AVR-Chat

At 16:07 02/02/2005, you wrote:
>(replies to multiple messages interspersed below)

ditto

> > From: James Hatley
> >
> > A word of caution ... any voltage used where people might
> > contact across it must be properly installed and shielded.
> > Any voltage higher than about 30volts is considered
> > potentially lethal.

Not quite, in the EU, US and Oz any voltage below 60Vdc is considered to be 
SELV (safety extra low voltage), in other words safe.   Above 60V the 
typical contact resistance of skin is such that currents considered 
unhealthy will start to flow if you come in to contact with them.

If you are going to design any power supply that people can inadvertently 
come into contact with be it directly or indirect via your circuits then 
your power supply shall comply with SELV requirements.   This means that 
you must comply with creepage and clearance and basic electrical safety in 
your power supply design.   Obtain a copy of your countries relevant safely 
standard and follow it explicitly.   You would not believe the difference 
between the relevant big players, US, EU, Asia & Aus/Nz :-)

> > From: Robert Adsett
> >
> > ....
> > Consider 24V, It's a common industrial DC bus voltage so
> > there is a fair amount of support for it
>
>I think 12 VDC is my current preference if the current draw requirements will
>support it since then 12 volt items like light bulbs, motors, etc. can be
>powered directly which would simplify things.  My next preference would be 24
>VDC since, being a multiple of 12 VDC power can be supplied by common 
>batteries
>with solar cell chargers, trickle chargers, etc.
>
> > (also 24VAC).
>
>I'd like to stay away from AC if I can to minimize parts requirements.

I'd give your connections about 2-3 months of reliable service then the 
corrosion gremlin will come to visit, afterwards you'll be pulling your 
hair out trying to figure out what is going on.   Having any DC flow and 
dissimilar metals present will generate/accelerate corrosion faster than 
you would consider possible.

If you are serious at using DC then you can use sacrificial anodes and 
negative rails (ie -50Vdc) to ensure that you force the anodes to be 
depleted and not your cabling.  This is what a lot of Telco's use here in 
Australia in their POTS exchanges.

Otherwise a AC supply would be a better bet.   I would seriously consider a 
24VAC system.   The HVAC industry has been using this for years and for a 
very good reason :-)   I would keep somewhere around 24-30VAC (don't forget 
to take into consideration max mains voltages on transformer primary!) that 
way your rectified DC voltage is kept below 50V.   The suggestion of the 
national simple switchers LM2574 et al is a good suggestion, get the -HV 
parts for good measure (55V max input voltage).

If an AC power system is used then make sure that you use full wave 
rectification not half wave.   This is to prevent unwanted DC biases 
flowing due to the current imbalance generated by half wave 
rectifiers.   Any DC flowing in your supply circuit will negate the 
effectiveness of the AC supply.

 > From: John Samperi
> > ....
> > At 05:12 AM 2/02/2005, you wrote:
> >
> > >As I see it, I'll have to bury two cables along the track, one for
> > >power, and one for the RS-485 signal bus (either 2-wire or 4-wire with
> > >signal ground to control common-mode voltage differences).
> >
> > Can you use the tracks as a ground wire ...just like the real thing?
> > You will need to make sure there is continuity, but with the
> > size of the tracks you should get low voltage drop on the
> > common, so your power wires can just be used for positive/active.

<Snip>

RS485 might not be a good option in this scenario for two reasons.   The 
first is the physical length of your wires are very long @1000' meaning 
lightning and other electrical discharges are not your friend.   Second I 
wouldn't bother trying to lightening proof your system from a direct 
strike, but near strikes will induce large currents in long wires buried in 
the ground.   The effect of this is very high differential voltages with 
respect to ground.   I've found that RS485 transceivers can be notoriously 
unreliable under these conditions.

For data communications why not consider the old faithful 4-20mA current 
loop ?   From what you have described your data speed requirements would be 
low, so a 75-300baud 4-20mA current loop would be well suited to your 
needs.   Did I mention this is what a number of "full sized" railways still 
use today for communication over long wire runs.   Better yet, no current 
flows in the idle state, so no corrosion problems :-)   If you keep the 
average bit ratio to 0.5 (ie successive '1010' combinations) then there is 
effective no DC flowing and the data is an AC signal.

Since we are on the topic of railways and safety if your club puts more 
than one train on the line and you are thinking of using signals to prevent 
collision then please always think safety, safety & safety when designing 
your systems.   If you have never done a Failure Mode & Effect Analysis 
(FMEA) on your circuit then have a read of the following link.

http://www.1stnclass.com/risk_analysis.htm

I've worked on Railways signalling systems here in Australia and every 
component from the lamp, electronics, software to the relay that 
electrically switches the lamp are included in the FMEA.   The most complex 
computer controlled signalling system in use here always ends up in a very 
crusty open frame relay to drive any lamp or switch/contact closure.  The 
design of this relay is some 90 years old and has yet to be ever 
changed.    The reason this relay is in use is it will *always* fail open 
circuit, 90 years of testing has supported this decision.   The effect of 
this "safe relay" is that no lamp or switch can ever be put into an unknown 
state, meaning the green lamp will never illuminate when the red should be 
shown.

I'm pretty sure that it would be rare for more than one train to be on the 
track at once, but I felt it worthy of noting, having spent a number of 
years designing safety critical systems that use embedded processors :-)

>Thanks to all for your suggestions.  For those that have no idea what I'm
>talking about when I say "live steam" tracks, here are some photos of my
>locomotive:
>
>http://www.whitetrout.net/Chuck/844

That is so cool !   What is the maximum pull under good track conditions ?

>I'm sure I'll have more specific AVR questions down the road (track?) as I
>progress.  Even though I'm new to AVRs I enjoy the list and am learning a lot
>just "lurking".  Hopefully some day I'll be knowledgeable enough to contribute
>:-)

Just keep the pictures of the trains with AVR's in 'em rolling in and we'll 
all be kept very happy.

Hope that this helps... YMMV

Regards

Matt