Turning Off Servo Power While Not In Use?

Thread Starter

liamneeson248

Joined Jan 11, 2016
36


I am using a servo in my project to turn from 0 degree to 135 but it keeps making sound and consuming ampere when on 135 and sometimes on 0 too, so i just want to design a transistor switch kind of circuit to turn off servo when i don't need it and turn it back on 3 second before i send my PWM signal into it, so the circuit that i have drawn here is it right ? or not.
 

djsfantasi

Joined Apr 11, 2010
8,563
RC Servos continually check to see that they’re in the commanded position and automatically correct if that position has drifted.

Without power, the axle of a servo can rotate. The degree of rotation depends on the mechanical coupling to its load. So, if you were to remove power, you may find the servo ends up drastically different than what you last set it at.
 

Thread Starter

liamneeson248

Joined Jan 11, 2016
36
RC Servos continually check to see that they’re in the commanded position and automatically correct if that position has drifted.

Without power, the axle of a servo can rotate. The degree of rotation depends on the mechanical coupling to its load. So, if you were to remove power, you may find the servo ends up drastically different than what you last set it at.
RC Servos continually check to see that they’re in the commanded position and automatically correct if that position has drifted.

Without power, the axle of a servo can rotate. The degree of rotation depends on the mechanical coupling to its load. So, if you were to remove power, you may find the servo ends up drastically different than what you last set it at.
that is not a problem for me as on 135 degree it doesn't or can't move because of the design of my project i just want it to stop making noise and stop consuming ampere, so if there is any safe way to do that?
 

djsfantasi

Joined Apr 11, 2010
8,563
that is not a problem for me as on 135 degree it doesn't or can't move because of the design of my project i just want it to stop making noise and stop consuming ampere, so if there is any safe way to do that?
Ok.

In my opinion. If the current increases and it makes a noise, it’s moving.
 

jpanhalt

Joined Jan 18, 2008
11,088
Whether it will hold depends on the type of servo. A coreless servo will not hold. Just the minimal weight of a control surface will sage it to the stop.

A more serious problem is that the Drain or your switch is attached to ground of the 3-lead plug. How do you expect the mosfet to do anything?

If you want to stop the humming at stop, reduce your throw and/or get rid of binding. Turn off the signal. When signal is reconnected, you will likely see it jerk a little.
 

djsfantasi

Joined Apr 11, 2010
8,563
A more serious problem is that the Drain or your switch is attached to ground of the 3-lead plug. How do you expect the mosfet to do anything
Huh? The mosfet is switching the ground lead of the servo. Thus cutting power to the servo when he wants. The drain isn’t connected to ground. Or did I miss something again?

The schematic is a little confusing. The 8V line is duplicated in several places. In effect, the diode parallels the cap and is placed between the servo power supply pins. And the 8V line simply goes from the power source to the servo positive pin; the mosfet drain simply goes to the servo negative pin.
 

jpanhalt

Joined Jan 18, 2008
11,088
I missed, actually imagined, something. I was thinking the system had a common ground for the servo, servo driver, and switch (that is, the servo had a path to ground without the mosfet). Obviously, I just read it wrong. Thanks for pointing that out.

John
 

Thread Starter

liamneeson248

Joined Jan 11, 2016
36
what will be a good way to keep it from buzzing? and even if we forget about the buzzing and i just want to turn the power off the servo should the mosfet works ? i have seen people on the internet using mosfet for both low side switch and high side switch.
 

jpanhalt

Joined Jan 18, 2008
11,088
As previously stated, "Whether it will hold depends on the type of servo. A coreless servo will not hold. Just the minimal weight of a control surface will sage it to the stop," and "If you want to stop the humming at stop, reduce your throw and/or get rid of binding. Turn off the signal. When signal is reconnected, you will likely see it jerk a little."

Let me add that expensive servos with a very narrow "dead band" (i.e., ≤ 3 or 4 us) may tend to hunt a little more than those with dead bands of 6 to 8 us. Of course, a lot of that is dependent on the driver you use. With an MCU and a very clean pulse, I have run digital servos that go between assigned positions, stop, and don't buzz. With a cheap/simple, single 555-based controller, I have had digital servos become almost uncontrollable.

So, look at your servo hook-up, linkage, and the way you are driving it. Do not drive it to a physical stop. In this example (sorry for the small picture), the digital servo is driven to a fixed location, but note that the latch is spring loaded. That servo sits for hours without changing positions and remains under power the whole time. It does not buzz. The driver is a simple PIC12F509.

upload_2019-3-23_6-24-53.png

The servo is obscured by the steel brace. You can see its two mounting screws. The servo horn has a pin attached to it that lifts the lever.

EDIT: I added a better picture. The rust is because this thing is over 10 years old.
 
Last edited:

djsfantasi

Joined Apr 11, 2010
8,563
I thought I had mentioned a technique that might work in post #4 (don’t look; I must’ve forgotten to add it).

As jpanhalt mentioned, a servo with a larger dead band may solve the problem. He has given you several other parameters to check.

Or you can experiment. Perhaps another position near 135° may still work for you without the servo buzzing. For example, try a range of values from 130°-140°.
 
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