Im an electronics engineer student, and im doing a project where im designing a circuit which will be used in automotive applications. More specific it contains a PIC controller which has 16 digital/analog inputs. It also has 16 outputs which will control 24V devices. It also has CANbus to connect several of these devices together.
Im having a problem grasping what kind of Transient voltages the circuit might be exposed of. Since this circuit might be used as a data collector e.g.temperature sensor read or as a controller in which an operator can press buttons that control another unit through canbus, I imagine that there might be different Transient types I need to consider. Many TVS devices state conformity to IEC 61000 standards. My initial plan was to use a TVS array as close to the input connector terminal on pcb. But as Ive read further Im really uncertain whether I can use TVS arrays. Most of these arrays are meant for High speed data lines and not MCU GPIOs. The requirement of the input lines are 5V,4-20mA. The MCU can handle a maximum of 5.3V,25mA on each pin. But these TVS array are meant for very small currents(?). And Im also afraid that they might disturb ADC readout(?).
Example of ESD event I can think of: Operator steps out of vehicle and carries a charge. Does not discharge until he touches buttons on a controller which might be inside a plastic chassis.
Im not the one designing the button panel, but I should be able to provide recommendations which will protect the controller. Which makes me think whether I should recommend protection on each button instead?
Im using a Can transceiver which isolates the Bus side from the MCU side. Im also using a DC-DC converter to power the 5V devices which isolates up to 1500V which should be enough protection against power surges coming from the 24V battery on the truck?
If someone could help me down the right path I would be very grateful.
Ive tried to elaborate my problem as good as I can. Hope it is clear.
Best Answer
A 4 - 20mA current loop would normally be sensed, not forced into a controller pin (which is how I am reading your post).
TVS arrays for high speed data lines are very high performance because they are extremely low capacitance, and they should work fine for MCU pins, but you have the option of using lower speed devices, so the first thing is to define what you need to protect and from what (which is a bit hazy in your question) and then choose an appropriate device.
It is difficult to give further guidance on ESD protection without further details (a system level architecture diagram, or even better, a schematic with the expected sort of interfaces (there is a schematic editor widget in the edit toolbar).
I do not know what country you are in, but the latest international standard for unswitched vehicle applications is ISO16750, which should give you sufficient guidance on whether you have taken the correct precautions.