Voltage retransmitter

Good afternoon everyone,
I am looking for an analog voltage retransmitter. This will be installed between a sensor and a legacy piece of control equipment. The reason for the transmitter is to improve the input impedance of the legacy control. Currently, when I connect the sensor to the control (under certain circumstances related to sensor age and operating temperature), the measured voltage is lower than the reading I get when connecting directly to a Fluke DVM.

My sensor will output a 0-2 volt DC signal. I need to exactly duplicate this signal out to the legacy controller. If I feed it 937 mV, I need 937 mV back out the other end. I don't want 4-20 mA, or a scaled 0-5V signal. Just a duplicate, accurate to the mV, signal.

To properly take a reading, the retransmitter will need to have in the neighborhood of 10 Meg ohms (the Fluke meter that works properly is listed to have this input impedance.) I have tried a retransmitter similar to what I describe in the past, but it is simply listed as > 1 Meg ohm input impedance and it has issues.

I can supply the retransmitter with either 120Vac or 24Vdc for operating power.

I'm currently searching the web for such an animal, but the input impedance seems to be a problem. So, I thought some of you might have come across something like this and could point me in the right direction.

Thanks,
Brian

Brian123 said:

I don't want 4-20 mA, or a scaled 0-5V signal. Just a duplicate, accurate to the mV, signal.

Click to expand...

I don't know why a scaled 0-5 VDC wouldn't be acceptable.

What I would do myself is get 2 signal conditioners. Set up 1 to convert 0-5VDC input to 4-20mA output, then in the receiving station put the 2nd conditioner receiving 4-20mA input & outputting 0-5VDC. This way whatever input is received by the 1st will be precisely output by the 2nd, with a current signal between over the distance.

And I've never had any issue with input impedance problems when using Phoenix Contact's signal conditioner.

Aabeck said:

I don't know why a scaled 0-5 VDC wouldn't be acceptable.

What I would do myself is get 2 signal conditioners. Set up 1 to convert 0-5VDC input to 4-20mA output, then in the receiving station put the 2nd conditioner receiving 4-20mA input & outputting 0-5VDC. This way whatever input is received by the 1st will be precisely output by the 2nd, with a current signal between over the distance.

And I've never had any issue with input impedance problems when using Phoenix Contact's signal conditioner.

Click to expand...

0-5 VDC isn't a problem as long as it isn't scaled. What I mean by that is some converters/conditioners are available with a 0-2VDC input, but only available with a 0-5 V output. That would mean that if I fed it with 1 V, it would spit 2.5 V out the other end. This won't work for me as I have no access to the legacy controller to change it's interpretation of the incoming signal.

I don't have a need to change from V to mA and back as the impedance problem is not distance related. The retransmitter sits about a foot from the controller. The problem I am having is entirely due to the controller's input impedance.

Do you have a part number or model line in mind for the Phoenix contact conditioner? Everything I am finding on their website lists a max input impedance of 1 Meg ohm. From past experience, that isn't enough.

The conditioner I mostly use is https://www.phoenixcontact.com/onli...tail:pid=2810913&library=usen&tab=1#Inputdata
which has many configurations of input/output settings.

But I checked & it has a listed input impedance of 1Meg, but like I said it's always worked for me without issue.

Another thought:

Is the sensor so critical that it can not be replaced with a newer sensor that will be more reliable?

just use the phoenix and scale them 0-5 out with 0-5 in and you won't have any problem, 2 volts in would be 2 volts out

I didn't do an extensive search but, these folks

http://www.autoctrls.com/acatalog/remo_modu05.html

do list 10M input impedance devices.

Thanks for the responses guys.

@Aabeck, I could swap out the sensor with a new, replacement sensor of the same type, but it will eventually have the same failure mode. Basically what I am trying to do is allow the sensors to be used to the same life-cycle and eliminate hard to diagnose temperature related problems. If I can get the input impedance up, these problems should mostly go away and this machine will behave like others we have here. Unfortunately, I am unable to change the underlying sensor technology for something that isn't prone to these problems.

@maxketcham, I will likely be using a 0-5 in, 0-5 out retransmitter. I won't be able to use the omega retransmitter, though, as it has insufficient input impedance.

@Doug-P, thanks for the link; I hadn't run across them yet. I did notice that RedLion sells a retransmitter that looks like it will fill the bill. If the RedLion doesn't pan out, I'll see what Automatic Controls can supply.

We use these...

http://www.automationdirect.com/adc...nt/Signal_Conditioners/Universal_Input/884114

for just what you are talking about where the PLC input impedance pulls the sensor signal down. They are rated at 10Mohm, although it may vary depending on which voltage range you choose.

They work great and are pretty inexpensive. Red Lion also sells the exact same thing with their name on it.

1 opamplifier with rail to rail and a few resistors will do

Old No. 7 said:

We use these...

http://www.automationdirect.com/adc...nt/Signal_Conditioners/Universal_Input/884114

for just what you are talking about where the PLC input impedance pulls the sensor signal down. They are rated at 10Mohm, although it may vary depending on which voltage range you choose.

They work great and are pretty inexpensive. Red Lion also sells the exact same thing with their name on it.

Click to expand...

Huh, I looked at AD's website yesterday, but somehow didn't get to that page. The Red Lion version is what I am going to try, but it's good to know AD carries them too.