Colin MacKenzie ( wrote:

-> I feel foolish, I have never fooled around with battery backup systems
-> but it is required for a project I have designed. I would like to
-> recharge the battery while the power is on. When the power goes off
-> the battery has to switch over quickly since it is powering a
-> microprocessor. That the circuits used in emergency lights are
-> probably simple but do they switch over fast enough?

-> How do I go about doing this?

The 'standard' way of doing this involves a battery, cap, and a few diodes.

              resistor      diodes   
Vin  O-------*------------|>|----------*-------------O  Vout
             |                         |
                               ---     Battery  
                               ---   Ground

Note:  -|>|-  is a diode with the anode on the left.  

Basically, the way it works is when power is applied, it passes thru the
top diode during normal operation, and charges the battery thru the
resistor and diode.  When power fails, the Vin side voltage falls below
the battery voltage, and the battery then takes over the load on the
Vout side thru the diode.  The switching is instantaneous, and probably
faster than a few uS.  Add a 100uf or so cap between Vout and ground to
take care of any diode switching transients.

You can remove some of the diodes if you don't need 'em; i.e. if you're
taking the output of a rectifier and running it to the Vin terminal,
the diode(s) in the rectifier will stop any appriciable current flowing.
I've used this circuit with much success in my designs.

PS: forgive the non-lucid theory of operation- I've just finished routing
the Board From Hell.  ;-)

| \_ \_  \_\_\_  \_  \_  \_   \_  \_           B5(6) t w g+ s-- r-    |
|  \_\_   \_\_    \_\_    \_   \_\_\_   &h48,&h65,&h6C,&h6C,&h6F,&h21 |
|   \_ \_  \_\_\_  \_      \_   \_  \_   __ __ |
|     "It's 5:50 AM.  Do you know        \/__Can *YOU* write 8085__\/ |
|     where your stack pointer is?"        \/      assembly?     \/   |

Date: 24 May 1996 09:24:35 GMT

Original Subject: Re: Making a battery backup.

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