LM317 Current Loop (LM34DZ)

In the data sheet suggestions, it has a de-coupling circuit. Have you tried this? If you are not sure how to add this to your circuit, I made a drawing on how to do it. You can see it at

LM34

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appreciate

Thanks Brian, talk about service. :)

My understanding of the (4-20) current loops was that they provided a two wire interface immune to noise. But if I still have to decouple the sensor, then I might as well run 3 wire over the 100 feet?

What has to be done, depends on how noisy the environmet is, it has to work in. Sometimes, this can only be found by trial and error.

The diagram does not show it, but you will have to bypass that 317 supply with something like 0.1uF minimum. That application circuit is out of spec for the 317 anyway- not enough minimum current worst case for regulation. There are other diagrams on the LM34 datasheet

-immediately after the specification pages, that show how to set it up for remote sensing through twisted pair cable- these are also temperature dependent current sources that overcome line length voltage drops and they are simpler than the 4-20mA loop circuit.

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I ran 4-wire for a serial device, but it got RMA'd. So I'm using that for my LM317/LM34, while the serial device get's fixed up. I realize that this is not the ideal wire, but I didn't plan on this problem. I guess I should ground the two unused wires in the mean time.

The circuit output is not the terminal labeled OUT on the datasheet circuit, that OUT refers to the LM34 OUT terminal. The circuit output is the total current drawn from the 5-30V power supply and not a voltage.

Hi Bill,

Have you used twisted pair for your wires? CAT5 cable is very handy stuff, and cheap too.

Cheers Terry

Since this is a relatively slow dc signal you are looking at, probably all you need is a rc decoupling network at the receiving end. Something like a 2K resistor and a 4.7 uf capacitor. Brian

I have no idea what "4-wire" is. Use twisted pair, and yes do ground unused wires

Cheers Terry