Backup a PIC16F870 in normal mode with baterry?

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neuralc

Joined: 06 Nov 2001
Posts: 274
Helped: 6

11 Sep 2002 14:38 Backup a PIC16F870 in normal mode with baterry?

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

Joined: 28 Aug 2001
Posts: 249
Helped: 13
Location: Argentina

11 Sep 2002 16:30

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. Shocked

neuralc

Joined: 06 Nov 2001
Posts: 274
Helped: 6

01 Apr 2003 14:40

OK,

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

Other opinions?

ThX

NeuralC

ME

Joined: 14 Mar 2002
Posts: 1523
Helped: 13

01 Apr 2003 15:11

......................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.

ME

Joined: 14 Mar 2002
Posts: 1523
Helped: 13

01 Apr 2003 15:18

http://www-s.ti.com/sc/psheets/slvs339b/slvs339b.pdf

http://www.maxim-ic.com/appnotes.cfm/appnote_number/170/ln/en

Last edited by ME on 01 Apr 2003 15:19; edited 1 time in total

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neuralc

Joined: 06 Nov 2001
Posts: 274
Helped: 6

01 Apr 2003 15:19

OK

But like that baterry will not be charged.

ThX

NeuralC

ME

Joined: 14 Mar 2002
Posts: 1523
Helped: 13

01 Apr 2003 15:26

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

Backup a PIC16F870 in normal mode with baterry?

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

Last edited by ME on 01 Apr 2003 21:23; edited 2 times in total