Sending 4-20mA down a phone line

JulianDave

Member
J
Join Date
Dec 2010
Location
San Diego Area
Posts
14
We have a transducer at a tank site to give us level. This 4-20mA signal is converted to 1-5V with a resister and fed into an "M-Unit" box that converts it from 1-5V to 1-5mA. This is sent down a dedicated dry pair phone line to another M-Unit that goes from 1-5mA to 4-20mA and into an analog card.

This receiving M-Unit no longer seems to be working, as I can only get about 2.3mA max out of it. I can't seem to find a similar unit for sale as it appears to be discontinued.

My question is: Do I really need these, or can I just run 4-20mA down the phone line? I would guess the distance is about 1.5 miles. If I do need something to transmit this down the line, any suggestions?

Thanks,
Dave
 
To protect your transmitter, another possibility might be to use a signal isolator like THIS ONE from Phoenix. I've never done it, but I would imagine you could use an one of these isolators at each end of the circuit to protect everything.

Steve
 
IMO, you will be better to convert to digital and send via modem. I believe that the cost would be justified.
 
Thank you for the replies. I'll have to do some research, but I think wireless could be a problem because it's hilly here. I'll look into the modem idea, but I'd still love to find a direct replacement part that converts 1-5mA to 4-20mA.

Dave
 
If it were me I would just run the 4-20 directly down the phone line. Our well field is all controlled by 4-20 loops on 19AWG communication wire. Some run as far as two miles back to the RTU. Our loops are all 24VDC as well. The 4-20 to 1-5 seems like an unnecessary step to me. As long as the voltage drop doesn't exceed the minimum voltage for the transmitter you should be fine.
 
Last edited:
Running 4-20 works as long is there is no fiber in the circuit.
Many years ago I had to replace time pulse equipment because the phone co upgraded the hardwire lines to fiber.

A Micrologix 1100 or 1400 configured as a DF1 master and a Micrologix 1000 configured as a DF1 half-duplex slave would work nicely
 
Is price an issue? GE Makes radios(NetIO) that I think will handle this and has range for up to 30 Miles. Not sure on the price of this but i would look into this.
 
I'm very curious. Is the "phone line" that is referred to a telephone line in service by a local telephone provider with a telephone number that can be dialed?

Or is everyone here talking about telephone wire; solid wire with those insulation colors that the phone company standardized on, wire that is not connected to a telephone system?

Dan
 
danw said:
I'm very curious. Is the "phone line" that is referred to a telephone line in service by a local telephone provider with a telephone number that can be dialed?

Or is everyone here talking about telephone wire; solid wire with those insulation colors that the phone company standardized on, wire that is not connected to a telephone system?

Dan
Click to expand...

Hi Dan, this is a phone line provided by the telephone company that is just a dry pair from point A to B. I Think I'll have to call them and see if they can reduce the resistance in the loop. 1600ohms seems like too much.
 
This receiving M-Unit no longer seems to be working, as I can only get about 2.3mA max out of it. I can't seem to find a similar unit for sale as it appears to be discontinued.

Hello,

are you using the existing circuit to obtain the 2.3mA max? have you used a generator to input a known controlled signal into the converter box? sorry i was just wondering iv had that bite me before. Thought the conditioner had gone bad but the signal wires were nicked and bleeding off.
 
matthew1 said:
This receiving M-Unit no longer seems to be working, as I can only get about 2.3mA max out of it. I can't seem to find a similar unit for sale as it appears to be discontinued.

Hello,

are you using the existing circuit to obtain the 2.3mA max? have you used a generator to input a known controlled signal into the converter box? sorry i was just wondering iv had that bite me before. Thought the conditioner had gone bad but the signal wires were nicked and bleeding off.
Click to expand...

Hi, I ordered a box to generate analog test signals yesterday, so I will try it when it comes. The defective M-Unit is suppose to take 1-5mA and convert to 4-20mA. I read 3.15mA or so coming in to it, so it should be turning that into something like 13-14mA or so. Changing the span and zero on the unit, I can't get it to spit out anything higher than 2.3mA.

So I'm thinking that regardless of test signals, it isn't working correctly.
 

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