Analog Input Calculations

I am working on my first project involving analog inputs and would apppreciate it if someone could verify my math before we make a purchase.

I have a 10k Ohm Pot that needs to control an analog input which requires a 0-20mA signal. I will be using a 12V supply voltage, and a calculated 475 Ohm resistor. The input card has 125 Ohms impedance.

So the circuit goes as follows:

12 Volt Supply
10K Ohm Pot
475 Ohm Resistor
125 Ohm Analog Input
0 Volt Ground


When Pot is off:

I = 12V / (O Ohm + 475 Ohm + 125 Ohm) = 20 mA

When Pot is fully on:

I = 12 V / (10000 Ohm + 475 Ohm + 125 Ohm) = 1.13 mA


Does this sound like it will work? Are there any caveats I should be aware of? Thank you for any imput!

P.S. I am not an electrician, but a CAD designer thrust into the world of PLC's.

We use something very like this for testing panels and logic. It will work, but there are a couple of cautions:

1) the actual signal will be very sensitive to your power supply regulation. If you have a 10% dip in power supply voltage, you will get a corresponding dip in the analog signal even if the pot position is unchanged

2) if you have bad connections or long wire lengths your signals will also be affected.

3) EMI and RFI might be a problem (but probably not if you are staying inside a control panel.

If you want an extremely robust and accurate system, you should probably invest in a signal conditioner that converts the potentiometer to a 4-20 mA signal (M-Systems, Action Instruments, A-Pi etc. etc.) Remeber that a true potentiometer transducer works off the diferential voltage between the wiper and the two legs. That makes it more repeatable and stable than your circuit, which is actually a variable resistor not a potentiometer (picky difference, but true anyway).

I would also reccomend using a signal conditioner and 4-20ma signal.
It will save you a lot of time and trouble, with the plus that these things rarely fail.

Just saw one from Pheonix Contact yesterday, but there are plenty of other companies with these things.

Hope this helps,

Doug

calcs

yes i use same setup only as i have 24 volts on plc i use that power supply with pots of 20kohm but hey that is same.
i use it a lot for simulating transmitters to check borders and limits.
i only not use 475 ohms but 1000 ohms for series as now i will get a little more as 20 mA input which gives me the control at sensor break.
and yes had to go back to store as the resistors burned out on me as the use 400 mWatts. but one size bigger was fine.
same is for potmeters you have to watch the current in these as when current is high dissipation is only on very small part of potmeter.
just try it and look for smoke.

paul deelen
computer shooter

Thanks everyone,

The input was much appreciated and I am following up on a few leads. One question, though. Why would analog inputs be designed to read changing *currents* when most analog devices I have seen specify a *voltage* range as their output signal? Just curious...

Brad

Why would analog inputs be designed to read changing *currents* when most analog devices I have seen specify a *voltage* range as their output signal?

Click to expand...

Brad,

just consider this: if you have a sensor with an internal resistance of 100 ohms at a distance of 100m with a poor quality cable of 0.5 ohms per meter. If thid sensor gives you a range of 0-10V, what do you measure at your analog input? Reconsider with a current source sensor of 4-20ma. See the difference?

Also take into account that voltage measurements are also more influenced by noise than current measurement.

Kind regards,

Jean Pierre Vandecandelaere

First - It doesn't cost much for the PLC manufacturer to provide an analog input module with both voltage & current inputs. Depending on the manufacturer this is handled via DIP switches, jumpers, or extra terminals to add a current sense resistor (typically 250 ohm) across the voltage input channel. This answers the first part of your question.

Second - It sounds like the second part of your question is "when would you use a sensor with a voltage output vs a current output?"

This is really up to the application engineer. Typically, 4-20mA devices are used because they are immune to line loss effects of long wiring runs. They also have a much higher noise immunity than a voltage output. Downside? Depending on how you terminate the senor, it can be difficult to trouble-shoot for a maintenance tech since they have to 'break open' the loop to install an ammeter to check the signal, assuming that you haven't provided those fancy terminal blocks designed for 'on the fly' current measurement. If your analog input module is based on a +/-10v input range and doesn't support individual channel configuration, you give up a lot of available resolution i.e. only using 1 to 5 volts out of a +/- 10 volt span.

Sensors with voltage outputs are generally easier to work with for a general maintenance tech using a simple voltmeter, and the sensor can also directly interface to a number of other devices simultaneously.

Most sensors are available with either voltage output or current output. Some sensors are strictly passive devices i.e. just a passive wheatstone bridge and produce a millivolt output which requires an external signal conditioner. Why would you use this? If you have an extremely harsh environment (i.e. temperatures above 200F) the passive sensor without electronics can survive where as the active device (voltage or current output) might not survive. Your signal conditionor module then resides in the control panel where it is protected from the harsh environment. I'm sure some of the other experts here can chime in with their thoughts.

Very good question for a newbie! It shows you are looking beyond just the PLC aspect and looking at the whole picture.

Thanks Jean and Greg,

That makes a lot of sense. Unfortunately, most of my electronics experience so far has been bookwork and theory, so I often miss a lot of the practical implications of "the real world" ... but I'm slowly learning.

Brad

Here is a spreadsheet that may help a little

One other advantage of 4-20 mA over a 0-10 VDC transmitter is the "live zero". With a voltage output at 0 VDC you don't know if your sensor is dead or your process variable is really zero. With a 4 mA signal you know your process variable is zero. With 0 mA you know your signal wiring is open or your transmitter is dead.

Reading current for maintenance

One way to measure current without breaking the loop would be to install a diode and run the 4-20ma signal through it. Connect your meter across the diode and the current goes through the meter instead of the diode. Also, most loops have a resistor installed such as a 250 ohm so you can measure voltage across it.

Great tip Andy, but alas, paulc20 has beat you to the punch on that one... bonkhead

See THIS POST

As Terry would say, "Neener, Neener, Neener!"

Since you're new here, we'll let you slide on this one...

beerchug

-Eric