AAI135 =8 CHANNEL ANALOG INPUT MADULWhat kind of I/O card is it?
thanks very much david it is intresting i will examin this issue, tooTypical 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?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 i will check this subject .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 are that could be burning up your AI cards?
hi david thanks very much david i will check it again> 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.