NI 9401 output current limitation

[Cedric1993]

Hi all,

I need to trigger some hardware with 3.3V TTL / 3.3V LVCMOS input using the NI 9401 module which is 5V TTL. Since there's no room for an extra power supply I'd rather use passive components to realise the transition from 5V to 3.3V instead of a voltage level shifter. I was thinking about using a 3.3V zener diode in series with a resistor to create a simple voltage regulator as shown in the image. On the datasheet of the 9401 module I couldn't find the maximum output current so I was wondering if the 9401 module can produce 20mA which is the testcurrent (Iz,t) mentioned in the diode datasheet. Anyone who has done something simular? I could also build a voltage devider with less current in the network but ofcourse that's less stable. The output is to be used as a positive edge trigger.

Thanks in advance

Cedric

[ghp_1]

I have the same question, did you find the answer yet?

[Cedric1993]

After not really getting anything here I just tried it out. I used the zener diode and resistor solution for downscaling the voltage level since I could only use passive components (if you are in the possibility to use active electronics you can use a voltage level shifter which is actually the best solution) I build the circuit into a little black box with the connectors I needed and it works like a charm. Do take the characteristics for your zener diode into consideration though.  The small current and voltage difference between input and output can cause a lower voltage then the 3.3V needed. I used 180 ohms resistor with the 3.3v zener to get a larger current and it works with a short cable (30cm) that I found on some shelf. Longer cables don't work though cause of the voltage devider you create with the internal resistance of the cable and the 180ohms resistor. The components are cheap though so you can maybe try some different resistor values and diodes to check which suits your needs best. Alternative is a voltage devider with 2 resisitors.

Cedric

[ClausB]

I also could not find a maximum output current spec, but the Guide says:

"The overcurrent protection allows only a specified amount of current through the output channels to protect the NI 9401 from short circuits. If the NI 9401 goes into an overcurrent state, the module sets all the DIO channels to high impedance for approximately 280 ms."

However, it does not say what the "specified amount of current" is.

So I placed a 511 Ohm resistor across a 9401 output and measured 4.25 V continuous. That works out to 8.3 mA, more than enough for my application.

The circuit in your diagram only draws ( 5 V - 3.3 V ) / 10 kOhm = 0.17 mA in addition to whatever the output load draws, so you may be safe.

[ClausB]

The 9401 circuit board has an NXP HCT244 buffer on each DIO nybble. Its output current limit is 35 mA and supply limit is 70 mA.

http://www.nxp.com/documents/data_sheet/74HC_HCT244.pdf

The output lines go through SMDs labeled RT1 - RT8, maybe PTC devices for overcurrent control.

[Guenter]

Thank you, now THAT is a useful answer!

Sad that you have to look that up by going into the circuitry rather than getting a clear answer from NI.

Nice greetings, Günter