Detecting short circuit using 6221

[ATMA]

Has anyone tried using a 6221 to detect short circuits, as you would using a DMM? I have an equipment with quite a few limit switches, so i would like to use the 6221 to detect whether they are open or close.

Previously what i did was i have a DO set to high and connect it parrallel to another DI and the limit switch. If the switch closes the DI will detect GND and i would know the switch has been closed. But for this equipment there is far to many switches to make sense using 2 DIO each (creating a wiring disaster).

Any ideas?

(Yes, i have a current limiting resister at the DO )

Message Edited by ATMA on 09-25-2007 09:00 AM

[Mark_E]

Hi ATMA,

If all you want to do is detect whether a circuit/switch is open or closed, the method you proposed will work. With an M-series card you will be able to access more channels than with a DMM, so if you are not concerned about the benefits of a DMM (high accuracy, precision, common-mode rejection, etc.), the 6221 should be just fine.

[ChrisR.]

ATMA:
I don't see why you couldn't just wire your resistors to +5V (available on the I/O connector) instead of a dedicated DO for each switch. That would cut the number of DIO's you use in half.

Another option would be to take advantage of the built-in pulldown resistors on each DI. Wire the limit switch between +5V and the DI, which will then sense high when the switch is closed, and low when it's open. The disadvantage of this approach would be decreased noise immunity because the built-in pulldown resistors are pretty weak, according to the specs.

Chris

[ATMA]

Hi Chris, yes wiring it to the +5V instead of another DO would be much simpler. But i didn't want to overload my 5V as it's already driving other components.

[Mark_E]

Hi ATMA,

Maybe you have more things in your setup that need to be considered before fully answering your question. Will you outline your entire setup so we can take all of that into account?

To successfully detect a short circuit on a switch you need two things:

1.     A known supply on one end of the switch

2.     An input line (analog or digital) to measure the voltage

If there’s a reason why you can’t use the supply on the DAQ board itself (for fear of overloading, etc.), use any external supply available. Again, if there’s more to consider in your application, please let us know.

[K C]

Hi ATMA,

I have to agree with Mark that you have to tell a bit more about your circuit.

Is it possible to group the switches or to place them in a matrix so you can reduce the number of inputs you need to detect which switch is closed ?

[ATMA]

Well, it's really nothing much. I just didn't want to overload the 5V supply from DAQ, although i could have 10k current limiting resistor.

The reason i didn't want to use the 5V supply in the first place is that i already have 2 encoders drawing current from that supply, i wasn't sure would connecting a couple more limit switches affect anything.

[Mark_E]

Hi ATMA,

How much current is being drawn from everything else? The NI 622x Specifications manual lists the current limit on the +5V terminal as 1 A (page 11). If you’re not pulling more than that total, then you could use the power from the 6621 itself rather than an external supply.

[ATMA]

Well, not much actually. Currently it's only connected to a couple of encoders which draws around 50mA. I guess it's alright.

[Mark_E]

Yeah, you should be perfectly fine as long as you don't exceed 1A. Again, this is from the NI 622x Specifications on page 11 under "Power Requirements". If you have further questions, feel free to post again.

Message Edited by Mark E on 10-04-2007 12:44 PM