Hi John



 JT> There are many similar arrangements, that only have a simple square

 JT> wave output in the second stage. Generally, the above method is

 JT> used for the higher power inverters. The second square wave type I

 JT> mentioned is sometimes called a "modified square wave inverter"

 JT> although the ones I've seen are quite crude.



 JT>  The second stage switches like this:



 JT>      ----      ----

 JT>      |  |      |  |

 JT>     -|  |-|  |-|  |-

 JT>           |  |

 JT>           ----



 JT>  So much for "modified square wave".



aha - so that's what "mod squarewave" is... I'm familiar with it, but

I'd always lumped it in with the straight squarewave, and called it

variable duty cycle squarewave. I thought mod sq was the above with

partly rounded edges, to reduce rfi and ringing. Innocent me...



 RM> The choo choo inverters, btw, use method 1, but their main

 RM> purpose in life is to drive motors.



 JT> You mean the PWM type? The fancy nancy train controllers do yes.

 JT> The simpler ones just use a high power variable resistor in series.

 JT> (Bodgie yes, but cheap).



Odd, that. An electronic setup would be similar in price to a power

pot, wouldn't it? (although the assembly cost could be a bit higher,

I spose)



reminds me of a battery charger I saw on a boat with 240vdc (yes,

dc) mains. BIG resistor in a room outside, duty and standby 24V

battery banks, standby on charge, duty in use, big 2 pole centre

off changeover switch to swap banks. The little Decca transit

satnav (DS2? model before the DS4) had no standby power, and always

lost its startup info during the brief dropouts that happened when

the switch was thrown, so some genius hooked it up to the "charger"

terminal to give it continuous power. Before its fuse popped, a

tantalum cap protected the power supply from the 240 by shorting

with a bang. I like tantalums. (Hi Brenton!)





 JT> As I had missed the original message, I was going to ask what your

 JT> use was when I remembered I may as well have a look in my own

 JT> message base. I still can't work out exactly what you are after. So

 JT> far, I can tell you want a fairly efficient inverter to go from

 JT> battery power to mains, and probably three phase as well.



It's a mishmash at the moment. It started off as a bit of fun, both

a bit of practice with the PIC and curiosity about how the synth

sine idea would work in reality.



The 3 phase version (if ever get a round tuit) is to drive the

occasional bit of 3 phase control gear we fix. We don't get enough

of it to warrant getting 3 phase power in the workshop, so I

usually lug them around to some friendly electricians.



Did you read my Exploding Speed Controller story from a couple of

years ago? I troubleshot that one at the leccy's shop, with two 100

watt light bulbs in series with each incoming phase. Good thing,

too - when I bent the control board just so, all six bulbs got very

bright.



With a low power inverter (current idea is 60VA per phase), such an

overload would be less likely to cause a big boom. This inverter

would be a mod squarewave type (the zero crossings are well

defined, as opposed to a synth sinewave) and I'm not sure I'd even

bother with regulation, at least at first.



 JT> Another option is for you to do the pwm switching at the

 JT> battery voltage side, and use a standard mains transformer to step

 JT> it up to 240.



That's exactly what I'm doing. This is a minimalist approach. If I

make it too complicated, some of the midair joints will get tangled

together.



 JT> Although if you want to use it with proper PWM and

 JT> simulate a sine wave, you will have to do some real processor work

 JT> as far as regulation goes. I see you have used a PIC to go up to

 JT> 1.something KHz so far, I don't know if that was a proper simulated

 JT> sine wave, or just plain square wave.



Synth sine. The speed limitation is because the hi pulses at the

beginning of a half cycle (and the lo pulses near the middle if the

duty cycle approaches 1), can get quite narrow as the switching

freq goes up. The times are worked out by a C program, which writes

a data file for the assembler to read as a lookup table. With a bit

of work on this program, I could get around the problem by deleting

the narrowest pulses altogether. Later, though.



Regulation: The simplest way would be to depart from the rigid sine

specification, and just alter the pulse widths in a fudgy way, eg

inc the hi pulses in the first part of the quarter cycle by

multiples of 1, and in the second part by multiples of 2, etc.

There's plenty of processor time available, it can spend all the

free time in one half cycle setting up the offsets for the next

half cycle. The current program has an empty main loop, it only

does something if there's a timer interrupt, and that something

in the current program takes around 36 usecs using a 10MHz clock.



 JT> One PWM inverter I've seen,

 JT> fairly high power (around 2KW) used an eprom to generate the PWM

 JT> signal on the mains 340V dc side.



Good technique. With a big enough eprom, regulation could be done

by having a heap of time arrays, and selecting them by connecting bits

from an a/d converter to address lines of the eprom. Chip count is

a bit high for what I want, though. Eprom, a/d, at least two

counters, probably a latch, various bits of glue logic.



 JT> If it is just a square wave, you don't really need a PIC, as it can

 JT> be done cheaper with garden variety components.



Marginally cheaper. A PIC and crystal is typically less than $15.



 JT> I think EA had a

 JT> simple low power (40VA) 12 inverter that used this principal.



I have one of those somewhere, used it years ago, I think it was

EA's first. It had a bit of trouble driving a 40 watt soldering

iron, so I put a 60VA transformer in instead of the 40. The

transistors got hot, a combination of insufficient base drive and

the transformer approaching saturation (it looked like that on the

cro, anyway, the dc current tended to shoot up near the end of each

on period). Being a lazy sod, I heatsunk the transistors rather

than fix the real fault...



We fixed a 600VA DSE inverter recently (broken wire) that used the

"modified" squarewave. Methinks it was a tad overdesigned - there

were at least a dozen ICs on its control board.



Cheers



--- PPoint 1.88