Constant Photon resets when driving a servo with little load [SOLVED]


#1

Hi Guys,

I am trying to drive a servo with some medium load, but nothing considerable. This reset happens even when attempting to move the servo fast without a load.

I am guessing I am having some EMI or noise issues. I tried using a more powerful power supply, and issue persists. When the servo moves fast or under load, the Photon turns the LED red and then reboots.

This is how the power is hooked up:
— 5VDC —> |SERVO POWER|----> REGULATOR ----> |PHOTON|

The Servo is being powered before the regulator but after the 5V DC wall power supply.
The Servo ground goes to the common ground plane in my custom PCB.

I am attaching the schematics of my board:

The servo is hooked to the 5VUNR net.

This are the headers I use to connect to the servo.

Any help would be greatly appreciated.

UPDATE:
This only happens with a Futaba S9350 High Speed, High Torque Servo. If I use a standard servo when I try to stall it it allows me to stall it without resetting the Photon.


#2

@frlobo, you need to add a large electrolytic caps (470uF or more) on both the input and output sides of the regulator to remove fluctuations and spikes caused by the servos. This is typically recommended in 7805 designs :smile:


#4

Thank you so much… It makes sense! I need to damp the power surge requirement by the servo.
Question: Will this go instead of the C1 and C3? or besides C1 (0.33), C3 (0.1)? I understand I also need to change the kind to a Polarized Electrolytic.

To be honest I found this schematic somewhere and I just copied. I need to study more engineering background. I found a great course from open courseware in iTunes U. That should help tech me how to calculate things instead of guessing.

I am guessing the higher the value the better… Quick question… Would this work?

Thanks again for the help @peekay123


#5

@frlobo, you are getting the idea! The caps go in parallel with the smaller ones. Think of the small caps filtering the high frequency noise and the larger cap creating current “reserves” to handle supply high-than-normal demand (like when the servo starts). Also make sure that the 7805 can handle the max insurge current to the servos.

The Adafruit cap is fine for the purpose. Have fun!


#6

@peekay123 question:

Why does the 7805 need to be able to handle the insurge of the servos, if the servos are powered before the regulator?

The main power supply can take up to 5 amps… I am guessing that right now the voltage drops because the servos uses the power and the 7805 regulator can’t keep up with the input drop and shuts down… It’s only a guess. If the caps can take the power reserve, I guess that it should work.

Also: can I use aluminum capacitors for this porpoises? What are the advantages ?


#7

@frlobo, shiver me timbers, you are correct! Using polarized aluminium caps is the best approach due to their larger capacitances.


#8

Another quick question… I used 470uF and it worked better … Although still some minor resets here and then when trying to move too fast the servo… Using a larger cap would it help?? 1000uf for instance ?? Or is there something more going on? @peekay123


#9

@frlobo, more capacitance is better so you can parallel 470uF caps to double the value! Try that to see if you get better results.


#10

That did the Trick… Il use a 1000uF to be sure… :blush:

Thanks for the tip!!


#11

What’s the spec of the power supply? Servos can draw a serious amount of current and if you’re using a USB supply, it’s likely not beefy enough.

eg: https://www.servocity.com/html/hs-7950th_servo.html specs that this servo will take 3.8 AMPS when stalled.


#12

I am using an external power supply (Bench) it has around 10A of current available. However I don’t know if the speed of response is fast enough to keep up with it. A 800 uF cap did the trick. Even when the servo is fully stalled.

I am planning on using this servo:
https://www.servocity.com/html/hs-805bb_mega_power.html#.Ve5vI7RD1SU

But there is no stalled current spec. Only idle current.

In my final design I am contemplating two 1000 uF capacitors just to be sure… :blush: It should help.

Thanks for the info!


#13

Sounds like your PSU is fine and the issue was likely the inductance of the wires to the load; caps help deal with this, I was just concerned that your supply might not be up to the servo’s requirements.


#14

My final project does contemplate 3 servos, 2 of which might use about an 1.5 amps each… That’s 3 amps. The third servo will not be a high torque servo at all and will be negligible. However I will need to consider at least a 4-5 amp Power Supply… Do you know if I can get this kind of amps in a wall adapter?


#15

You can get wall adaptors in all sizes, they just get more expensive (and usually, when you get above about 5A, the connectors get strange because it gets harder to trust barrel jacks at those current levels). The cabling also gets thick.

An alternative would be to use a 12v, 2.5A adaptor (easier to find, no connector issues) and have an on-board DCDC which drops this down internally to 5v @ 5A+. Extra cost, though.


#16

Thanks!


#17

Does anyone object to me marking this one as [SOLVED] in the title ?


#18

Nope. It’s solved


#19

@frlobo, would you mind posting the schematics image here again, please? It seems they were deleted, and I would like to see where you installed the caps.
Thanks


#20

Check your PM

I installed it in the input to the power of the MCU and the same input to the power of the servo. As already stated it might be an overkill but it works!