audio extension cable for CT cables

Hi,

I recently received an EmonPi and have hooked it up with an extension cord outside for power.  I'm getting what appears to be good readings for both CTs and the Temp.

But I'd like to put the EmonPi inside my house for protection by putting everything but the sensors inside, and routing the sensor cables through a hole in the wall.  

Can I extend the CT cables with normal stereo audio cables?    If so, how long can I safely make it without losing too much accuracy?    I think 10 meters is ideal for my situation, but 3 meters might do if I must.

I was thinking about something like this:

 http://www.amazon.com/gp/product/B00RXNUUAW/ref=s9_hps_bw_g23_i8

 

Can I extend the temperature cable and if so, how long?

 

Obviously, I don't know much about electricity physics.   So I couldn't figure it out from other forum posts and pages here. 

Robert Wall's picture

Re: audio extension cable for CT cables

You don't need a cable with 4-pole connectors. If you're going to thread it through a wall, you might as well buy a length of cable off the reel and fit a plug (or joint it up) after it's gone through the wall. That way, you need a smaller hole!

For the CT, 10 m shouldn't be a problem, and it can't lose accuracy because it works on current. Get a length of two-core screened "microphone" cable, e.g. something like http://spiratronics.com/cable-and-connectors/cable-and-wire/screened-aud...
At the Pi end, don't connect the screen and use the two inner cores, connecting to tip and sleeve of the plug (which you've taken off the CT). At the CT end, earth the screen to your mains earth, on the CT cable, forget the screen and connect the red and white wires to the cores of your new extension cable. Solder the joints if you can, otherwise a small screw terminal block is fine.

For the temperature sensor, there's a lot about this in Building Blocks on the DS18B20 page.

Rhys's picture

Re: audio extension cable for CT cables

Hi Robert,
I found your reply regarding extending the CT sensor leads here and in another thread.
I have a couple of questions regarding this.
Is the screened extension cable important (for shielding or something else)?
You mention that the screening of the extension cable at the CT end should be connected to the mains earth, any particular reason?

If the screening is needed then my following question is probably moot...
I'm possibly going to want to extend 3 CT leads from where my (still to be ordered) emonTX would sit to my main DB in the house, probably a little under 3.5 meters in total, and was wondering if the following would be possible.

Remove the 3.5mm plugs from the CT's and attach new 2 core cable to the 3.5mm plugs and connect the wire ends to connector blocks that are wired to a RJ45 socket, then use use a standard LAN cable to extend to within range of the CT attached leads using another RJ45 to connector blocks "board" and attached the CT leads to the connector blocks. I'm looking to extend 3 CT's thus my diagram shows only 3 and therefore only using 6 pins of the RJ45...
This of course does not make use of any screening/shielding, hence my questions and remark above about screened cable.
The only reason I'm asking this is if it would work it means only having to run 1 LAN cable instead of 3 separate cables.

Thanks
Rhys

Edit - fixed link to inline image - Moderator, BT

emjay's picture

Re: audio extension cable for CT cables

@Rhys,

If you want similar results with the CAT5 cable, you need the screened version (it does exist!)

If you want to run a quick test with a pre-made CAT5 patch cable, look carefully at the pin outs on the RJ45 - you will see that the pairs are not in the simple sequence you might expect.  Matching a twisted pair in the cable to each C.T. is an important part of reducing noise pickup

 

Robert Wall's picture

Re: audio extension cable for CT cables

There should be no problem using screened CAT* (you don't need the super-fastest) but as emjay says, you must keep to the pairing in the cable. And I wouldn't necessarily use RJ45 connectors - I don't know their current rating, but I do know they don't last long outdoors.

I don't think there would be a problem with overall screened cable and 3 - 4 m, you cannot lose accuracy unless you have a shunt path for the current (remember a CT is a current source). But the proper place to earth the screen is at the source (the CT end) and not connect it anywhere else. It's all to do with where noise voltages get injected and noise currents circulate. Depending on what interference sources are nearby, I wouldn't like to offer odds on unscreened cable. OK, the impedance is low (22 Ω) but the ADC is capable of seeing voltages down to millivolts.

N.B. You can get multi-core 'microphone' cable, but it would not be worth it for 3.5 m.
This http://spiratronics.com/cable-and-connectors/cable-and-wire/screened-aud... is what I normally recommend.

Rhys's picture

Re: audio extension cable for CT cables

Thanks emjay and Robert for the replies.
I hadn't though about the amp rating of the RJ45 connectors, did a quick search and found a few mentions of around 1.5A so clearly not going to cut it.
I think I'll go with the microphone cable you recommend Robert, will source it locally.

Thanks
Rhys

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