D24V10F5 causing erratic servo movement

Support Voltage regulators
1

For my project I’m trying to power a generic 4.8-6V RC servo (link here) through a D24V10F5 5V buck regulator, itself powered by a 12V wallwart.

I’ve ensured the following:

  • The servo and Arduino Uno i’m using to control it share common ground.
  • A 44uF cap is between Vin and GND on the 5V reg to prevent LC spikes
  • A 100uF cap is between Vout and GND on the reg to stabilize output voltage
  • The reg pushes a max of 1A while the servo was tested to draw only 300mA peak at 5V.
  • The 12V wallwart outputs max 1.2A.

I tried running a simple turn and return code as below:

#include <Servo.h>

Servo thisServo;

void setup() {
  thisServo.attach(10);}

void loop() {
  thisServo.writeMicroseconds(900);
  delay(3000);
  thisServo.writeMicroseconds(1800);
  delay(3000);
}

this resulted in extremely erratic motion. From what I can observe the servo will hit the 900us position and immediately and rapidly oscillate between 900us and 600us for 3 sec, then move to 1800us and oscillate between 1800us and 600us for 3 sec, and continues looping like that. 600us is roughly the position of the clockwise stop for this servo, and telling the servo to hold any value seems to make it oscillate between that and 600us.

I tried two suboptimal workarounds that appeared to work:

  • Plugging the servo directly to a 5V wallwart works perfectly.
  • Curiously, extending the positive lead of the servo via a 60cm crocodile wire seems to work as well; the longer the wire, the less oscillations are observed. I discovered this when testing the current draw with a multimeter.

I feel like i’m missing something frustratingly obvious here, but i don’t know what, and unfortunately I don’t have an oscilloscope to test this for myself. Does the D24V10F5 generate a lot of EMI which interferes with the servo PWM?

2929bad3-13e8-4f71-90c8-d74155327a04
2929bad3-13e8-4f71-90c8-d74155327a041600×777 245 KB

2

I just ran my setup through an oscilloscope.

I don’t think there’s any EMI on the PWM signal since it still looks clean during operation:

PWM
PWM1287×940 49.2 KB

But apparently the D24V10F5 was failing to deliver a stable 5V to the servo:

Vout
Vout1295×944 56.4 KB

This was measured while the servo was tasked to hold a constant 900us position. It constantly and rapidly oscillated around the target position.

Is this a product defect or am I doing something wrong?

3

Hello.

A typical servo of that size can easily draw short bursts of current in excess of 2 A, which would be well beyond what that 1 A regulator can supply. How were you measuring the current when you concluded it was peaking at 300 mA? Can you look at the current draw on your scope and post some captures of that, both when it is powered from the regulator and when it is powered directly from your wall wart?

-Patrick

4

Solved the problem by getting a D24V50F5 instead. It seems the servo was drawing more current than a D24V10F5 can handle.

A bit confused why a 5V 1A wallwart works ok though.

5

I expect if you just look at it with your oscilloscope, you will get a better understanding of why you didn’t have any issues with the wall wart. (Probably the voltage drops were not as pronounced.)

-Patrick