7 replies to this topic

[Daniel Minnaar]

Hi there, I've been asked if I can report on the amount of voltage and current being used by an electrical substation at a factory. I'm not very clued up with electronics, so is there any sort of component out there that can read this kind of circuit, which the netduino can monitor without blowing up? Thanks

[Mario Vernari]

Reactive, do you mean *high* voltage and *high* current? Consider that for voltages greater than around 30V (yes, thirty), the things are getting harder and dangerous. Cheers

[Daniel Minnaar]

Reactive, do you mean *high* voltage and *high* current?
Consider that for voltages greater than around 30V (yes, thirty), the things are getting harder and dangerous.
Cheers

Probably well over 30V... it's a substation. I figured there might be a digital component I could plug into and read the values somehow?

[Mario Vernari]

Oh, well...at this point you only should look for industrial sensors, meters, and related. Usually these devices are remotable via RS485 (e.g. Modbus), so that would be a solution. I *strongly* discourage you to create your own circuit/hardware. Cheers

[JonnyBoats]

A power sub-station with voltages measured in kilovolts and 1000s of Amps is really a special case where safety is paramount. As suggested in another response only properly designed industrial equipment tested and certified for such an environment should be used.

For others who might be considering measuring voltages in a residential location such as 120V or 240V (still lethal) there is a good "general" model to follow.

A Netduino (or any other micro-controller) is generally designed to operate on 5V or less (3.3V in the case of a Netduino). It is not designed to survive the application of high voltages (think of it this way, if it would kill you, it will kill a Netduino).

The best approach is to keep the high voltage (or current) on a separate board from the Netduino,

There are many Digital Multi-Meters (DMMs) out there which will output their readings on a serial port. The DMMs are designed and tested to be safe and reliable for measuring voltages and currents within their design specification. You can connect such a DMM to a Netduino via the serial interface (you may or may not need a level shifter such as a MAX232 but that is another issue).

Here is a post where someone used a DMM from Radio Shack in conjunction with a computer: http://www.linuxtoys.org/dvm/dvm.html The price they paid for the DMM seems high (old post perhaps?), I know I bought one on sale (different model) for about $30.

[427v8]

"I'm not very clued up with electronics," Eeeeek!!!! Find someone who is!!! Measuring over 30v is non trivial

[Daniel Minnaar]

"I'm not very clued up with electronics,"
Eeeeek!!!!

Find someone who is!!!
Measuring over 30v is non trivial

Haha! Thank you all for your concern for my safety, but I will most certainly not be creating my own circuits for something like this - I was just interested to find out what sort of 'industrial' sensors were available and how I would interface with them.

As Mario and Jonny mentioned, using serial would be perfect for my needs. I don't necessarily want to connect these directly to the Netduino board, I just want to use it to get the values of the transducers.

For the sensors that don't have serial bus interfaces, what other options do I have?

P.s: Could someone explain how the max/min milliamps work on the Netduino? I'm a little confused as to how many each pin can handle (the datasheet mentions specific pin ranges?)

[Mario Vernari]

Hello Reactive. You should clarify what are the specs of the target substation: voltage range, AC or DC, current range, number of phases, etc. In the order to find out some meter, you must know quite well what the target need. Most of the industrial sensors have their own "intelligence" embedded. Basically they embed a MCU with several functions. From what I heard, in the US there is still a vast usage of the Modbus protocol (either via RS485 or via TCP/IP). Here in EU, the Profibus seems more used (probably because the Siemens power!) About the Netduino, just select the "Download" tab, at the top of the page. Then take a look at the Atmel datasheet, where all the electrical details are well explained. I personally find confusing/bad practice relying on the different capabilities of the I/Os. Unless you are in the optimization stage of a particular circuit (thus to save components/costs), I'd prefer to consider all the I/Os having the same characteristic, so the "worst" case. Hope it helps. Cheers