signal to feed to the MOSFETS in the bridge, via a suitable 'bridge driver' circuit. (I think I read somewhere that a full bridge 'may' be more suitable than a half bridge, I think it was in Steve's link, something to do with 'current sensing', in his case)

furnace

Tue Nov 18 2014, 03:19PM

Registered Member #4992 Joined: Wed May 23 2012, 03:57PM
Location:
Posts: 108

Shard wrote.....

an inverter, for outputting a sinus wave, will need to switch the DC bus at a duty cycle proportional to the 50/60 hertz signal so that the output, when filtered out by the parasitic inductance and filter of your system, will be a sinus wave

this is a switching system which behaves like a class D amplifier designed to work at mains frequency

if you want to design an inverter yourself the best thing to do would be to design a small class D amp and make it work with an input signal which will be the output of a small mains transformer

I've worked a lot on half bridge inverters for induction heating and is very familiarized with the pll driver, but these work on very high switching frequincy.
So now I'm trying this 50/60 hertz cycle and it seems like a whole different thing.
The link that Steve gave is very helpfull, but I would still have to read and study it thoroughly before I could understand everything on it.
Thanks a lot guys for all the info it really helps than to just jump into something that you almost have no clue as to what your doing.

Shrad

Tue Nov 18 2014, 03:30PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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Posts: 780

basically you vary the duty cycle according to your frequency

that's why a class D amp is such a simple example, as it will hash your DC in the same way and thus the output of your transformer will be sine

furnace

Fri Nov 21 2014, 03:08PM

Registered Member #4992 Joined: Wed May 23 2012, 03:57PM
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Posts: 108

Is it possible to get a pure sine wave on a class d amp with a simple drive circuit like 555 timer or tl494 at 50hz?

Shrad

Fri Nov 21 2014, 09:59PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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if I recall well enough there has been a simple 555 based class D amplifier here

as you don't need audio quality over a wide bandwidth and can optimize for 50Hz, I strongly suggest that you use something like ltSpice for simulating the basic principle of a sawtooth or triangle generator coupled to the sine wave input via a comparator

BigBad

Sat Nov 22 2014, 03:31AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600

Ash Small wrote ...

Here's a link to the Wikipedia page on class D amplifiers.

With class d there has to be a low pass filter somewhere. I mean, you're PWMing, you have to filter off the harmonics.

If you're driving a speaker, the speaker coil is an LR filter.

Driving the mains, presumably you'd need an inductor/transformer or something to smooth it out wouldn't you? You don't want huge currents flowing in and out of your inverter as it switches.

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