Digital voltmeter - 500m cable

[MikeyGH]

Hello,

I need to display voltage readings 500m from where the readings are being taken. I am reading a 0-5 V DC reference signal which I want to display on a digital voltmeter 500m away. I cannot use wireless transmissions (it must be hard wired).

 

[Arouse1973]

Hi Mikey, welcome to EP BTW.
Ok so what is your question? Have you tried something that doesn't work or you want some ideas? Also what are you using to measure the voltage? And what is the gauge of your wires?
Adam

 

[MikeyGH]

Hi,

Thanks for replying. No I haven't done anything yet. So I am starting from scratch.

Currently, the signal is just measured by a panel mounted digital multimeter on site. My first thought was to use an ADC and boost the signal to transmit the binary number or multiplexed BCD outputs and then convert it back to TTL to input into a 7 segment display. But this requires over 20 separate cables and in the end I plan to do this to over 30 different signals.

 

[Minder]

Usually the digital voltmeter has very high impedance, so there should be no problem with volt drop, I would use 20g shielded cable twisted pair.
This is similar in function to the DC power supplies that have remote sense, except these adjust the source so that the destination voltage remains the same.
M.

 

[MikeyGH]

Hi, Thanks for replying. You think that just moving the current voltmeter to the new location 500m away with a decent cable would not cause any errors in voltage reading?
Thanks

 

[Minder]

I don't see that it would for a high impedance meter?
I would expect the current to be in the mA range or fraction of.
M.

 

[MikeyGH]

I just assumed there would be an issue with such low voltages. I will try and do a test on monday and see what the results are. It will be in the worst of EMI environments so I will have to use heavily shielded cable, filters, and isolators but hopefully its okay or can at least be calibrated.

Thanks for your comments.

 

[Minder]

Here is info on the sense lines I mentioned earlier.
http://power-topics.blogspot.ca/2013/04/power-supply-remote-sense-mistakes-and.html
M.

 

[MikeyGH]

Thanks for the link. It makes more sense to me now. For me, I think the impedances should remain constant and so I can compensate in other ways.
Thanks

 

[duke37]

If you use a digital signals, they can all go over a pair of wires by time division.
For such applications, a current loop (4mA to 20mA ?) is normally used to reduce the effects of interference. With some cunning circuitry you may be able to multiplex the analog signals. It depends on fast you need the system to respond.

 

[AnalogKid]

A tradeoff is that the higher the meter impedance, the more susceptible it is to noise. If you don't need fast meter updates, add a noise filter capacitor across the meter input. The inductance of the cable will act as one half of a lowpass filter.

ak

 

[GPG]

common mode choke will reduce the noise.if its a problem.

Hi,

Thanks for replying. No I haven't done anything yet. So I am starting from scratch.

Currently, the signal is just measured by a panel mounted digital multimeter on site. My first thought was to use an ADC and boost the signal to transmit the binary number or multiplexed BCD outputs and then convert it back to TTL to input into a 7 segment display. But this requires over 20 separate cables and in the end I plan to do this to over 30 different signals.

Are all the signals from the same site ie only one cable? A lot more info reqd