AVR-Chat

Kat: Thanks for spotting my stuff-up.  It only really clicked when I 
read your post and I thought "well now, I'm an idiot" (although, 
realistically, I used much worse language!).  I do tend to forget 
things that are that obvious, guess I was in a hurry to help/'show 
off'/any excuse I can think of later.... at any rate I'll just go 
kick myself now (ow!).  

Jay: No, you won't need another supply for this circuit.  What I was 
attempting to describe (and I very rarely explain things properly 
the first time, so please bear with me) was thusly:

When the primary power is on, the relay is energised, and power 
passes through the primary pin to the switched pin which then feeds 
the on-board regulator, which feeds onwards to the circuit.  Of 
course, as Kat pointed out, you will need a diode to prevent my 
lovely caps from keeping the coil energised for a period.  The 
primary supply comes in along a track to your switching circuit.  At 
some point the track will split, as shown: (again, please excuse my 
crap ASCII art)

         _______  Relay coil
        /
-------<
        \----|>--- Pin supplying primary power to switched pin.

Please note Kat's diode, shown correctly on the 'supply pin'.  As 
Kat described, it will prevent the caps further down the lin (after 
the switched pin) from keeping the relay energised and not allowing 
the switching to occur until the caps are drained, and the micro has 
lost most of its power and likely reset.

Hopefully this will help you with your project, and I hope you 
understand what I'm trying to get at here.  If not, feel free to let 
me know.  We are all here to help each other, after all.

Cheers,
Adam.


--- In AVR-Chat@yahoogroups.com, "Kathy Quinlan" <kat-yahoo@k...> 
wrote:
> 
> 
> > -----Original Message-----
> > From: Adam Read [mailto:asread@b...] 
> > Sent: Thursday, 18 March 2004 5:10 PM
> > To: AVR-Chat@yahoogroups.com
> > Subject: [AVR-Chat] Re: back-up power supply circuit
> > 
> > 
> > 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. 
> 
> Ummmmmmm I would use an AC relay and put it before your bridge, or 
the
> extra caps to keep the supply up for switchover will also keep the 
relay
> energised, you could also use a diode after the bridge but before 
the
> filter caps and take the +supply for the relay from between the 
diode
> and bridge.
> 
> Regards,
> 
> Kat.
> 
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