How come my transistor does not switch using NI 9403 module?

[DBerry]

I am trying to switch over from using a PCI-DIO-96 card to a Compact Daq module NI9403 for switching a TIP120 transistor.  Both the PCI-DIO-96 and NI9403 are 5V Digital Outputs, my source voltage is +24VDC, I have a 1K resistor between the 5V DO signal and the "Base" terminal on my transistor, and a diode along the load path.  The transistor switches fine with the PCI-DIO-96 board but not with the 9403.   Any thoughts? 

If the two boards have different current outputs will this cause a problem? (or does the 1K resistor control the base current)?

Thanks

Dan

[K C]

It's possible that the TIP120 requires more base current then your digital output can drive. It depends how much current there must flow through your TIP.

Because it did work with a DIO96 I don't expect that the currents are high. So you can try to solve it by lowering the resistor value to 100E. The maximum base current for the TIP120 is 120mA.

[K C]

To add also a (real) solution:

You can replace the TIP with a MosFet or add a transistor to drive the TIP

[RVallieu]

Check the manual for the 9403 for the output current, the whole module can only source 64 mA maximum before going into overcurrent protection.

Message Edited by RVallieu on 02-19-2008 12:20 PM

Message Edited by RVallieu on 02-19-2008 12:22 PM

[J_Thomas]

Hi Dan,

I had a couple of thoughts:

Do you have any of the NI 9403 DO lines tied to ground?  If you so you might want to check out the following link:

NI 9403 Over Current Protection

I have seen another case where a user at first had a similar problem when he switched from a PCI-DIO-96 card to a NI-9403.  In this case he had a custom circuit board which switched 5V to 24V.  The problem turned out to be a grounding issue.  Some of his system components were not tied to the same ground.

It might be a good idea to take a digital multi-meter and measure the voltage across different points in your setup.  When the output of a DIO line set to high, do you measure 5 V between the digital output pin and the com pin? Can you measure a non-zero voltage across the 1K resistor?

You might review your circuit setup for any grounding issues?  It may be helpful to post a simple wiring diagram of your setup.