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Looking for a very good industrial digital inputinterface

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Junior Member level 2
Mar 27, 2002
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For a project, which shall run in industral environment (high noise and EMC) i have to design a very good switch closer input interface for a microprozesor.

It shall be very low cost.
Also the input voltage range need to be 24V (20-28V)

And it need internal pull up against 24V that with a external switch a low potential (GND) can be used.

The output shall be 5V for a microcontroller.

I have already tried out several different solutions but any haven't solute my noiseproblem.
I can't used optocopling solutions :-(

I have used a standard pullup solution (4,4K against 24V) wit a length resistor 10K protected with s zenerdiode 4,7V (with additional ceramic C's)
Also the very cheap solution with the diode in length and 2 different pull-up potentials (with ceramic C's).

Have anybody a really good ( or the best )idea?

Regards Max :lol:

The main problem it's not the circuit but the ground loop which is the pathway for noise to disturb the MCU. One solution it is to have 2 power source, with only one point of conection between the two common grounds, at the digital input board. Close your electronic completelly in a metal case grounded, and design your input PCB with at least 1 ground layer. Use fast clamping diodes to ground and Vcc at the input (BAT46 for example), not the usual zeners. Design your inputs with lower impedance (to draw around 10 - 20 mA).
Good luck and welcome to the EMC club :)

Nice idea.

But there are the next problems.
My power is driven by a 3 phase 400 Volt AC source without Middle (N)
with this source a transformer is feeded to generate 230V ac potential free.
This 230V is transformed by a transformer on my PCB to 18V AC.
18V AC to 24V DC then via linear regulator to 5V DC for the Prozessor.
So no ground to shield and also no case is available :-(((

It is now the question is it common or differental noise.



You have to ground to earth the equipment (case) by regulation, even if the 3 phase power is without neutral. (is the neutral of the main transformer isolated, or just the power cable have only 3 wires?)
You can still conect your minus from the 5 v power supply to this ground, and add a second 24 V ac transformer for the digital input power supply. The idea is to separate the (presumed) long cables of the digital input from your logic, and to conect them only in one point that is conected also to the ground. The voltage of the equipment conected to earth is low (below 24V) and it depends from EC standards if it's OK or not.

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