battery backup circuit.

[peterthenovice]

I am looking for a simple circuit that switches to battery power to power a Geller voltage reference.

[yanir]

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

[peterthenovice]

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.

[yanir]

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.

[MacAttak]

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.

[peterthenovice]

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

[ptricks]

If you use schottky diodes check the reverse bias limits as schottky are pretty low in that area.

[yanir]

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.