4 -20mA Analogue input help

[andybarrett1]

Hiya

Just a quick one ....need to interface a 4-20mA signal to a pic

I know I can do it with probably a 250R resistor and Opamp LM358

Wonder if anyone done anything similar

Or any pointers / pitfalls

Thank you for reading

Andy

[MOUNTAIN747]

Andy, a 250 precision Res is what I always used in the field for years. Just check the voltage across the res with the pic. I would think it depends on the accuracy you need.
wayne

[Ioannis]

I'd recommend 0.1% or best and also an input protection for overvoltage on the PIC side. If for any reason the ground side of the resistor is disconnected, it is certain that the PIC will be destroyed.

Ioannis

[sayzer]

If you will use 10-bit ADC to read the voltage on 250R resistor, why would you need an opamp?250R will give you 5V at 20mA max.and, 1V at 4mA lowest.Your ADC reading will be from 205 to 1023.And remember, most 4-10ma sensors provide lower than 4mA and higher then 20mA; Mostly, 2mA to 25mA is the field.So you may want to keep your voltage range between 0.5V and 4.5V. I would not go for 5V at max mA. You should make it 4.5V at 20mA. So if it goes higher than 20mA you still have room to detect it.

[Ioannis]

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

Ioannis

[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