Reading different hardware with 1 AI channel 1

[guilio2010]

What device would I need where I can use a digial output to select what device I want to read into a single AI channel and then switch to another device to read into the same AI channel? The DO is 3.3V and 4mA and I was thinking that using a ssr that can work off that would work, but unsure if that would be the right thing ot use.

 

[itsmoked]

You need to look at devices called ANALOG SWITCHES or MULTIPLEXORS.

A classic example is the HC4051. If you really mean a differential analog in you would look at the 4052 instead.

Randomly grabbed example of an HC4051

There are lots of different types and some gotchas that need to be analyzed.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[danw2]

If you only have two signals then use the contacts of a DPDT relay to switch either signal to your AI.

Your digital output signal drives the coil of the relay.

I don't know what relays are available with coils rated at 3.3V4mA but if pickings are slim, drive an SSR and use its output to drive the coil relay.

 

[guilio2010]

I'm looking at more than 2 actually. The multiplexer is what I was thinking just forgot the name but I'm looking for something already built that I can use and not have to built/design it myself. Din rail mount, fast switching, and if it's 16 to 1 that would be ideal. I seen a few, like the omega but not sure about the performance.

Also, are there combiners that will take 4-20, 1-5, and dio singles and combine them together? I found fiber optic modules but those are expensive. If it's to 485, that would work but would like to combine the signals, transmit the stat and then seperate at the control cabinet.

 

[guilio2010]

So I been trying to brush up on my circuits and want to verify my line of thinking. Please excuse the lack of circuit descriptions.

What I am looking to do is send 8 x 4-20mA loop power transmitters into 1 AI channel. Using NC/NO relays, I can do this with 2 x 4-20mA transmitters bench test and had no issues. I will have a DO that will every sec select channel 1-8. Looking at the multiplexer, I can visually see where I could connect my (+) of the transmitter to my power supply ,then connect the (-) of the sensor to the inputs of the multiplexer. This would select the channel based on the DO and then tie the COM of the multiplexer to the AI channel. However, when I start looking into this more, I get doubts that it is that easy.

1) I'm looking at the datasheet for different multiplexers and if I am looking at this correctly, it can't handle 20mA going through it. Correct? Would this mean I should only use the output of the multiplexer drive a transister that can handle larger currents and then tie each transisters common to the AI channel?
2) I want to keep the tranmitters powered so if the multiplexer has channel 1 selected, channels 2-8 would go to (-) of the power supply and the sensor stays powered up. Other wise, the sensor would take ~10-15 seconds to power up. I'm guessing if I am right about 1), then I owuld need a P and N junction transister so that I get ON and OFF state?

With relays, I know I can do this fairly easy, but I was hoping to do this in a smaller package and possibly not have issues of replacing relays due to end of life.

Any advice is appreciated.

Thanks,
Guilio

 

[BrianE22]

Analog inputs measure voltage. You'll have to convert your current to a voltage before sampling it.

 

[btrueblood]

You need an array of precision resistors, and let each transmitter power one of those resistors, then use the multiplexor to switch the measurement circuit to measure the drop across each resistor in turn. That's ok though, because opening (turning off) the 4-20ma transmitters, then reconnecting them will incur a certain amount of settling time as each transmitter turns on again when the relay closes.

 

[guilio2010]

Brian: The AI card we use has internal resistor that you select ON if you have a 4-20mA signal. If you have V signal, then you turn OFF the resistor option.

btureblood: So basically, run each 4-20mA across a 250ohm precision resistor to convert it to 1-5V, then take that into the MUX. Then from the MUX, run the COM into the AI channel measuring the voltage (From above, turn OFF the internal resistor in the card). If I am picturing this right, then all transmitters are on and the MUX is just picking up the voltage drop. What do you mean turning off the 4-20mA? I want to keep the transmitters on all the time. I would think if I'm measure the Vdrop across the resistor, I shouldn't have to turn off the transmitter?

Thanks,

 

[danw2]

If you're still intent on rolling your own, you can skip this, but these little panel mount 1/8 DIN vertical boxes work well for mux'ing up to 8 analog 4-20mA signals at a slow rate suitable for local indication.

The process meter is the equivalent of your analog input. You'd have to be able to deal with 105Vdc signals, because dropping resistors are built-into each channel in order to maintain the integrity of the current circuit so that if any of the devices are 2 wire loop powered transmitters, they don't have to re-cycle and re-intialize to recover from an open circuit power outage.

The indicator LEDs serve as identification of which is channel is switched through to the output. Keeping track of which is which is up to you.

complete page here:
Link

 

[guilio2010]

Yeap. That fits the intent. It mentions it can be tied to PLC. The only thing would be syncing it up with the clocks and or controlling it with the PLC. Only thing I can think of is using a DO cycling every sec on the Stop/Go while the unit is cycled at .6 sec.

What did you search to find that?

 

[danw2]

>controlling it with the PLC using a DO cycling every sec on the Stop/Go while the unit is cycled at .6 sec.

No, I read the remote change-channel function (quick scroll) as a DO/DI less than 0.5 seconds (greater than 0.5 seconds is "stop scan")

Quick-Scroll (page 24 LIM138.pdf)
To quickly scroll through the channels press the STOP/GO button quickly (less than 0.5 seconds).

The DI wants a dry contact (pg 25):
Connecting a N.O. pushbutton switch across all the STOP/GO & COM terminals in a multiple-unit system allows the operator to control the Minimux® II at a remote location.

 

[itsmoked]

To do this you would normally power all the 4-20mA devices continually. Turning them on and off frequently will be a problem. They may have to boot-up or settle or take a few readings and get their box-car averages going, etc., etc. Power cycling them constantly would be bad.

So you power them ALL, ALL the time.
You run each one thru a 250Ω resistor just as btrue suggested.
You then use REED relays to switch each voltage from each resistor tied to each 4-20 source individually into your A2D front-end.

You let the input settle.
Read the voltage.
Open the reed switch.
Close the next reed switch channel.
Process the data read from the last channel.
Read the next channel.
(repeat)

No Mux is used as the reed relays provide the same function.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[guilio2010]

danw2: I must of missed that on quickly scroll less than .5sec. Thanks.

itsmoked: Thanks for bringing up reed relays. I never thought of those and was focus on mechanical. The device mentioned above does keep the sensor on all the time from what I read. I agree turning them on and off frequently is a problem as settling/boot takes to long. The only thing I see from your description is what will control switching the channels on the reed switch? If this is automated, then from what I am seeing is your reducing lets say 8 AI channels on the PLC to 1 AI, but increase your DO channels on the PLC from 0 to 8. Where I was looking at on the Multiplexer is if I have a 8 channel MUX, I have 3 inputs for channel selection. Unless there are reed relays that come with a channel selection?

Thanks everyone...