[MRCG] 800Hz AC Power for Navy Radios

[Mark J. Blair]

On May 12, 2013, at 16:50 , Chuck <j-mcclurg at sbcglobal.net> wrote:
> I am keeping an eye on this as you know.

Since you mentioned it, here's a status update:

I received my inverter from Amazon yesterday. It's a Power Bright model APS-1000-24V:

  http://www.amazon.com/Power-Bright-1000-Watt-Inverter-110-Volt/dp/B002EA006S/

I opted for a 24V input inverter rather than a more common and cheaper 12V one, because the GP-7 would suck too much power for my 12V radio room supply. This inverter will run off my PP-4763A supply. I also selected one with a "true sine wave" output.

I took a gamble by ordering this one, without knowing anything about what's inside of it. So far, things look promising.

It does not appear to contain any highly-integrated components (i.e., no microcontroller, custom inverter controller IC, etc.). The control circuitry appears to consist of standard off-the-shelf 4000-series CMOS logic, linear ICs such as op-amps, and discrete components. This is about the best possible case with respect to having access to the innards and the ability to change the design, but on the other hand, my job will be more complicated than simply googling up a datasheet for a custom IC and then changing an R-C on the frequency-defining pin.

Initial blind probing appears to have verified my guess that it would generate the sine wave by pulse-width modulation with an output filter to roll off the switching frequency. It appears to switch in the ball park of 25 kHz. There is one crystal in an oscillator + divider circuit, and one trimmer potentiometer which turns out to adjust the output voltage.

That's where I am right now. I've identified an area on the PCB where I suspect the 60 Hz might be coming from, so I need to study the circuitry some more to figure out what's going on. I think I'm going to order a high voltage differential scope probe before I poke around much more, and I found that poking some nodes in that area with my regular scope probe made the supply emit a disturbing noise... luckily, the magic smoke stayed inside! Those HV diff probes are EXPENSIVE. The cheapest one I've found is made by LeCroy and costs around $390 new. Tektronix makes a particularly nice one which would interface well with my Tek digital scope, but those ones appear to cost $600+ used on eBay! I've wanted this sort of probe for a long time for working on power supply circuits, and I finally have a good excuse to take the plunge and get one.

I've been taking pictures, and I'm working on a web page post with my status so far. I don't know yet whether I'll publish it before I've dug into the circuit deeper. We'll all find out later this evening, I guess. :)

I presently suspect that I have three major tasks:

1) Figure out how the 60 Hz waveform is generated, and modify the circuitry to make an 800 Hz sine instead.

2) Make sure that the output filter can pass the 800 Hz sine without heating up too much. Hopefully the corner frequency is high enough that it will work without modification at 800 Hz.

3) Figure out the physical details of my modification, including tearing out the Edison plugs, installing something else in their place, possibly adding a switch for 60/400/800 Hz if practical, etc.




-- 
Mark J. Blair, NF6X <nf6x at nf6x.net>
http://www.nf6x.net/