Author Topic: battery backup circuit.  (Read 17083 times)

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Offline peterthenovice

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battery backup circuit.
« on: April 21, 2013, 06:34:54 pm »
I am looking for a simple circuit that switches to battery power to power a Geller voltage reference.
a craftsman multimeter, bk precsion scope
 

Offline yanir

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battery backup circuit.
« Reply #1 on: April 21, 2013, 06:48:10 pm »
I am looking for a simple circuit that switches to battery power to power a Geller voltage reference.

You can use a 2 diode selector circuit if your primary voltage source is greater than the diodes turn on voltage and the input voltage of the second diode. Schottky diodes are recommended for both their low lower turn on voltage (typically 0.4v) and high switching speed.

there is an example circuit here:
http://saeedsolutions.blogspot.com/2012/12/how-to-switch-between-power-supplies.html
« Last Edit: April 21, 2013, 06:53:30 pm by yanir »
 

Offline peterthenovice

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Re: battery backup circuit.
« Reply #2 on: April 21, 2013, 06:55:09 pm »
I am looking for a simple circuit that switches to battery power to power a Geller voltage reference.

You can use a 2 diode selector circuit if your primary voltage source is greater than the diodes turn on voltage and the input voltage of the second diode. Schottky diodes are recommended for both their low lower turn on voltage (typically 0.4v) and high switching speed.

there is an example circuit here:
http://saeedsolutions.blogspot.com/2012/12/how-to-switch-between-power-supplies.html

can you use a transistor with the plug pack connected to the base.
a craftsman multimeter, bk precsion scope
 

Offline yanir

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battery backup circuit.
« Reply #3 on: April 21, 2013, 07:08:46 pm »
Quote from: peterthenovice

can you use a transistor with the plug pack connected to the base.

Yes provided your voltages are all within the range the transistor can handle. I think the attached circuit would work. I've used something like this in a recent design and now the main and backup voltages can be equal. The transistor in the diagram is a PNP.
 

Offline MacAttak

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Re: battery backup circuit.
« Reply #4 on: April 21, 2013, 09:03:01 pm »
Diodes have a voltage drop that needs to be accounted for. Another way to do it is with complementary FETs. A nice example is in the power source switching used here (the part of the schematic labeled "Power" which switches between +5v and +3v sources):


(from http://tiny-circuits.com/products/tinyduino/asm2001/#design-files_517452430bf34)

It uses a dual MOSFET (http://www.onsemi.com/pub_link/Collateral/NTZD3152P-D.PDF) to cutoff the lower voltage source when the higher source is present.

Of course, the two weaknesses here are 1) these are low-power parts (intended for extremely low current device), and 2) it relies on the battery backup power source having a lower voltage than the external power source.
« Last Edit: April 21, 2013, 09:10:11 pm by MacAttak »
 

Offline peterthenovice

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Re: battery backup circuit.
« Reply #5 on: April 21, 2013, 09:14:08 pm »
Diodes have a voltage drop that needs to be accounted for. Another way to do it is with complementary FETs. A nice example is in the power source switching used here (the part of the schematic labeled "Power" which switches between +5v and +3v sources):


(from http://tiny-circuits.com/products/tinyduino/asm2001/#design-files_517452430bf34)

It uses a dual MOSFET (http://www.onsemi.com/pub_link/Collateral/NTZD3152P-D.PDF) to cutoff the lower voltage source when the higher source is present.

Of course, the two weaknesses here are 1) these are low-power parts (intended for extremely low current device), and 2) it relies on the battery backup power source having a lower voltage than the external power source.
since a 15 volt power pack would be used with two 9v's in serise that would not work
a craftsman multimeter, bk precsion scope
 

Offline ptricks

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Re: battery backup circuit.
« Reply #6 on: April 22, 2013, 12:12:36 pm »
If you use schottky diodes check the reverse bias limits as schottky are pretty low in that area.
 

Offline yanir

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Re: battery backup circuit.
« Reply #7 on: April 22, 2013, 06:11:25 pm »
Left out a diode. Here is a circuit I'm using for switching between a battery and adapter (power pack to you aussies) for input to a lcd backlight driver. It employs a PNP mosfet to prevent current from being drawn from the battery when there is a voltage (provided by the adapter) present on the Gate.
« Last Edit: April 22, 2013, 08:02:44 pm by yanir »
 


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