NANO-10 Analog Input Protection

[garysdickinson]

I've run into a bit of a problem with the NANO-10 Analog Inputs.  I have damaged the inputs on 2 of my NANO-10 boards.

I have attatched a schematic of a possible input protection circuit.  I am looking for feed back.

The sensors that I am working with output a 4 to 20 mA current source.  The 250 ohm resistor converts this to voltage between 0 and a bit less than 5 VDC.  I am aware of the 2K input impedance of the analog inputs.

The 5.1V Zener diode is an attempt to limit the voltage that the analog inputs "see". The Zener is supposed to clamp the voltage to about 5.1 VDC max and if the input voltage goes below ground the Zener will behave like a normal diode and clamp the low voltage to about -0.7 VDC.

The 120 Ohm resistor just limits the current that flows in the circuit and does not affect the accuracy of the measurement (the sensor outputs current and not voltage). The 0.01 uF capacitor is an attempt to bypass transient events to ground.

Gary Diuckinson

[support]

We think your input protection circuit should work pretty well.

We would like to mention that from our knowledge, all of the Nano-10 analog inputs have 20K ohm input impedance and should be pretty well protected against transient voltage applied to its input, such an momentary wrong connection to 24V, or electrostatic discharge.

Are you pretty sure that the Nano-10 analog input on your Nano-10 was damaged? You may want to request an RMA to send them back to TRi to check.

[garysdickinson]

I am very certain how I destroyed the 1st PLC.  I am using inexpensive 4..20 mA signal generators that can, also, output 0.20 VDC. I screwed up and sent 20 VDC into both ADC inputs.  Not too bright on my part.

I tried to be much more careful with the 2nd NANO-10.  Both ADCs worked with 4-20 mA signals. I then worked on refining my digital signal processing software using ADC[1].  I then added in ADC[2] to my test code snd found that ADC[2] was not working.  I don’t know how I damaged the 2nd PLC, but I did.

I have found a 3rd NANO-10 in my collection of PLC stuff. I will see if it works and add the input protection circuitry.  If I can’t get it working I will order another NANO-10.

Regards,

Gary Dickinson

[garysdickinson]

Nano-10 analog inputs update.

I did actually destroy one NANO-10 by connecting both ADC  inputs to 20 VDC.  This seems reasonable but stupid on my part.

The second and 3rd NANO-10s have not been damaged.  I simply wrote bad TBASIC code and the "flailing" ADC was the result of a a coding error.

Just wanted to come clean.

Gary Dickinson

[BetterBath]

I ran into the same issue protecting ADC inputs on soil sensors and solved it with a 10k resistor, 3.6V Zener, and small cap — worked well in practice.
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