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4-2omA loop
- Thread starter rainsong
- Start date
- Thread starter
- #3
Skogsgurra
Electrical
You may need to understand the difference between a signal generator and a calibrator before proceeding.
Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
Gunnar Englund
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
You can just hook a DMM mA scaled meter in series with the line.
Keith Cress
Flamin Systems, Inc.-
Keith Cress
Flamin Systems, Inc.-
This is an application note by action Instruments to do exactly what you want.
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
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
- Moderator
- #9
Hi ScottyUK;
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
respectfully
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
Skogsgurra
Electrical
To that excellent observation I would add: Do not concentrate on a single thing like 4-20 mA loops. Learn electrcity and electronics from the basement up to the chimney. The 4-20 mA is, after all, only a tiny portion of what you need to know.
Gunnar Englund
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
Gunnar Englund
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
automatic2
Industrial
with that being said, it might help to know that 4-20 is a constant current source. Varying the loads, varies the voltage from the source. A dummy load confirms the source, a calibrated source confirms the load circuit. Your circuit is a simple loop from source + to source -. You may branch off with isolation drivers, which become a seperate loop in thier own.
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?
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?
It's resistance to voltage noise(the most common).
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.-
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.-
Skogsgurra
Electrical
Hi Smoked,
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
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
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
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
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