I am working on a mobile robot and have damaged 2 RPis as a result of what appears to be a transient over-voltage situation. Please see the attached drawing of the basic setup. The failure is not repeatable so its tricky to figure out what exactly is causing it (after the first failure, I changed the DC-DC converter). Is there anything that appears to be missing? The only common action between the two failures I can recall is the e-stop button is used. The motor controller from Roboteq (FBL2360) allows regeneration but it has been capped at 35V which the DC-DC converter is able to handle. These over-voltage situations also damage the USB devices connected to the RPis and there are some expensive components so any recommendations to further protect those? Here's the DC-DC converter
Suppose the relay interrupts the motor current, 20 amperes, in 20 nanoseconds.
Suppose there is a wire running from the relay to the motor controller, and that wire couples into the RPi regulated power; the wiring between regulator and RPI forms a loop with VDD and RTN. <- EDIT
Suppose the distance from wire to loop is 1cm. Suppose the loop area is 1cm by 10cm (4"). What voltage is induced, on top of the regulated RPi VDD?
$$Vinduce = [MU0 * MUr * Area/(2 * pi * Distance)] / dI/dT$$
What is the induced voltage? anything higher than 1 volt transient is bad news. Now insert value for MU0 of 4 * pi * 1e-7
Vinduce = 2e-7 * area/distance * dI/dT
Vinduce = 2e-7 * 1cm*10cm/1cm * 1amp/nanosecond
Vinduce = 2e-7 * 10cm * 10^+9 amp/second = 2e-7 * 0.1 * 1e+9
Vinduce = 2e+1 = 20 volts induced
To avoid this (e.g. reduce the induced voltage from 20 volts to 0.2 volts),
(1) keep the MCU PCB a meter away from the high-current fast-off motor wires (hot and return wires).
(2) place a steel plate adjacent to the MCU PCB, insulated to prevent shorts; have the plate be larger than the MCU PCB
(3) insert a PI filter (C-L-C) at the MCU PCB, as shown
The one Ohm across inductor is for dampening. Rdampen = sqrt(L/C) =sqrt(1mh/0.1mF) = sqrt(100 = 3.16 ohms ideal. The 1 ohm is over dampening.