4 -20mA Analogue input help

[sayzer]

In case the ground return is somehow opened, won't this destroy the pic?

Ioannis

Yes, I think so; Most likely it will burn the i/O circuity of the pin.
If not, you get reset.

And, most of the time, people tend to connect sensors without powering-off the reader device. So there is high possibility that what you said is indeed a highly possible scenario.

[CuriousOne]

Accidentally delivered 12VDC to GPIO.5 of 12F683. Port is now dead, but other ports and IC itself work properly.

[Ioannis]

But I do not see an input protection circuitry. Only the 250Ω resistor.

I think better to use a 240Ω resistor, then a series resistor with diodes to Vdd and ground for the least protection.

Better use these:

1. A ferrite ring (say Murata BLM21, 1K@100MHz) in the input wire. Then a small capacitor of 47-100pf to ground. This will give a protection to very short transient input voltages.

2. Then 1K in series and 100pf to ground for a strong low pass filter so a 500V pulse will give a 5V/ns slew rate on the 100pf.

3. Two LND150 with a common resistor of 1K will give a two way current limit of 10mA so the diodes mentioned above can handle the excess current to power supply.

Ioannis

[andybarrett1]

Hi All

Thank you... Thank you ......

I am more confused than ever now.....But all good advice.... I was thinking I was keeping it simple with my 250R and Op-Amp

But lots of good advise here......like the 0-5v to 4.5v Idea.... Never thought :-)

Precision resistor is good ... But again liking the 240R plus Series 10R with diodes

Just swamped...... Thank you Again all

Processing the help

Andy