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Backup a PIC16F870 in normal mode with baterry?

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neuralc

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Hi all,

If some one have some info about how to backup a PIC16F870 in normal mode (not powerup, not sleep) (consumption arround 2 mA).
With baterry or Super capacitor?

ThX


NeuralC
 

penrico

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The way to power backup with a capacitor is connecting two diodes in this manner:

..............D1.....................D2
VCC ------>|------+-------->|--------|uC|
..........................|
.......................|.--.|
.......................-----
..........................|
.........................gnd

If you use a Capacitor of 0,33F, and your circuit drains 2mA VCC=5V, it will still working about 5500 seconds (>1 hour).

D1, and D2 is best if they are schotkly diodes. 8O
 

neuralc

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OK,

But with 1000uF (2mA), in 2 sec the cap will be discharged

Other opinions?

ThX

NeuralC
 

ME

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......................D1.....................
VCC (5V) ------>|--------------+-----|uC|
............................................|
............................................|
...............Vbat (4.5V)....D2....|
..........................-------->|---+
..........................|
.......................__|__+
........................___ ...... 1,5V battery cell
..........................|
.......................__|__+
........................___ ...... 1,5V battery cell
..........................|
.......................__|__+
........................___ ...... 1,5V battery cell
..........................|
.........................gnd

Use Schottky diodes

When VCC is on D1 conducts and no current runs from the battery because of the lower battery voltage (4,5V).
When VCC is off the voltage between D1 and D2 drops to 4,5V - Vf(D2), then the battery starts to conduct.
 

neuralc

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OK

But like that baterry will not be charged.

ThX

NeuralC
 

ME

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If you want to charge the battery when power (VCC) is on you can use this schematic:




Remember to use NiCd or NiMH cells. You have to make some minor modifications to use it with 5V. Use shottky diodes instead of 1N4007 if you want a lower voltage drop across the diodes.
You can connect 5,8V to the circuit then you got a 5V output if you use schottky diodes with 0.4V voltage drop. If you connect 4 NiCd cells you got 4*1.2V-Vf=4.8V-0.4V=4.4V in backup mode.

Description: http://www.electronic-circuits-diagrams.com/telephonesimages/telephonesckt10.shtml
 

neuralc

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OK
Thanks

Will try it


ThX

NeuralC
 

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