Analog Input circuit for 0-5/10v and 0/4-20ma

Thread Starter

jonfarrugia

Joined Feb 19, 2010
71
I've designed an analog input circuit to accept 0-5/10v or 0/4-20ma. User selectable with a few jumpers. The signals frequency will be low, no more than 150hz. This will be used in an automotive environment. I need more eyes on this circuit to see if I missed anything.

P19 in and P18 in will be used for a 0/4-20ma field input circuit. at 20ma, 10v is at R24, a voltage divider between R23 and R21 will provide ~0-3.3v. The Bat54s is used for over voltage and reverse voltage protection.

P19 out and P18 in will be used for 0-10v field input circuit. A voltage divider between R23 and R21 will provide ~0-3.3v. The Bat54s is used for over voltage and reverse voltage protection.

P19 in and P18 in will be used for 0-5v field input circuit. A voltage divider between R23, R21 and R22 will provide ~0-3.3v. The Bat54s is used for over voltage and reverse voltage protection.

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Thread Starter

jonfarrugia

Joined Feb 19, 2010
71
hi jon,
What is the impedance of the load the 5V/10V is driving into.?
P19 1 & 2
E
P19 and p18 are jumpers. The signal from the field is an unknown source... It could be anything that outputs and analog signal.
The processor on the left side will read the 0-3.3v signal. The processor will be a teensy mcu.
 

dendad

Joined Feb 20, 2016
3,987
I would add a 50mA Polyswitch in line with the input terminal. In my experience, someone will hook 24V onto the 4-20mA input!
 

dendad

Joined Feb 20, 2016
3,987
Also, if you put P19 on the other end of R24, a 3 pin link can be used. Off = 0-10V, one end = 0-5V, other end = 0-20mA.

Have you calculated the Rs for a bit over the required ranges? Like 0-6V, 0-12V and 0-24mA?
I did not work them out myself. But you need over range readings to check for errors.
 

Thread Starter

jonfarrugia

Joined Feb 19, 2010
71
I added the 50ma polyfuse. I also changed the selection jumpers. I also needed to lower the resistor value for current mode. I change the resistor value so that the voltage accross it would be 0-5v instead of 0-10v. I needed to change this value so that it would work with the three position manual selection jumpers.

Now if you place a jumper between:
pins 1-2, the analog input is 0-20 ma or 4-20 ma
pins 2-3, the analog input is 0-10 v
no jumper, the analog input is 0-5 v

So with the jumper in the 0-20 ma position, left the voltage divider in the 0-5 v position, so I needed to adjust the current resister to produce 5v across it.

As for your comment regarding over range, I really don't see the point of over range detection. In fact, if I add it, I loose resolution across the range I am most interested in.

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dendad

Joined Feb 20, 2016
3,987
Over range is very important. How will you know if the sensor has gone faulty?
For home use, I s'pose it is ok to go without, but in industrial applications, you need to make sure things are working.
Also, you need to allow for resistor tolerances. So, each device needs to be calibrated and the reading set in EEPROM.
Just make R19a 11K or12K and you get some headroom.
 
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