AVR-Chat

but if i use a relay, i still another supply for the coils. that means i'll have another supply. one battery for the back-up power and another battery to supply the relay. am i right with that? :)
 
thanks.
-jay

Adam Read <asread@bigpond.com> wrote:
Jay and anyone else interested:

Personally I'd use a relay to switch between supplies, thusly.

1 o----  /------o 4
       ) |
       ) |   /---o 5
       ) o | o
2 o----    |
           o
           3 

   Hook pin 1 to your input dc supply.  Pin 2 goes to ground.  Pin 3 
is your input to your DC regulator.  
   Pin 4 is the pin that will be short to pin 3 WHEN THE COIL IS 
ENERGISED.  This means, naturally, that pin 5 is short to pin 3 WHEN 
THE COIL IS NOT ENERGISED.  (Please excuse the "shouting", I'm just 
trying to emphasise the point).  Pin 4 is therefore the pin which 
you connect to your DC supply, and pin 5 is connected to your backup 
supply.
   If you use large enough caps on the input to your DC regulator, 
the time taken to switch between supplies should be dampened, with 
no or little loss of power to the circuit, depending on load, of 
course.  Should you be concerned about this, it should be relatively 
simple to check the "down-time" by setting up a 'scope on pin 3 (I'm 
making the assumption that you have access to a 'scope, if you don't 
have one it could be a little difficult to check).
   The advantage of this is that it requires few parts, is 
relatively simple, and completely isolates the backup supply from 
the primary supply (so you can use lithium batteries as described in 
other emails).
   Personally I'd go with this one, particularly if you are 
concerned with the possibility of charging batteries which shouldn't 
be charged ie alkalines, some lithiums, etc.
   Also, I apologise for the ordinary ASCII art, I can't draw with 
pen & paper either for that matter.  I've also chosen arbitrary 
values for the pins, they will of course vary between devices.  
Sorry about the late reply, I've been busy with work related crap, 
didn't have a decent chance to check email or reply sooner.

   If anyone wishes to comment (about the circuit, not my 
tardiness!! :)), please feel free.  

Adam.

--- In AVR-Chat@yahoogroups.com, "jaythesis" <jaythesis@y...> wrote:
> hi!
> 
> does anyone have a circuit design on back-up power supply? like if 
> the main power is cut-off, the battery automatically takes over to 
> supply the MCU. can i have the design?
> 
> thanks in advance.
> 
> -jay


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