Re: gEDA-user: gEDA, where are the loadable examples?

[Gene Heskett <gene.heskett@xxxxxxxxxxx>]

On Sunday 13 March 2005 23:14, Charles Lepple wrote:
>On Sun, 13 Mar 2005 15:42:22 -0500, Gene Heskett
>
><gene.heskett@xxxxxxxxxxx> wrote:
>> I don't have to have the 82c55, but if I did, I could maybe do two
>> boards and let ngspice do some optimizing.
>
>There is an 8255A symbol in the micro subdirectory of the
> geda-symbols package, but it's just a graphical symbol. That is,
> you can draw a schematic with it, and if you have a generic 40-pin
> DIP footprint, you can put it on a PCB. (This particular symbol
> file has pin numbers associated with it, which lets you use it with
> a generic footprint.)
>
>If you try to make a spice netlist, I don't know what would happen,
>but I would bet that it's not what you intended.
>
>> How is ngspice at simulating analogue circuitry thats supposed to
>> be running in pwm mode?
>
>Not sure about ngspice in particular, but in general, spice-like
>modelling systems can easily handle simulating transistors operating
>in saturation (as most PWM systems are designed to run). If you have
>an integrated PWM driver chip, you probably won't be able to easily
>simulate overcurrent limiting, but if you pick a transistor model
> with similar output characteristics, you can model the non-limited
> operating region.

The existing circuit I have uses an lf357 as a comparator, which when 
the sensing R's in the ground leg of the l298 bridge driver detect an 
over-current, pull the enabling input back down if it can.  With an 
82c55 pulling it up, and a 270k resistor between the 82c55 and the 
rest of the circuit including this pulldown, the darned thing is 
doing an analog regulation, so apparently I need another rolloff pole 
in the feedback loop to make it actually oscillate.  Or figure out 
how to put a lot more hysteresis in the thing somehow.  That may be 
the best bet in the long run, but my choices of pf sized caps here at 
home isn't much.  I'll have to see if I can work that out in my head.  
I have done similar stuff before so maybe I can again.

Humm, looking at the schematic, (I wish you had one too, so you could 
follow this one sided conversation better) the lm358 (each half) that 
is being used as a current comparator, has a 10 meg R from its output 
to its + input, which with a 10k R from the setting pot, also a 10k 
unit with the full 5 volts across it, means it has about a 4.5 
millivolt positive feedback to establish some hysteresis.  Watching 
it just now with a scope across both of the .5 ohm sensing R's, I can 
see its switching moderatly cleanly when only one coil is being 
driven, but goes a bit berzerk when both coils are on.  But the 
switching frequency is something above 120khz too.  When the source 
current turnon delay is about 1.25 usecs, thats a lot of time spent 
in the high heating transistions of the switching.  The offtime 
delays are much faster of course at around .1 usec.

So the waveforms are ringing pretty bad. When only one coil is on, it 
is about a 60% duty cycle square wave, from zero to an amp, but the 
top of the waveform isn't the usual ramp of ever increasing current 
one would expect to see when driving an inductance, its essentially 
flat.  I'd think what I should be seeing across the sensing R's would 
be much closer to a sawtooth, but I guess its steering the 
freewheeling shutdown current not thru the chip, but thru that 8 pack 
of si diodes, type FR106's.  Humm, google search on them puppies to 
see how fast they are.  Cause they need to be fast for power diodes.  
Humm, 800 volts, but only 1 amp, fast.  The curves go up to 3 amps 
though, but the killer to me is the >.5 usec reverse recovery time, 
no wonder there is a nearly 10 amp inrush for about 1.3 usecs when 
the bridge turns back on.  No curves on the forward breakover time 
furnished but I'd bet its 100ns or more.  They need to be schotkeys 
so they wouldn't need the 800 volt rating, 75 would be sufficient if 
they were fast enough.  And to be current sensed correctly, those 4 
diodes in the lower half of the circuit should return not to ground, 
but to the top end of the sensing R's so the motor current is being 
monitored full time as opposed to only when the bridge drivers in the 
L298 are on.  That poor diode recovery time explains the square wave 
I'm seeing, with the ten amp leading edge spike.  The spike is the 
reverse recovery time of the diodes.  I'd bitch about the design, but 
SGS/Thompson shows it exactly that way in the data sheets.  Monkey 
see, monkey do.

A star topology for the grounds would have been nice, I wonder how 
much I'd have to cut & jumper to do that, no pcb artwork handy & the 
pcb's are all bolted down, and its getting late.

That and lowering that 10 meg feedback to even as low as a meg would 
slow the pwm regulator switching speed by half I'd guess.  I'll look 
at the actual voltages on a winding tomorrow and see whats going on 
there.

One thing I don't like about this board is the lack of a centralized 
single point ground, radiating from the common point of the sensing 
R's.  Theres at least a volt of noise sitting on the pin they call 
ground at the input header when compared to the ground at the power 
supply header.  I think thats the first thing I'll fix on one board 
for effects, then play with the feedback R.

And my thinking out loud has probably bored you by now, so I'll say 
goodnight.  And I'll let you know what results I get for the cut & 
jumper operation to make a single point ground out of it.

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
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