# LM339 differential voltage?

I am making a bldc controller and i am going to use a comparator to check the back emf of every phase i have chose this LM339 because i have used this in previous projects too but its maximum differential voltages are 36 and i want to make a controller for 48 voltage bldc motor, will it work? Because emf will come for like a second or so it won't be constant i think, and if not should i look at some other comparator or should i just use voltage divider?

Just use a voltage divider to keep the input voltage within the common mode input range for the comparators. For LM339, that will be from 0V to a couple of volts from the positive supply.

dl324:
Just use a voltage divider to keep the input voltage within the common mode input range for the comparators. For LM339, that will be from 0V to a couple of volts from the positive supply.

from 48V to 5V is right ? is the back emf of a motor is always equals to the provided power supply?

Mark248:
from 48V to 5V is right ?

Only if the positive supply to the comparator is at least 7V. The upper range of the common mode input voltage is about 2 volts from the positive supply. Details are in the datasheet, but that's my recollection.

is the back emf of a motor is always equals to the provided power supply?

No. It can be significantly higher. Does your drive circuitry already include a snubber (flyback, back EMF) diode?

dl324:
Only if the positive supply to the comparator is at least 7V. The upper range of the common mode input voltage is about 2 volts from the positive supply. Details are in the datasheet, but that's my recollection.No. It can be significantly higher. Does your drive circuitry already include a snubber (flyback, back EMF) diode?

No it does not have a flyback diode as i need BACK EMF in order to have my algorithm know the position of rotor.

Mark248:
I am making a bldc controller and i am going to use a comparator to check the back emf of every phase i have chose this LM339 because i have used this in previous projects too but its maximum differential voltages are 36 and i want to make a controller for 48 voltage bldc motor, will it work? Because emf will come for like a second or so it won't be constant i think, and if not should i look at some other comparator or should i just use voltage divider?

No, it will burn up with 48V.

What you need to do is scale the voltages down with resistive dividers, then you can use a lower supply voltage for the LM339, if that makes things simpler.

Mark248:
No it does not have a flyback diode as i need BACK EMF in order to have my algorithm know the position of rotor.

You need to do something to limit the voltage to the comparator. Post a schematic of your circuit.