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Digital Input Current Drawing Problem in PCI-6220

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We are having NI PCI-6220 Card mounted on PCI-Raiser Card in a mini windows PC. We are using 8 Digital Input Channels P1.4-P1.7 and P2.2-P2.5 for taking some inputs from our Input Card.

 

When the computer is on then everything is working fine and we are getting Digital Inputs in Labview. The problem is that when computer is turned off the Digital Inputs on PCI-6220 Card start drawing current and due to this voltage regulator IC on our input card starts to Heat up.

 

This problem only occurs when the PC is turned off , in on state everything works fine.

 

We will be greatful is anyone can suggest a solution to our problem.

 

Regards

Shourya

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Solution
Accepted by topic author ssingh1

I am not familiar with this special NI part. But maybe there are protection diodes on the outputs. Usually these diodes are connected from the inputs to the supply rail. 

 

In case the supply rail is at 0V (or below the input voltage fed into them), the diodes will turn on and feed current into the supply rail of the PC, thus drawing current from your external device. If this device has a push-pull output configuration, in HIGH level there will be excessive current drawn from the supply in case the outputs are shorted or almost shorted as it will be in the configuration described above. 

 

I have no immediate solution except for additional hardware such as optocouplers, decoupling the outputs of your device from the inputs of the NI device. 

 

 

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For this reason, I always use digital buffers with an enable.  The opto-couplers would also be a good solution.


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Dear Buechsenschuetz & crossrulz,

Thanks for your replies.

Please let me know if Opto-Couplars can provide solution as ultimately these will also be providing Digital signal to DAQ Card Input which in turn will again draw high current from Opto-Couplar when device is turned off.

Can this problem also be due to Pull-down configuration on Digital Input of DAQ Card?

As Given on NI Site :-

 

M Series     Pull-down     50 kΩ

Please also let me know if there can be any other solution.

Regards

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As mentioned, I have no personal experience with this NI board and, of course, do not know your peripheral device in detail. 

 

A pull-down resistor of 50kOhm will not cause any trouble to any peripheral device. The current drawn by such a resistor is less than 1mA. 

 

An optocoupler will interrupt the direct current path between your device and the NI card inputs. So your device will not be affected whatever happens on the NI card side. 

 

You have to make sure that the output of your device can drive the input side of the optocoupler. Usually optocouplers require 1...10mA on the input side. Make sure that the output of your device can deliver sufficient current. Also, in most cases you will need a current limiting resistor between the optocoupler and your device. 

 

There are optocouplers available which will interface directly to a digital input. The drawback is that you will need a supply voltage for the internal circuitry of the optocouplers. Many NI boards provide +5VDC outputs which can be used to supply some external circuity. 

 

It seems as if you do not have too much experience with electronic circuitry. As mentioned, I will not be able to answer your question ultimately and in detail. Desigining an optocoupler interface is not too difficult and anyone with some experience should be able to do so. I think there are also ready-made modules available. 

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Something like this will probably work for you: 6N137S-TA1.  You can use the 5V from the DAQ card to power the DAQ side of the chip.  You just need to supply the digital output and a return from your UUT.  As mentioned, you may need a current limiting series resistor to protect your UUT.


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Dear Buechsenschuetz & crossrulz,

Thanks once again for your valuable inputs and sorry for the delay in replying.

I would like to inform that our Hardware Team tried various suggestions by you and found that reducing the Digital High Output of our Input Card to around 2 Volts (using potential divider) removes the Current Drwaing problem in DAQ Card when the PC is turned off  and we are still able to drive DAQ card as expected in PC on state. We are still testing the solution to see if it is reliable.


Regards

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