AAI835 AND AAI135 PROBLEMS

Our cards of dcs cs3000 centum analog aai835 and aai135 get damaged a lot, a channel burns or the card burns at all. Does anyone have an idea? why this cards fail very much
 
Typical of DCS hardware - no hits on a spec sheet in the first 20 hits on Google. So I'm blind as to what their specs are.

I can tell you what I discovered when troubleshooting failed 4-20mA input modules where the dropping resistor that was built-in to AI module, burned out. In both cases, the dropping resistor was not a separate, external dropping resistor, but internal to the AI.

In one case, the pressure transmitter was located subterranean in a vault and vault flooded with very dirty, conductive water, which shorted the (+) to the (-) terminals on the transmitter. The cable gland was 2 sizes too big for the cable and the rubber grommet was not screwed down tight around the cable, allowing water entry.

In the 2nd case, a technician thought he could measure the loop current on head mounted temperature transmitters, which unlike pressure transmitters do not have a pair of test terminals, which can be used to check the loop current without opening the circuit to insert a milliammeter. The low impedance of the DVM in the milliamp measuring range was a near loop short circuit putting most of the 24V from the power supply directly across the analog input's dropping resistor.

The internal dropping resistors are by design, precision resistors in order to get an accurate voltage drop representing the loop current. But they are never, ever power resistors, designed to heat up and take much more than 30mA, maybe 40mA.

A direct short in the field wiring forces a 250 ohm resistor to take 96mA, which is 2.3 watts, which is not the rating of the internal dropping resistors.

The two graphics below show a normal and a loop with the field wiring shorted, putting 24Vdc across the analog input resistor.

framed, 2-wire, 4-20mA loop powered transmitter Loop.jpg


framed, Short Circuit, 2-wire, 4-20mA loop powered transmitter Loop.jpg

What do you think the problems are that could be burning up your AI cards?
 
Typical of DCS hardware - no hits on a spec sheet in the first 20 hits on Google. So I'm blind as to what their specs are.

I can tell you what I discovered when troubleshooting failed 4-20mA input modules where the dropping resistor that was built-in to AI module, burned out. In both cases, the dropping resistor was not a separate, external dropping resistor, but internal to the AI.

In one case, the pressure transmitter was located subterranean in a vault and vault flooded with very dirty, conductive water, which shorted the (+) to the (-) terminals on the transmitter. The cable gland was 2 sizes too big for the cable and the rubber grommet was not screwed down tight around the cable, allowing water entry.

In the 2nd case, a technician thought he could measure the loop current on head mounted temperature transmitters, which unlike pressure transmitters do not have a pair of test terminals, which can be used to check the loop current without opening the circuit to insert a milliammeter. The low impedance of the DVM in the milliamp measuring range was a near loop short circuit putting most of the 24V from the power supply directly across the analog input's dropping resistor.

The internal dropping resistors are by design, precision resistors in order to get an accurate voltage drop representing the loop current. But they are never, ever power resistors, designed to heat up and take much more than 30mA, maybe 40mA.

A direct short in the field wiring forces a 250 ohm resistor to take 96mA, which is 2.3 watts, which is not the rating of the internal dropping resistors.

The two graphics below show a normal and a loop with the field wiring shorted, putting 24Vdc across the analog input resistor.

View attachment 983


View attachment 984

What do you think the problems could be that are burning up your AI cards?
thanks very much david it is intresting i will examin this issue, too
AAI835 IS A I/O analog card of yokogawa centum Dcs cs3000 that i think has a internal design problem and yokogawa has designed another version of it but we are in iran!!! we can not contact with them. we are on sanction we designed that card ourselves
 
The most common problem is excess heat. high temperatures kill electronic components and it only takes one component to fail to kill the card. Clogged air conditioning filters is the primary suspect. Analog outputs generate far more heat than analog outputs and could contribute to the combination card failures if cooling is marginal.

The 2nd most common problem with AI cards is near lightning strikes (nothing survives direct lightning strikes). Various vendors sell surge protection that can help protect 4-20mA circuits from induced EMF from near lightning strikes.

Can you have the bad cards analyzed by whomever designed them to tell you what failed?
 
we are working on this problem nowadays... if we solve the problem, I inform here, maybe someone will benefit later
Currently we see that for example a capacitor burns at the input of 24 volts or a fuse burns one amp at the input
 
Typical of DCS hardware - no hits on a spec sheet in the first 20 hits on Google. So I'm blind as to what their specs are.

I can tell you what I discovered when troubleshooting failed 4-20mA input modules where the dropping resistor that was built-in to AI module, burned out. In both cases, the dropping resistor was not a separate, external dropping resistor, but internal to the AI.

In one case, the pressure transmitter was located subterranean in a vault and vault flooded with very dirty, conductive water, which shorted the (+) to the (-) terminals on the transmitter. The cable gland was 2 sizes too big for the cable and the rubber grommet was not screwed down tight around the cable, allowing water entry.

In the 2nd case, a technician thought he could measure the loop current on head mounted temperature transmitters, which unlike pressure transmitters do not have a pair of test terminals, which can be used to check the loop current without opening the circuit to insert a milliammeter. The low impedance of the DVM in the milliamp measuring range was a near loop short circuit putting most of the 24V from the power supply directly across the analog input's dropping resistor.

The internal dropping resistors are by design, precision resistors in order to get an accurate voltage drop representing the loop current. But they are never, ever power resistors, designed to heat up and take much more than 30mA, maybe 40mA.

A direct short in the field wiring forces a 250 ohm resistor to take 96mA, which is 2.3 watts, which is not the rating of the internal dropping resistors.

The two graphics below show a normal and a loop with the field wiring shorted, putting 24Vdc across the analog input resistor.

View attachment 983


View attachment 984

What do you think the problems could be that are burning up your AI cards?
we should test if wires of a transmitter be short circuit can burn card out?
 
> we should test if wires of a transmitter be short circuit can burn card out?

I don't know if you need to test a field short circuit if you believe in Ohms Law because it should be obvious what a short circuit does to the dropping resistor. Unless the capacitor is a filter cap on the front end, I can't imagine how a field short circuit would affect an internal capacitor. As to a fuse, I haven't a clue what that AI card's fuse does, so who knows? But if you want to test . . .

If your cards use external dropping resistors, then these field short circuit faults should not damage the AI card, the faults should only burn up the external resistor.

I would not want to risk burning out a card that uses an internal dropping resistor by testing it. If the cards use an internal dropping resistor, then I would make a test circuit with an identical resistor (resistance and wattage) like the diagram above, and then do two tests:

1. Put whatever brand/model DVM your techs use on the 1A DC range and put the probes across (+) and (-) and see if the resistor survives the test. The DVM's internal fuse will probably not blow at 1A but the resistor is still likely to get hot and burn up. If you try it with the DVM on the 20mA DC range, the circuit will draw more than 20mA and the DVM's internal fuse will probably blow/open. Some DVM's have a circuit breaker for the current test circuit rather than a fuse, which open and needs to cool down before it resets.

2. Short the transmitter (+) and (-) terminals with a wire and see how quickly the resistor gets hot and burns up.

It be more prudent to explain to techs that they're not allow to attempt (to futilely) measure loop current by putting their meter across the (+) and (-) transmitter terminals. They can measure voltage across the (+) and (-) terminals, but not current.
 
thanks very much david
i know ohm and his law;))
we test short circuit in input and out put and in input current becom 29to 30 mA and on output nothing happen i gess cards have protection aboat short circuit but as you said i check filters of rittal rack and they were very dirty and dusty we should change them or clean them
we change fan of them last year but we didnit noticed to filters
thanks again with your help
god bless you
 
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