[aprssig] backup pwr systems

Bob Bruninga bruninga at usna.edu
Mon Dec 11 22:33:21 CST 2006

>> My thought is just a sensing circuit that only fires 
>> the inverter MOSFET's on the rising side of the 
>> 60 cycle waveform.  And then fires them for 50ms 
>> or so on each cycle. If there is no waveform, then 
>> the inverter transistors don't fire.  Should 
>> be very simple. 
> You would have the power company beating down your
> door that way.  Switching the MOSFETs on only a 
> half cycle would introduce even more harmonics 
> into the output power and get fed back into the grid.  

No, that's what every light dimmer does,  And if it is running a 400 watt lamp load like a dining room light then it is the same harmonics.  A simple RFI filter can smooth it up so even a HAM can run on the same power.

THink about it.  The load of the "grid" is something like maybe 0.01 ohm or much less.  There is little that a single mosfet (or a single light dimmer) can do that would cause any discernable gliches on the waveform.  All the mosfet would do would be to contribute a flea power of current, and it would never leave my house, since with only 400W going in, and my houose drawing 1 kW, then nothing leaves my system.  I just reduce my current is all.

> Best to just stick with a standard UPS for each 
> item (or group of items) ... or go with a 
> grid-tie inverter system.

Yes, that was the orginal thread.  I'm looking for a synchronous inverter (call it a grid tie system) in the 400 watt class that should be able to be built for under $100 in electronics.  The cheapest grid-tie systems are in the thousands of dollars.  Hence by post.

>  Having something that plugs into the wall as you 
> describe is a sure-fire way to electrocute a line 
> worker when the power goes out (because you don't 
> have an anti-islanding relay installed as would 
> normally come with a grid-tie inverter). 

No, you didnt read my post correctly.  I said a synchronous inverter which means it cannot generate power on its own with out the presence of the 60 Hz grid in the first place.
It is inherently safe by design.


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