Matrix user forums

Hi guys,
I'm re-visiting a project and want to design in robust input over voltage protection.
The inputs are mostly for slow digital switched inputs in an automotive environment.
As ever..dont want to go crazy and over the top of parts (cost, board space and time of assembly) but dont see the point in making something that will be poorly protected.

The basic starting point is.
Microcontroller with a Capacitor to ground, series current limiter resistor and pull up resistor to 5V supply.

I would like to clamp both constant and transient/ESD over voltages, thus looked at adding a TVS and/or Zener.
Do I need both? Looking at a 5V TVS specs ( for microcontroller inputs) they have a breakdown voltage (start of voltage clamp) at 5.6V and full voltage clamping at nearer 8V. This is obviously way over the Max voltage limit of the microcontroller inputs @ Vcc+0.3V (so around 5.29V)

If I want to do this properly, do I need to run the TVS to deal with the high voltage short duration spikes/ESD, then use a Zener to bleed of any constant DC voltage over ~5.2V.

Any help appreciated.
Regards,
J.

Hello Jay,

You can do something like this to give fairly good input protection.

input voltage -> series resistor -> diodes -> series resistor -> input pin

The series resistors can be between 200R and 10K depending on how much current you need to go through to the input pin. INT pins need a bit more current to allow the Interrupt to work.

The diodes in the input chain shown above refers to a diode connected between the signal chain (A1) and VCC (K1) and another diode connected between the signal chain (K2) and VSS (A2). This allows any negative or excess positive voltages to be fed onto the supply rails which should have enough capacitance to overcome any large spikes from getting through to the microcontroller. These diodes could be standard Schottky rectifier diodes.

Thanks Ben,
So a Diode clamp will be sufficient? I guess that advantage of using Schottky diodes is a fast responce and a tight clamping due to the small forwards voltage drop?
I have added a capacitor as well since since I wanted the resistor/capacitor combination to give some hardware filtering for switch bounce.
Is there any reason to use this twin diode clamp as opposed to a single zener?
J.

Hello,

A single zener requires a certain amount of current before the zener will start conducting and protecting your circuitry, this should be a bit lower with a schottky.
Also a zener will not protect against any negative voltage spikes and can effect analogue readings near the top of the scale.

I've had some bad experiences with Zener's not working as expected and I know the clamp approach works very well. Saying this a Zener would probably do the job if you don't want to go for the full clamp approach.

Cool,
So my general purpose digital input circuit is looking like this; I've added a pull up R3 to keep my input logic stable, I'm thinking something weak like 100k ?
R1 @ 10K
R2 @ 1K
The Switch will pull R2 to ground.
R2 should be low enough to ensure it can pull down both R1 and R3
D2 and D3 will be Schottky diodes.
The R1 and C1 combination will determine the severity of the switch denounce.
This looking about right ?... my resistor values are bit of a guess.
Thanks,
J.

Hello,

That's looks great. I would maybe rig it up just to be sure there aren't going to be any problems but that should give you very good input protection.

Thanks Ben,
Top support.
J.