4-2omA loop
4-2omA loop
(OP)
How do you trobleshoot a 4-20mA signal.
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RE: 4-2omA loop
RE: 4-2omA loop
RE: 4-2omA loop
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: 4-2omA loop
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 4-2omA loop
Peter Nachtwey
RE: 4-2omA loop
Action use to have a very good application book which makes you an expert in 4-20 ma systems in notime. I don't see it on their website now. There was an outfit in Flordian that allson had good applicationn literature.
BJC
RE: 4-2omA loop
Start by identifying the problem. 'Troubleshoot' covers countless possibilities. Add some detail and maybe someone can point you in the right direction.
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Sometimes I only open my mouth to swap feet...
RE: 4-2omA loop
I agree with you completely.
I was called in to look at a loop that consistently failed on high alarm. Not a good thing for a high alarm shutdown loop.
Continuity was good and the instrument mechanic had calibrated and re-calibrated every device in the loop.
It turned out that the input resistor of the alarm relay was a considerably higher value than stated in the data sheet. As a result of the higher resistance there was a higher voltage drop at 20 ma. The power supply could not supply enough voltage to push 20 ma through the loop.
The solution was to pad another resistor in parallel with the input resistor to bring it into spec and then recalibrate the relay.
The point of this;
if you have to ask then your first step is to spend a few hundred hours learning the basics of 4-20 instrumentation. If you don't understand the system you usually can't trouble shoot it.
respectfully
RE: 4-2omA loop
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: 4-2omA loop
In any loop you will only have one driver, which can be confirmed with a dummy load (250 ohm)and measuring voltage across. In most cases you will have a single load, which will be calibrated using a calibrated source (variable). Everything else is wires and terminations, which by the way create the most problems.
Question for all though, why has the 4-20ma become so popular over the other standards?
RE: 4-2omA loop
It's ability to automatically excite a transmitter.
It's only two wires.
Voltage drops are only a secondary problem.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 4-2omA loop
Funny you mentioned resistance to voltage noise.
I came back home from a motor testing lab yesterday. Fairly large test rigs with 800 kW braking power. PWM and active front ends.
This is a very modern facility and there are all supervision, warnings and alarms you could think of - and then some.
Problem: The vibration transducers that are used to warn the operators if anything starts getting ugly (they cant hear anything because everything has been soundproofed and they are not allowed to enter the test cells when running the tests), these vibration transducers were live and kicking until they activated the brake motor (generator, as it is). The the 4-20 dropped below 4.00 mA and the warning went off.
It was the PWM residues on the signal cable that entered the current source output and probably got rectified inside so that the output current dropped below 4 mA. I could see it clearly on the scope. Remedy: 100 nF capacitor across transducer and 100 ohm resistors in both + and signal lines. So, that transducer got extra 2+2 V to feed. But since supply was 24 V and the load was 500 ohms, it worked.
Interesting side note. When I had fixed this problem, the next problem was that they got alarms from the transducer on the drive side. Too much vibration!
We will be looking at this more in depth next week. There are about fifteen such rigs on this site and all show low vibration amplitudes. We start to think that vibration is higher than indicated but that the EMI from PWM has taken the signal down a bit. That's why the one I fixed suddenly says vibration is high - perhaps...
But, otherwise, it is a convenient way of powering a transducer and also getting signal back in an easy-to-use form. Voltage drop is seldom an issue, so a 10 V signal does also work well. But then you mave common mode problems. Also, you cannot power the transducer via a two wire cable any more. But, I prefer 10 V. A lot easier to filter and also to trouble-shoot. No need to break the loop ur use other tricks. Just connect voltmeter and measure.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...