Low voltage Cutoff circuit

[Kentucky]

I'm looking for a simple LVC circuit that I can use to run a DC motor (speed400) directly from a Lipo, prob 2S (no ESC).
I would like to use it for an electric control line model.
Basically the motor just needs to run flat out with a LVC to save the battery.
Found this circuit on the net but have no idea what the current rating would be.
Any ideas?

[Kernwrech]

The power dissipation of the TO-220 housed IRF510 is 43 watts. This means that as 7,4 volts you'd have 5,8 amps of continuous current. It depends largely on your cooling how much amps can your FET take...

[jeffs555]

Using that IRF510 mosfet, the current rating would be very low. Using a better mosfet, you could get the current rating you need. You need to look for a logic level N-channel mosfet with a low RDS(on). The FDP8874 is one example, but there are hundreds of others that would work.

[jeffs555]

The power dissipation of the TO-220 housed IRF510 is 43 watts. This means that as 7,4 volts you'd have 5,8 amps of continuous current.

The power dissipated in a Mosfet does not depend on supply voltage, only on the voltage dropped across the mosfet. The voltage dropped across the mosfet is equal to the current times RDS(on), so the power dissipated is calculated by multiplying RDS(on) times the square of the current.

[Dan Baldwin]

The IRF510 is not a good candidate for controlling a speed 400 motor. It has an RDS(on) of about .4 ohms, and an absolute maximum continuous current rating of 5.6 amps. I have used the STP55NF06L in some of my circuits. It has an RDS(on) of about .018 ohms, an absolute maximum current rating of 55 amps, and a gate turn on voltage of 1.7 volts. It's $1.17 from Mouser electronics (Mouser part number511-STP55NF06L).

The problem that you will probably have with the circuit you posted is that it has dosen't have hysteresis, so when it shuts the motor down, the voltage on the battery will rise, and it will turn the motor back on. It may even operate in linear mode meaning that it will partially turn the transistor on when the battery pack gets to the turn off voltage. If the transistor ends up running in linear mode, it will get EXTREMELY hot.

Dan

[jeffs555]

Dan makes a very good point about the hysteresis and the output going into linear mode. You could solve those problems by connecting the anode of D1 to the drain of Q3 instead of ground. This would provide a hard cutoff without any restart. You would then need a momentary switch across Q3 to start the motor and arm the circuit.

[vintage1]

You can achieve the same by adding a resistor from theQ2 collector to the base of Q1.

No need for a switch.

Depending on the resistor value, it will drop out at some given voltage, and come back at some slightly higher voltage.
I'd divide all the resitor values by 10 as well..they are ridiculously high.

[jeffs555]

For his application, I would think a hard cutoff with arming switch would be very desireable. I can just picture the battery recovering enough to overcome hysteresis just as he was walking up to the plane.

At the bottom of this page is a 555 based circuit. It is for car battery discharge protection and uses a relay, but could easily be modified to use a mosfet. http://www.barneymc.com/lite_wir/lite_wir.htm

[Chippie]

I'd divide all the resitor values by 10 as well..they are ridiculously high.


Hmmm....I thought that too, especially the 1meg resistor....

[Kentucky]

Thanks for all the input guys.
Lots of good advice and plenty of very helpful folks here at rcgroups. :D
Hard cutoff without restart is fine (just like when that glow motor runs of fuel)
Delayed startup would be a great addition, just enough time to walk over and pickup the handle :rolleyes:
I'll start putting something together soon, just need to dig out those old brushed motors. :)

Thanks again.

P.S this circuit was never intended to run a DC motor, something I found on the net, a good starting point maybe?

[z-matrix]

use an irfz44/irfz48 that can switch and hold 10-12A current without any additional heat sinking.

but take care the oscillations somehow, give larger hysteresis voltage, or use an RS flip flop.

(high turn on-off frequency will result in the mosfet switch explosion)
additionally connect a schottky diode anti-parallel with the motor, or else the fly back voltage at under voltage lockout will avalanche the mosfet switch.