Strange voltage drop on INFI90 DI's

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Thread Starter

TonopahJoe

System powered digital inputs on NTDI01 termination units show mysterious voltage "losses" no one has been able to explain (including ABB)

Maybe someone can shed some light on this "mystery". The system sourced 48vdc to our NTDIO1 termination units (which are connected to IMDSI14 slave modules) measures about 49 volts from the positive field input terminal to ground when the field device contacts are open. (The dip shunt configurations are 1-6-8.) The common terminal measures 0 volts to ground (open field contact), however, the voltage across the two terminals is about 33.5 vdc. Where did the 15.5 volts go? Interestingly, when the field device contacts close, both terminals read about 49 volts to ground. I would expect to maintain the 15.5 volt loss, but it doesn't happen.

On two of our PCUs they're using 25.5 vdc to power the NTDIO1's which are connected to IMDSI13's. (Again, the 1-6-8 dip shunt configuration.) In this case the open field contact positive field input terminal reads 10 volts to ground. (And there's definitely 25.5 volts being supplied to the E1 terminals.)

The open field contact voltage across the input terminals is 10 volts and the voltage from common to ground is 0. When the field input device contacts close we measure 0 volts to ground on both terminals and across the terminals. The minimum turn-on voltage for the opto isolator is 19.5 volts so it shouldn't work at all, but it does.

I would expect the open contact voltage on the positive to be 25.5 volts, and maybe that 15.5 mysterious volt loss from positive to common we see on the 49 volt TU's, but what's up with the 10 volt thing? And dropping to zero on both terminals upon field contact closure?

We had an ABB tech rep out at the power plant this week and his position was that the 1-6-8 dip shunt configuration wouldn't work and was to blame! I don't think so. That's a standard system powered dip shunt configuration and these systems have been working for 7 years.

It's not a question of functionality, it's more a question of how it works at all.
 
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Glenn Clarke

Hi Tonopah,

The situation you describe for the DSI13 is correct. The input circuit of the DSI13 is basically a 1.3 k resistance in series with a 15V zener and an opto-isolator. When you connect a multimeter across the input terminals when the field contact is open you are essentially creating a circuit comprising of a 1.3k ohm internal resistance in series with a 15V Zener and an opto-isolator and finally the 10M ohm resister (meter). The current at this point is (25.5-15-.7)/10,001,300 = 1µ amps. The voltage across the 10Mohms, the meter, will be =~ 10V. The minimum turn on voltage refers to the voltage required across the terminals when using the "field powered" dip shut configuration, straps 5/6.

The same "theory" should apply to the DSI14. The internal resistance of the DSI14 is approx 5k ohms. Once you connect the meter, the circuit is completed and the current would be 49-15-.7)/10,005,000 = 3.3µ amps and the voltage across the 10M ohm meter would be 33.3 V. Then you make a couple of statements that don't make any sense to me at all. With the field contact OPEN you should read the same voltage when measuring from the positive terminal to ground and across the terminals. Also, I can't see
how, when the field contact is CLOSED, that you could measure 49 volts from both terminals to
ground.

Regards,
Glenn
 
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