Question-MOSFETs

125

01:22:20

Hy!
My project is to handle 21 servo motors with a microcontroller, I must switch all the servos off, when the battery is running low. So, I measure the battery's voltage with the microcontroller, and when the voltage in the battery runs low, I switch the 21 servos off.
In this schematic, a simple switch simulates the microcontroller, and a lamp simulates the servo motors ( i didn't want to place 21 lamps).
One servo need 11.1V voltage, and all the servos drain about 10A from the battery in summary.
My question is the following:
Is this schematic correct?
Can you tell me some concrete power-MOSFET types, that can handle this voltge, and current?
Thank you!

published 11 years ago

faceblast

11 years ago

the N MOSFET isn't doing much in this schematic, it's just sending the gate high on the P MOSFET to turn it off. The P MOSFET is doing all the work. you'll need a P MOSFET capable of 450W of power (11.10V x (10A x 4 for motor startup)), i don't know of such a thing.

Sine_eyed

11 years ago

Quick question: you have one battery, that, until it is drained passed a certain voltage will be powering 21 10Amp servos in continuous operation- right? That's a lot of juice- you got a car battery or something?..

faceblast

11 years ago

total drain of all motors is 10A so probably 15 min running time out of a 12V 3300mAh battery

faceblast

11 years ago

if the microcontroller is running the servos, just connect the battery to an analog pin through 1megohm and programme it to read the voltage and turn off the motors when it goes too low. if your pic has no analog input, just use a comparator and a ref voltage to digitise it

markellamate

11 years ago

Thanks for the answers! So, the 21 servo motors belongs to a Kondo Robot. My project is to control this robot with a Silicon Laboratories C8051F580 microcontroller. All the things will run from a 11.1V, 800mAh, Li-Po battery... I can measure the battery's voltage through an analog pin, and I have to switch the servos off when the battery runs too low. The total drain of all servos is about 8-10A, and each servos need 11.1V. So 1 POWER-MOSFET enough for me?

Mamish

11 years ago

I'd use a low-side nmos instead of a high-side pmos. This isn't a concrete rule but they generally have lower on resistance so you get less power dissipation. If you can't find a mos with good enough specs you could always split the servos among a couple of mosfets or even one servo per mosfet. But I am curious why you can't just have the micro stop sending positions to the servos when your battery gets low.

markellamate

11 years ago

Thanks, it's a good idea, that i can stop the microcontroller sending positions to servos... But if the battery's voltage gets low, it isn't damage the servos or the battery? I mean: I don't switch the servos supply voltage off in this case, but i apply a lower voltage to them. It doesn't hurt the servos or the battery?

faceblast

11 years ago

For li-po battery use you'll need to cut power to the entirely assembly, not just servos, to prevent damage to the battery. even tiny currents can kill li-po batteries; leave one plugged into a laptop for a year in storage and it will die.

faceblast

11 years ago

but the microcontroller is by far the easiest component to use for power management, especially lipos. all you need to do is have it sample the battery volts, and shut down when power is close to the low threshold

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