Hi,
I accidently destroyed the controller board inside of a MG996R 360 servo motor. The motor still works, and the servo is rated for 4.8 to 6v, maximum 7.2 . I was wondering if i can feed the servo more like 8,5 or 12 v? If i use it with a mosfet, would i need a diode?
thanks
No
It should be ok for 12v
What have you got to lose
If you mean you just want a motor and gearbox not working as a servo then you might get away with higher than standard voltages.
But the main problem is the current and that depends on the load on the motor. So it may work just sitting there not connected to anything but then die in seconds if you actually try to drive anything with it.
Steve
Yes sorry, i meant i would run the DC motor alone, not as a servo. I'd connect it with a mosfet to control it from a Leonardo.
What do you have to lose? Try it. Sometimes making mistakes is what it takes us to learn.
1 Like
Maybe you should do the conversion from
servo operation to gear motor operation?
If you run just the motor from a PWM output , set the maximum output at say 50% , then the average would be 6v with a 12v supply and should work fine .
herbschwarz:
Maybe you should do the conversion from
servo operation to gear motor operation?
Not sure what you mean there.
wolframore:
What do you have to lose? Try it. Sometimes making mistakes is what it takes us to learn.
Actually the motor as it is is still usefull for my project, because i don't need it to spin in both directions.
Before i could only get 1.1A draw (stall current) on 5v and 1.6A on 7.4v, now feeding the motor directly i got 1.8A at 5v, so i think i can get 2.5A (which is the maximum stall current according to datasheet) at 6 or 7.2v
But i heard that when using motors with mosfet i have to add a diode because of the magnetic effect of the motor?
It seems that at least some servos have a mechanical stop on
the main gear that prevents continuous rotation per
Servo and Accessories.
Also, www.instructables.com/id/Converting-Micro-Servos-for-
Continuous-Rotation.
Herb
I believe the OP posted MG996R 360
Which is a 360 degree servo meaning continuous rotations
And yes a dc control using a mosfet will require the use of a fly back diode, or use an H-bridge if you want to reverse it also.
Specifications
• Weight: 55 g
• Dimension: 40.7 x 19.7 x 42.9 mm approx.
• Stall torque: 9.4 kgf·cm (4.8 V ), 11 kgf·cm (6 V)
• Operating speed: 0.17 s/60º (4.8 V), 0.14 s/60º (6 V)
• Operating voltage: 4.8 V a 7.2 V
• Running Current 500 mA –
• Stall Current 2.5 A (6V)
• Dead band width: 5 µs
• Stable and shock proof double ball bearing design
• Temperature range: 0 ºC –
4.8 V a 7.2 V
– 900 mA (6V)
double ball bearing design
55 ºC
Yeah, go for it. What could possibly go wrong and specifications are written for the less that brave. Seriously? I wouldn't do it but hey, it's your motor.
Ron
hammy:
If you run just the motor from a PWM output, set the maximum output at say 50% , then the average would be 6v with a 12v supply and should work fine.
And that is clearly not the case! 
Feeding something double the voltage implies quadrupling the power dissipated. So a PWM of 50% at twice the voltage means twice the power dissipation.
Or would be if it was not - a motor! 
Realise that controlling a motor with PWM is extremely inefficient, the above proposal, if used to operate the motor at its full speed would cause substantial overheating. A motor's speed is governed by its back EMF. If you are running the motor at the speed corresponding to a back EMF of somewhat less than 6 V and powering it with bursts of 12 V, then the extra 6 V is being dissipated in the winding resistance and will not be contributing to the motor work. During the 50% of the time it is powered, the current will be far more than twice what would be drawn at 6 V (because it is limited mainly by the winding resistance and not the back EMF) and the motor would be overheating badly.
amadeok, the people who are suggesting you just try operating a 6 V motor on 12 V are "pulling your leg".
Paul__B:
And that is clearly not the case!Feeding something double the voltage implies quadrupling the power dissipated. So a PWM of 50% at twice the voltage means twice the power dissipation.
Or would be if it was not - a motor!
Realise that controlling a motor with PWM is extremely inefficient, the above proposal, if used to operate the motor at its full speed would cause substantial overheating. A motor's speed is governed by its back EMF. If you are running the motor at the speed corresponding to a back EMF of somewhat less than 6 V and powering it with bursts of 12 V, then the extra 6 V is being dissipated in the winding resistance and will not be contributing to the motor work. During the 50% of the time it is powered, the current will be far more than twice what would be drawn at 6 V (because it is limited mainly by the winding resistance and not the back EMF) and the motor would be overheating badly.
amadeok, the people who are suggesting you just try operating a 6 V motor on 12 V are "pulling your leg".
Hi, thanks that's very informative. What if i used a DAC on the mosfet gate?
DAC to mosfet is not the way to use them. There’s a Vgs threshold you have to meet and using DAC sounds like you will be in Ohmic region or you’re still switching high and low with no benefit to using the DAC.
If you have a broken servo just test it on 12V and see what happens, if it burns out what have you lost.l. Just check and see if it gets hot and unplug it if it does. I’m guessing that the 6v limit was for it’s internal control board. The motor might be fine at 12v but no way of knowing without testing. It should be much faster.
amadeok:
What if i used a DAC on the mosfet gate?
You could do that but the concern is that you will be dissipating a lot of power using the FET in effect as a variable resistor.
My recommendation is that you accept that it is a 6 V motor (because if it is in a servo that is designed to operate on 6 V, then that is what the motor is designed for) and operate it on 6 V.
Good point about the power with 12v pwm - I got that wrong by the sounds of it