# using power fet as a low battery cut off

#### prototype1905

Joined Oct 5, 2011
1
hi i need help in designing a circuit using a power fet and that cuts off the load when the battery voltage reaches 9 v and turns on the load when it is 12.5 v i am using sealed lead acid battery of 12v and 1.4 ah and the load is 21 10mm leds

#### praondevou

Joined Jul 9, 2011
2,942
Hi prototype1905, are you still there? I know it's a few days old, this thread, but if you still are interested, tell us. Shouldn't be so difficult to solve your problem..

If you still want a solution, please provide the maximum current you are going to draw.

#### chrisw1990

Joined Oct 22, 2011
551
21 10mm LEDS.. be around 420mA..? if you run at 20mA...

Joined Dec 26, 2010
2,148
21 10mm LEDS.. be around 420mA..? if you run at 20mA...
That assumes that none of the LEDs would be connected in series, which may or may not be the case. With a nominal supply of 12V and minimum 9V, then depending on the LED forward voltages, at least two and possibly three devices in series may be possible. This would at least halve the current drain.

Actually 9V is a low discharge voltage for an SLA battery. It may be preferable for battery life to end the discharge at a higher voltage, at which it may even be possible to use some types of white LEDs in series threes, particularly if low dropout constant current control can be used.

#### chrisw1990

Joined Oct 22, 2011
551
That assumes that none of the LEDs would be connected in series, which may or may not be the case. With a nominal supply of 12V and minimum 9V, then depending on the LED forward voltages, at least two and possibly three devices in series may be possible. This would at least halve the current drain.

Actually 9V is a low discharge voltage for an SLA battery. It may be preferable for battery life to end the discharge at a higher voltage, at which it may even be possible to use some types of white LEDs in series threes, particularly if low dropout constant current control can be used.
good idea.. on another note why use a sealed lead acid battery for 21 leds?
seems quite a large battery to be using for such a task?