I have to deliver power to a sensor which is 500 meters far away from the mains. Sensor needs an 12V DC power supply, and the DC power supply needs to be powered from the mains. Battery is not an option and I have to use SMPS for the sensor. The power delivery(power cables to the sensor) will be indoors(inside a bridge). Sensor sinks less than 10mA current. No voltage regulation needed.

I have two choice:

Either I will power the DC supply in the vicinity of the mains, and send the power to the sensor which is 500 meters far away by using twisted shielded cable. This is in Figure 2 below. Or I will power the DC supply through 500 meters of AC cables and power the sensor at the vicinity of the DC supply. There will be no direct lightning strike ect. but EMI might occur.


simulate this circuit – Schematic created using CircuitLab

In terms of noise the closer the power supply to the sensor it is better because the sensor output quality is affected by the power supply noise.

As in Figure 2, I was planning to send the power to the sensor by 500m DC cable and was planning to install TVS diodes and caps at power supply rails before the sensor.

In terms of noise and EMI, is it better to use the wiring scheme in Figure 1 instead? If I use the 500 mains cable do I still need astra components for over voltage or noise? Which way requires less effort or caution?

  • \$\begingroup\$ Will that sensor also send data back via wire? In that case having only one wire would be a priority for me \$\endgroup\$ – PlasmaHH Apr 12 '18 at 6:36
  • \$\begingroup\$ Yes that sensor output will be sent back all the way back to the left 500m. \$\endgroup\$ – GNZ Apr 12 '18 at 14:18
  • \$\begingroup\$ What did you mean by having one wire? \$\endgroup\$ – GNZ Apr 12 '18 at 14:19
  • \$\begingroup\$ would you rather have someone laying down the power and data wire, or a single one? \$\endgroup\$ – PlasmaHH Apr 12 '18 at 14:29
  • \$\begingroup\$ The details of the cable and complete wiring per sensor is in this question electronics.stackexchange.com/questions/367743/… \$\endgroup\$ – GNZ Apr 12 '18 at 14:45

I would propose a third option. Consider running a higher DC voltage (voltage determined by regulator dropout and wire losses) through the 500m of wire and regulate down to 12v at the sensor end with a 7812 or similar. This keeps regulation at the sensor end, but allows you to run DC instead of AC.

  • \$\begingroup\$ There is no voltage attenuation current is soo small. I dont want to use regulator and I also dont need. \$\endgroup\$ – GNZ Apr 11 '18 at 18:37

From a safety point of view, #2 is far, far better. You need to think very carefully about what happens if somebody accidentally breaks your long wire. If you want to try AC, a metal conduit is a must. And if water gets into it, you need to think about that as well.

Also, you have not specified your current levels. You'll need to use wire gauge specs to determine the voltage drop between the power supply and the sensor, and if this varies a lot you may well need to put a local regulator at the sensor, with a somewhat higher DC output from the SMPS.

  • \$\begingroup\$ Current is around 10mA and I'm sure DC voltage will not attenuate I tested it. I'm not worried about safety I can take care of it. How would your answer be in terms of EMI? Regarding Figure 1, would AC cables still need a MOV for example at the DC power supply inout? \$\endgroup\$ – GNZ Apr 11 '18 at 18:36
  • \$\begingroup\$ "I'm not worried about safety I can take care of it." Dude. You really DO need to worry about it, just not nearly as much at 12 volts. As for EMI, there's no easy answer with 500 meters of wire acting as an antenna. Shielded twisted pair is a good start, double or triple shielding may be needed, and even then you're likely to find problems. There just isn't an easy answer. \$\endgroup\$ – WhatRoughBeast Apr 11 '18 at 18:41
  • \$\begingroup\$ Okay I see now I started to think I should consider safety more because I need at least 30 of 500 meters of mains cables in that case because there will be 30 sensors. Human life is more important than a stupid noise issue. \$\endgroup\$ – GNZ Apr 11 '18 at 18:43
  • \$\begingroup\$ @GenzoWakabayashi - Don't get me wrong. It is possible to use AC (although if you only need 10 mA @ 12 VDC it doesn't make much sense). And a stupid noise issue isn't stupid if it makes your data collection impossible. I just said you need to think about it (before somebody gets hurt). Get paranoid, but not too paranoid. \$\endgroup\$ – WhatRoughBeast Apr 11 '18 at 18:48

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