Interview Question

I had an interview today, and i was asked, why would you choose a transmitter with a 4-20ma output signal instead of a 0-5v output signal. Me being a sparky, wasn't sure what the benefits of 4-20ma are over 0-5v, can any Instrument types help me.

Cheers

Sparky

Looks familiar, same interviewer?

Check out this thread


http://www.plctalk.net/qanda/showthread.php?threadid=9584

oops, wrong thread, just a minute I'll find it...

I guess this is the thread I was thinking of, I combined the two in my faulty memory bank...

http://www.plctalk.net/qanda/showthread.php?s=&threadid=7489


as I wrote in preceding thread
"One of the great advantages to 4-20mA as oppossed to 0-20mA (or 0-5V)is that you can verify circuit integrity with 0% output. If you have 0ma (0V) you know you are looking at a wiring problem."

Brian

Two main advantages come to mind:

1) being a low impedance system it's much less susceptible to induced noise than an inherantly (comparatively) high impedance voltage source.

2) with 4 - 20 mA you get easy checking for wire breaks - you get this with 1 -5 V as well, of course, but it's not so easy withh 0 -5 V or 0 - 10V setups.

another advantage is that given isolated inputs you can pass the current through several devices, for example panel meter, trend recorder, etc. as well as the PLC input. How many depends on the available voltage, but up to three devices is usually no problem.

Personally, given the choice, I would always go for 4 - 20 mA, unfortunately, I don't always have the choice! bawling

PS just noticed you're in Newcastle, my Dad lives in Consett, I was over visiting last March - not far away.

The answer he was most probably wanting to he is:

Voltage signals will allways have an associated signal loss the magnatude depending on the length of the wiring.

the 4 - 20 ma current signal does not have any signal losses.

guest said:

The answer he was most probably wanting to he is:

Voltage signals will allways have an associated signal loss the magnatude depending on the length of the wiring.

the 4 - 20 ma current signal does not have any signal losses.

Click to expand...

Bingo.

KCL says that the current in a loop is equivelant at any point. Period. If you're reading 12mA at your input card, you can guarantee that there is 12mA going out of the transmitter. Voltage signals, on the other hand will always be affected to some extent by resistance losses in the wire (obviously the losses will be greater with longer wire)

Live zero

"One of the great advantages to 4-20mA as oppossed to 0-20mA (or 0-5V)is that you can verify circuit integrity with 0% output. If you have 0ma (0V) you know you are looking at a wiring problem."

Click to expand...

This is called a live "zero" and if you do have a complete break in the circuit, you will get a real zero. Also, this allows you to set the "zero" just a little below 4 ma to hold a valve or other device completely off. Doing that with a real value of zero ma is not possible.

Thanks for the replies.

The answer i gave to the interviewer, was that the 4-20ma has what i called a "positive zero", and this helped in fault finding, and that the 4-20ma signals could be transmitted over longer distances.

From the replies that you've all given, it looks as though I at least got that question partly right, although the interviewer just sat there with a blank expression on his face.

Cheers
Sparky

sparky64 said:

although the interviewer just sat there with a blank expression on his face.

Click to expand...

Maybe, that's because the answer you gave him was not the exact one written down on his 'answer' sheet provided him by an engineering colleague and he didn't know enough to coax you into expanding on your initial answer

Just a thought.

Paul

Well the interviewer must have liked the replies i gave, as the agency that represents the company have been in touch, and i've got through to the second stage of interviews.

cheers

Sparky

This just might be where they ask you to explain your experience with unitary, rack, and modular PLC's?

LOL!