Cut off switch circuit / Dual voltage

Hello, I'm currently building a system with a rechargeable battery in it. The power supply is 12v dc 3a and the battery is 12v 5mah. The battery is NiMH, it cannot discharge below 10v, if i do so i'll ruin it. I would like to design a circuit where, when i'm using the power supply the battery stop supplying and when I unplug it the battery takes over. Also, I want the battery to stop supplying if its voltage is below 10v . I would like to use IC 741 as a comparator. I really need help. Thanks in advance!

by samybee1
April 15, 2017
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1 Answer

Answer by mikerogerswsm

There is a type of power plug that does just this. When you plug in your power supply the battery is mechanically disconnected and when you pull out the plug your battery is switched on again. Here is one that does 12V 3A:

https://www.bitsbox.co.uk/index.php?main_page=product_info&cPath=225_232&products_id=1696&zenid=793604o3g13ggf5bss3vp9fs02

The battery low voltage cutout is fairly simple to arrange, but remember the circuit itself will draw a few mA from the battery when it is off. A typical circuit is here:

http://www.homemade-circuits.com/2013/10/low-battery-cut-off-with-indicator.html

To minimise battery current when off you need a low current opamp, like this:

http://electronicdesign.com/site-files/electronicdesign.com/files/archive/electronicdesign.com/files/29/6166/figure_01.gif

You will need a different values of R1 and R2.

+1 vote
by mikerogerswsm
April 15, 2017

PS - here's a similar power connector in use:

http://mike-wsm.org.uk/psv.html

Note the battery negative is switched (centre pin positive).

by mikerogerswsm
April 15, 2017
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