There are a couple of possible reasons i can think of:
- Does your relay board has an optocoupler? its important to keep the output eletrically isolated from the relay, it draws way too much current for the output, and its a good idea having an diode to protect against switching surges (reference
https://resources.pcb.cadence.com/blog/ ... protection)
- Can you check if the AMS1117 (voltage regulator) is working? maybe the relay draw too much current for it, usually when it burns you can see a small spot on top of the component. Servos and relays are inductive and draw a lot of current when turning on, this could have killed the AMS1117. I always like to use an eletrolityc capacitor in parallel with the servo. If this is the case, the AMS1117 is very cheap and you can just replace it.
An altenate solution to the relay is to use an transistor (i like to use logic level mosfets). this will save power compared to the relay, be quieter and last way longer. (add the optocoupler to drive the mosfet). This video can give a good reference:
https://www.youtube.com/watch?v=vudSUDFWoQE
Try checking if when you kill the PWM output the servo goes to idle, if it does, you wont need any additional circuit, just use detach (if you are using the arduino library) or disable the PWM output.
Tip about noise: i had noise issues in the past while working with 3DPrinters and a long cable, if i remember correctly i fixed it with an 10nf or 100nf ceramic capacitor very close to the servo, from the PWM signal to ground (or vcc, its been too long)
