AVR-Chat

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

Take a look at Texas Instruments Power MUX chips.  These are idea for
automatic power switch-over applications.

I'm using the TPS2115 and it rocks:

http://focus.ti.com/docs/prod/folders/print/tps2115.html

Barry

-----Original Message-----
From: jaythesis [mailto:jaythesis@yahoo.com] 
Sent: Saturday, March 13, 2004 9:55 PM
Subject: [AVR-Chat] back-up power supply circuit

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

 
Yahoo! Groups Links

I like the KISS approach... Depending on your needs, you can
use a SPDT relay with a mains powered coil. When the power
is on, the relay is energized and the N.O. contacts closed to make
the DC circuit from power supply to your MCU board.
(common to the MCU board, NO to the power supply +
and NC to the battery +).
 If the mains power cuts out, the relay will release and make
the NC contacts circuit which can is wired to the battery. A big
electrolytic cap across the DC input can be selected to maintain
the supply voltage while the relay switches over so that your
board doesn't drop out and also to help absorb the switching
voltage spike.

Hugh


jaythesis 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
>  
>

Here's a rough sketch:

http://homepage.mac.com/johnatl/FileSharing25.html

Regards,
   JJ

On Sunday, Mar 14, 2004, at 00:55 US/Eastern, jaythesis 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
>
>
>
>
> Yahoo! Groups Links
>
>
>
>
>

Sorry, it's the backup.jpg file.

Regards,
   JJ

On Sunday, Mar 14, 2004, at 08:33 US/Eastern, John Johnson wrote:

> Here's a rough sketch:
>
> http://homepage.mac.com/johnatl/FileSharing25.html
>
> Regards,
>    JJ
>
> On Sunday, Mar 14, 2004, at 00:55 US/Eastern, jaythesis 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
>>
>>
>>
>>
>> Yahoo! Groups Links
>>
>>
>>
>>
>>
>
>
>
> ------------------------ Yahoo! Groups Sponsor  
> ---------------------~-->
> Upgrade to 128-bit SSL Security!
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>
>
>

Battery ---------->|----o----o MCU +VCC
Main 5V6         Diode  |      Main +5V, Backup +4.8V
                        |
                        |
Battery----------->|----'
Backup 5.5V      Diode


I guess simpler than that is impossible.
Does it solve your problem?

jaythesis 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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isn't the back-up battery always conducting on that state even if the main power is on?

thanks.

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At 05:09 PM 3/14/2004 -0800, jay marante wrote:

>isn't the back-up battery always conducting on that state even if the main power is on?

No, the diode in series with the battery is reverse-biased.

If the backup is a lithium, you need dual diodes, or a current limiting scheme to some very small current, so that a single component failure won't give you an exploding lithium battery.

David VanHorn wrote:
> At 05:09 PM 3/14/2004 -0800, jay marante wrote:
>
>> isn't the back-up battery always conducting on that state even if
>> the main power is on?
>
> No, the diode in series with the battery is reverse-biased.
>
> If the backup is a lithium, you need dual diodes, or a current
> limiting scheme to some very small current, so that a single
> component failure won't give you an exploding lithium battery.


That's another point.
If backup usage current is below 500mA, then use plain and cheap Alkaline
ones.
They are safe, shelf life is much longer than anything else.
Of course, lithium current limit is a good idea, perhaps a small fuse? :)


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>
>That's another point.
>If backup usage current is below 500mA, then use plain and cheap Alkaline
>ones.
>They are safe, shelf life is much longer than anything else.
>Of course, lithium current limit is a good idea, perhaps a small fuse? :)


Fuses that small are WAY expensive, and I don't think you can go that low. 
I once talked to a Panasonic apps guy about a design we had, that put 600nA into a lithium cell. His comment was that it probably wouldn't explode. 

If the steering diode on the battery output fails, then you need to still have essentially zero current into the battery, by some means.

David VanHorn wrote:
>> That's another point.
>> If backup usage current is below 500mA, then use plain and cheap
>> Alkaline ones.
>> They are safe, shelf life is much longer than anything else.
>> Of course, lithium current limit is a good idea, perhaps a small
>> fuse? :)
>
>
> Fuses that small are WAY expensive, and I don't think you can go that
> low.


You mean 250mA and 500mA small fuses?  That's pretty common, radial or axial
socketed ones.  For sure the fuse will open much sooner than the battery
detonates into a Radiologic Thermal Event... :)


> I once talked to a Panasonic apps guy about a design we had, that put
> 600nA into a lithium cell. His comment was that it probably wouldn't
> explode.


Are you talking about recharging current?


> If the steering diode on the battery output fails, then you need to
> still have essentially zero current into the battery, by some means.


That's the Alkaline advantage, but a 20 megaohm resistor in parallel will be
something to think and avoid the battery leaking or contacts corroding.


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At 10:10 AM 3/15/2004 -0500, Wagner Lipnharski wrote:

>David VanHorn wrote:
>>> That's another point.
>>> If backup usage current is below 500mA, then use plain and cheap
>>> Alkaline ones.
>>> They are safe, shelf life is much longer than anything else.
>>> Of course, lithium current limit is a good idea, perhaps a small
>>> fuse? :)
>>
>>
>> Fuses that small are WAY expensive, and I don't think you can go that
>> low.
>
>
>You mean 250mA and 500mA small fuses?  That's pretty common, radial or axial
>socketed ones.  For sure the fuse will open much sooner than the battery
>detonates into a Radiologic Thermal Event... :)

I think we're talking at cross purposes. I'm thinking of the failed diode case, where the normal supply starts trying to charge the battery. 


>> I once talked to a Panasonic apps guy about a design we had, that put
>> 600nA into a lithium cell. His comment was that it probably wouldn't
>> explode.
>
>
>Are you talking about recharging current?

Yes.


>> If the steering diode on the battery output fails, then you need to
>> still have essentially zero current into the battery, by some means.
>
>
>That's the Alkaline advantage, but a 20 megaohm resistor in parallel will be
>something to think and avoid the battery leaking or contacts corroding.

Then you're pulling current on the backup unnecessarily.
Not much, I'll grant you, but it's still a sloppy solution.
A proper switchover circuit isn't hard, but it's more than a couple diodes. 

A couple diodes works for hobby purposes though.

David VanHorn wrote:
> I think we're talking at cross purposes. I'm thinking of the failed
> diode case, where the normal supply starts trying to charge the
> battery.

[snip]

>> That's the Alkaline advantage, but a 20 megaohm resistor in parallel
>> will be something to think and avoid the battery leaking or contacts
>> corroding.
>
> Then you're pulling current on the backup unnecessarily.
> Not much, I'll grant you, but it's still a sloppy solution.
> A proper switchover circuit isn't hard, but it's more than a couple
> diodes.
>
> A couple diodes works for hobby purposes though.


Oh yes, a failed diode could cause a mess in the system, but a mess would
also caused by a bad contact at the backup battery leads by corrosion, then
no backup will be available.

A pack o 4 x AA alkaline batteries would supply 1.5A/h, lets say that after
one hour supplying 300mA it will still with enough juice to feed power for
some more time.

20MOhms resistor in parallel with the battery (6V) will drain 300nA/h, what
means one million hours to reach the same equivalent 300mA/h, that is 115
years.  I guess a 1MOhms resistor will do a much better job, and still 6
years to reach the same discharge level.

I don't believe in backup systems without maintenance.  Even without use,
the 4x Alkaline batteries should be replaced every 6 months or so.  The
replacement sees to keep fresh batteries, exercise the contact springs and
provide visual inspection.

Backup systems use to be a real pain in the neck, since the only moment you
can be sure it is working, is when you most need it.  If not working, you
wasted a lot of effort for nothing.

This is almost the same as hiring someone to be your bodyguard, to feel safe
you need to see him from time to time watching over you.

Of course, there are many ways to produce an efficient backup switch, but as
much components you insert into the thing, more complex it turns to be,
thus, more subjected to fail.  In all my prof life, I saw much more, much
more bad and oxidized contacts than open or shorted diodes where low current
is involved.

Just as information, several metal traffic bridges use a small AC current
through the estructure, just to reduce oxidation and rusting at the screws,
bolts, joints, etc.  This small AC current also serves as to measure the
oxidation level of the bridge.  There are some kind of gadgets that transmit
such information via RF straigh up, so it can be read by a county airplane
or chopper.


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>
>Oh yes, a failed diode could cause a mess in the system, but a mess would
>also caused by a bad contact at the backup battery leads by corrosion, then
>no backup will be available.

That's a survivable failure. What I'm thinking of here, is a failed diode charging a lithium backup battery, which definitely can lead to an explosion, even at relatively low current. UL requires double protection mechanisms for this.


>Backup systems use to be a real pain in the neck, since the only moment you
>can be sure it is working, is when you most need it.  If not working, you
>wasted a lot of effort for nothing.

Yes, and the added complexity makes it MORE likely to fail. 
Dammit.


>This is almost the same as hiring someone to be your bodyguard, to feel safe
>you need to see him from time to time watching over you.
>
>Of course, there are many ways to produce an efficient backup switch, but as
>much components you insert into the thing, more complex it turns to be,
>thus, more subjected to fail.  In all my prof life, I saw much more, much
>more bad and oxidized contacts than open or shorted diodes where low current
>is involved.

I won't use slide-in lithium coin cells for that reason. Not enough wetting current, even with high contact force.  Solder-in only for me.

>Just as information, several metal traffic bridges use a small AC current
>through the estructure, just to reduce oxidation and rusting at the screws,
>bolts, joints, etc.  This small AC current also serves as to measure the
>oxidation level of the bridge.  There are some kind of gadgets that transmit
>such information via RF straigh up, so it can be read by a county airplane
>or chopper.

Cute!  I see dashboard application. Unsafe bridge ahead! :)
Fish floating in the water, due to open circuit on the bridge might be a bad sign..

For production use I still would recommend using an auto-switching power
MUX.  They rock.  They will auto-sense the voltage differential and
automatically switch between sources.

Their cheap too.

TI makes a few of them.  Again, I'm using the tps2115. Here's the
datasheet:

http://focus.ti.com/lit/ds/symlink/tps2115.pdf

They use integrated MOSFETS for a low voltage drop.

Otherwise, you could use the circuits below but I would suggest using a
Schottky diode which will give you a lower voltage drop and be more
efficient.

-----Original Message-----
From: Wagner Lipnharski [mailto:wagner@ustr.net] 
Sent: Sunday, March 14, 2004 1:48 PM
Subject: Re: [AVR-Chat] back-up power supply circuit

Battery ---------->|----o----o MCU +VCC
Main 5V6         Diode  |      Main +5V, Backup +4.8V
                        |
                        |
Battery----------->|----'
Backup 5.5V      Diode

I guess simpler than that is impossible.
Does it solve your problem?

jaythesis 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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Yahoo! Groups Links

At 08:34 AM 3/15/2004 -0800, you wrote:

>For production use I still would recommend using an auto-switching power
>MUX.  They rock.  They will auto-sense the voltage differential and
>automatically switch between sources.

Good for a hobby project, if you don't want to take the time/trouble to design your own.

Maxim/Dallas make a lot of money on that market.

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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

> -----Original Message-----
> From: Adam Read [mailto:asread@bigpond.com] 
> 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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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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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.
> 
> ---
> Outgoing mail is certified Virus Free.
> Checked by AVG anti-virus system (http://www.grisoft.com).
> Version: 6.0.618 / Virus Database: 397 - Release Date: 9/03/2004

> -----Original Message-----
> From: Adam Read [mailto:asread@bigpond.com] 
> Sent: Friday, 19 March 2004 8:28 PM
> To: AVR-Chat@yahoogroups.com
> Subject: [AVR-Chat] Re: back-up power supply circuit
> 
> 
> 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!).  

No worries mate, we all make mistakes, I have made some major ones (like
sending a PCB to be fabbed upside down, so all the IC's had to be
mounted on the under side (these were the good ol days of tape and film
lol)


 
> 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.

I have uploaded a gif to the files section showing how it is done.

Regards,

Kat.



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I havent been following this back-up power supply circuit very closely so I
may be way off track here but...

If I had a LOAD that was feed from 110 VAC POWER SUPPLY and wanted to supply
emergency power to it in the time of a power outage here is how I would do
it.

I would start off with the POWER SUPPLY voltage a little bet larger than the
BATTERY.
Read that sentence again, its a must. Four alkaline batteries (4.8 = 4 x
1.2) and the output of a 5 volt regulator should work fine.

On a piece of paper write horizonallly the words BATTERY, POWER SUPPLY and
LOAD
Individually circle each one
Put a lead out of the top and bottom of each
Mark the top leads + (positive) and bottom leads - (negative)
Connect all the negative leads
Connect the POWER SUPPLY through a diode to the LOAD
Connect the BATTERY through a diode to the LOAD

Because the voltage on the positive terminal of the LOAD from the POWER
SUPPLY is greater than the voltage from the BATTERY current WILL NOT flow
through the diode connected to the BATTERY.

As soon as you unplug the POWER SUPPLY the BATTERY will take over supplying
power to the load. This will continue until the BATTERY runs down or power
is restored to the POWER SUPPLY. The LOAD will never know if is is on POWER
SUPPLY or BATTERY. It will on so never know when the POWER SUPPLY drops out
or comes back on.

All you need, besides the BATTERY, POWER SUPPLY and LOAD is 2 diodes.

Mike

PS Like I said at the start. I have missed a lot of this thread so I don't
know everything that has been, and is being, talked about. Designing many
custom circuits over the past 40 years, power supply circuits are duck soup.

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> -----Original Message-----
> From: Mike Bronosky [mailto:Mike@Bronosky.com] 
> Sent: Friday, 19 March 2004 11:11 PM
> To: AVR-Chat@yahoogroups.com
> Subject: Re: [AVR-Chat] Re: back-up power supply circuit
> 
> 
> I havent been following this back-up power supply circuit 
> very closely so I may be way off track here but...
> All you need, besides the BATTERY, POWER SUPPLY and LOAD is 2 diodes.

Been here lol, this was the first idea, but it was pointed out a diode
failiure on the battery line could blow the batteries up (and after some
of the crap NiMh I have found over the last couple of months that
explode even in normal use.....)

The relay Idea was floated as unless the contacts weld (not likely with
only a couple of 100 mA) it is relatively safe.

For Commercial designs I use 2 diodes on the battery line (this has UL
approval on one of my marine designs (and also VME and others)

It has also been suggested to use a power management chip or reg with
battery input etc. 

Regards,

Kat.

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Outgoing mail is certified Virus Free.
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Oh, I remember reading something about that awhile back. Someone suggested a
fuse, and with the double-diode you mention, that should make it fairly
safe.

For those that are not electronic technicians, there are no electronic
circuits that beyond failure. By all means a mechanical relay and its
contacts are more likely to fail than the PROPERLY rated diode.

Life is full of risks. The only way I know of to do away with all risks is
death. Ain't may folks jumping on that wagon.

Mike

----- Original Message -----
From: Kathy Quinlan <kat-yahoo@kaqelectronics.dyndns.org>
To: <AVR-Chat@yahoogroups.com>
Sent: Friday, March 19, 2004 10:36 AM
Subject: RE: [AVR-Chat] Re: back-up power supply circuit


>
>
> > -----Original Message-----
> > From: Mike Bronosky [mailto:Mike@Bronosky.com]
> > Sent: Friday, 19 March 2004 11:11 PM
> > To: AVR-Chat@yahoogroups.com
> > Subject: Re: [AVR-Chat] Re: back-up power supply circuit
> >
> >
> > I havent been following this back-up power supply circuit
> > very closely so I may be way off track here but...
> > All you need, besides the BATTERY, POWER SUPPLY and LOAD is 2 diodes.
>
> Been here lol, this was the first idea, but it was pointed out a diode
> failiure on the battery line could blow the batteries up (and after some
> of the crap NiMh I have found over the last couple of months that
> explode even in normal use.....)
>
> The relay Idea was floated as unless the contacts weld (not likely with
> only a couple of 100 mA) it is relatively safe.
>
> For Commercial designs I use 2 diodes on the battery line (this has UL
> approval on one of my marine designs (and also VME and others)
>
> It has also been suggested to use a power management chip or reg with
> battery input etc.
>
> Regards,
>
> Kat.
>
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>
>
>
>
> Yahoo! Groups Links
>
>
>
>
>
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>
>

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