Confusion

schreef in bericht news: snipped-for-privacy@googlegroups.com... I have to design SCADA system for dam.I have water level sensor which has output between 4 and 20 mA in analog form and it is transferred to control room about 2 to 3 km apart from dam using OFC. Is it necessary to use amplification and waveform shaping circuit after current to voltage converter having input 4 to 20 mA and output 0-5v if output from converter to be connected to ADC(8 bit)?

Sending analog DC signals over this distance can hardly be done directly neither in voltage nor in current. Converting to light will not make any difference. So you wil need some kind of encoding or modulation. As you have to convert to 8 bits digital anyway, you'd better do the A/D conversion on the remote side and send that 8 bits to the control room. The old-fashioned ASYNC will do for the communication. On the remote side you will need only one simple micro that can do both the conversion and the communication.

petrus bitbyter

Hmm OK I know nothing about sending signals for more than ~50 feet, but why not? With 20mA and say 50V that's a few k-ohm of resistance. So 5 km (there and back) and say 200 ohm/km...24 gauge wire looks like it would work? I looks like a big antenna so I guess it'd need a shield.

To the OP, I'm confused by your question. If the I-V output 0-5V matches the full scale of the A/d, then no amplification. (otherwise adjust it.) I don't know what waveform you are shaping. Or do you just mean a low pass filter?

George H.

Hmm OK I know nothing about sending signals for more than ~50 feet, but why not? With 20mA and say 50V that's a few k-ohm of resistance. So 5 km (there and back) and say 200 ohm/km...24 gauge wire looks like it would work? I looks like a big antenna so I guess it'd need a shield.

To the OP, I'm confused by your question. If the I-V output 0-5V matches the full scale of the A/d, then no amplification. (otherwise adjust it.) I don't know what waveform you are shaping. Or do you just mean a low pass filter?

George H.

the first electrical storm to blow past will slag your sensor or scada system or both,

Your optical cable goes into an optical-to-current converter? Then a current to voltage converter?

If so, then you can go directly in an ADC, though you have to worry about the maximum possible output voltage from the current-to-voltage converter (if it's too high it could damage the ADC) and you might have to add an analog anti-aliasing filter.

Best regards, Spehro Pefhany

Ahh OK thanks, How did the telephone/ telegraph lines deal with lightening?

George H.

ut why not?

re and back) and say 200 ohm/km...24 gauge wire looks like it would work? I looks like a big antenna so I guess it'd need a shield.

ches the full scale of the A/d, then no amplification. (otherwise adjust i t.)

ow pass filter?

ng?

Direct hits melted the wires. Nearby hits induce large currents in the grou nd and hundred of volts voltage differences betweem local ground voltages.

These were dealt with by a combination of resistance, inductance and spark- gaps/gas-discharge tubes. The spark gaps/gas-discharge tubes limited the vo ltage difference between the circuit and local ground to a hundred volts or so, and the resistor-inductor combination limited the transient local curr ents to a survivable level.

If you dig deep enough you can find detailed publications on the subject.

Local area networks suffered horribly from this effect at first - pretty mu ch every prototype had to get blasted by a local lightning strike before th e inventors would realise that it was a problem.

Even the first Intel/Xerox/DEC for Ethernet didn't allow for protection aga inst local lightning strikes, but it got revised in very short order in add this in.

has output between 4 and 20 mA in analog form and it is transferred to control room about 2 to 3 km apart from dam using OFC. Is it necessary to use amplification and waveform shaping circuit after current to voltage converter having input 4 to 20 mA and output 0-5v if output from converter to be connected to ADC(8 bit)?

So, just convert to digital local to the sensor (say the normal 12 bits worth) and send the digital data back via FO cable and use a FO to serial converter for the PLC.

?-)

They didn't use bleach. :)