Hi Chris



 RM> I recently built a simple r-c low pass filter to allow my

 RM> multimeter to measure the mains freq of the choo choo inverters

 RM> I've been playing with.



 CB> WTF is a "choo choo" inverter - something to do with model trains?

 CB> I thought they ran on 12V DC?



a dual inverter for running aircons on the new 2800 series QR is

buying. 72vdc input. 68HC11 brain. One inverter draws 7A and runs

the control gear and the cool air fan. The other inverter draws 60A

and runs the compressor and the hot air fan. Two people can lift

one of these inverters with considerable difficulty, four people

can manage better, a forklift does it easily. Basic switching speed

is 3kHz.



 RM> The unfiltered waveform is complex, the meter thinks it's around

 RM> 10 kHz, so a lo pass helps the meter look for the lower freq.



 CB> If it's something like the "modified sine wave" inverters used to

 CB> provide 240VAC on 4WD's, trucks, etc, then you will certainly need

 CB> a lowpass to get a steady reading on a freq. measurement...



The last "modified sinewave" inverter I saw (a 600W DSE unit) was

actually a straight variable ratio squarewave with a small output

choke, although I didn't look at the actual output waveform at the

time.



I'm having a go at building a small one with a PIC and some fets at

the moment. The highest switching freq I've tried so far is 1.2

kHz, with fair results. Got a few voltage drop problems to iron out

before going higher (veroboard and 6 amps don't mix...). I'm

digressing a bit with the built-up sinewave idea, though, (for fun

and curiosity, mainly) because the original project idea was a

small low power 3 phase inverter to bench test motor speed

controllers. I'll have to use single pulse "sinewaves" for that,

because the speed controllers will be looking for zero crossings,

and doing 3kHz transitions in 3 phases would be really pushing a

10MHz PIC in interrupt mode, I'd have to go to (yechh) straight

line mode (delays done with program loops instead of with hardware

timers).







 RM> I suspect the "real fault" was poor layout.



 CB> I've has a few fixes over the years where crap layout/design was the

 CB> problem, and a modification rather than a normal repair was needed to

 CB> get it going.  The earlier version of the Tait T555 2 channel UHF

 CB> mobile was a common one in that line for a while...



We drifted away from Tait around then. We sold a few 496s, and got

to know the "pull the vco shields off and clean all the ground

contacts" cure. A couple of 500s are out there somewhere, and

they're both horrible.



The new 2000s look promising, except for one in one of our vans

that sometimes draws 2A even when it's turned off. The PA

transistor takes off at switchon, and stays that way until the

radio is keyed. I've slowed it down by reducing the value of a

damping resistor across one of the coils in the collector circuit,

but it still does it sometimes. Fun.



Cheers



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