There was a diagram I saw on the power switching.
Try to lookup for AVR butterfly. The power circuit has
such roll over.

http://www.atmel.com/dyn/products/tools_card.asp?tool_id=3146
Look for User guide.
Schematic 4 of 4.




John


--- Carl Denk <cdenk@alltel.net> wrote:

> If someone could come up with a replacement for the
> relay, that would be 
> great. Need to actually switch between the 2
> batteries OR'ed together 
> and the 12 VDC power supply. Thinking FET's or
> something, but, although 
> LOGIC, TTL, some OP-AMP design and Ohm's law is my
> level, my expertice 
> is more toward structural concrete. ~)
> 
> Carl Denk wrote:
> > I wish I could do  that, but The battery voltages
> are fixed at what they 
> > are, and the maximum voltages of the other parts
> is fixed too. Looks 
> > like the next higher voltage supply is 15 volts,
> that's a little higher 
> > than I would like to operate, plus the batteries
> float at 13.8 volts. 
> > That relay is quite beefy, with 10 amp contacts,
> and if the move maybe a 
> > dozen times a year, that would be a lot, plus it
> could be put on 
> > maintenance schedule to replace every few years.
> The whole system is 
> > based on the Kohler engine electrical which is 12
> volts (nominal), then 
> > all the sensors (pressure, voltages, temperatures)
> are selected and 
> > calibrated based on the 12 volts.
> >
> > What about adding a diode across the contacts with
> the capacitor ?
> >
> > Dwayne Reid wrote:
> >   
> >> I think that I'm still gonna beat the same drum.
> >>
> >> Eliminate the relay that switches the PLC power
> line.  Use a diode 
> >> array of 3 diodes so that whichever voltage is
> highest feeds the PLC 
> >> power input.
> >>
> >> Ensure that the AC power supply is higher than
> the float voltage 
> >> present on the batteries when fully charged.  No
> relay needed.
> >>
> >> Then try it and see how you make out.
> >>
> >> dwayne
> >>
> >> PS: the only problem with adding that large
> capacitor after the relay 
> >> contacts is inrush current eventually damaging
> the relay 
> >> contacts.  But I still don't see the need for the
> relay and think 
> >> that eliminating it will get rid of your problem.
> >>
> >> dwayne
> >>
> >> At 01:06 PM 4/10/2007, Carl Denk wrote:
> >>
> >>   
> >>     
> >>> Powered from the non-generator engine battery,
> all OK. If it's powered
> >>>     
> >>>       
> >> >from the engine's battery, upon starter
> energizing, voltage is pulled
> >>   
> >>     
> >>> down to 9 volts, and PLC resets, had that issue
> some time ago, and added
> >>> a second battery (from a homebuilt airplane we
> had dismantled) with the
> >>> diodes to select the higher voltage one as
> described earlier. No
> >>> problems when powered off either battery, except
> when starting. Issue
> >>> only when switching from the switching power
> supply to battery, when the
> >>> relay coil is deenrgized and spring pulls
> contacts open. No problem when
> >>> coil is energized and contacts are switching.
> >>>     
> >>>       
> >>   
> >>     
> -- 
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