cRIO 9403 output current

[cbelting]

I saw a question regarding the output current capability of the 9403 32-channel DIO but here's a different version of the question.

What device is being used to drive the outputs?

The user manual indicates output voltages for two output current conditions (100uA & 2mA) and a maximum sourcing current capability for the entire module, but does a poor job of accurately describing current limits. I'm driving a solid state relay that will operate reliably with 3mA of LED current, the number of outputs is limited to less than eight, and I can sink current through the SSR LED rather than source it.

So if the output driver device (or capability) is better defined I could determine the output voltage at 3mA and see if it will work for my application.

Thanks

Chris

[dsFlux]

Hi cbelting,

As per the user manual and the information on the product overview page, the max current output from each DIO line is 2 mA. The maximum current the module as a whole can output is 64 mA (32 channels of 2 mA each). The module would not be able to output 3 mA from a single channel. Let me know if you have any other questions.

[cbelting]

Well that didn't really answer the question but it is consistant with the manual that seems to imply by listing two different currents, the reader should assume the higher current is a maximum limit.

I'd say it is wasted effort on both our parts David.

[johnsold]

When you consider the overcurrent protection warning in the manual, I would say that attempting to get more than 2 mA from any one output is asking for trouble.

It appears that you are only using 8 of the 32 lines. Consider a group of 4 lines. Put a current limiting resistor in series with each line. Tie the other ends of the resistors together and connect to the negative end of the SSR input. Connect the positive end of the SSR input to +5 V. Drive all 4 lines in the group simultaneously. The current limiting resistors will cause the lines to share the total current. Each line then needs to sink only 0.75 mA to provide 3 mA to your SSR.

Lynn

[Dcarson]

Interesting solution using multiple outputs to drive a single relay.  Really defeats the purpose of having all the outputs.

I need to drive relays with the outputs of the 9403 is there any way to externally increase the current limitations like what  is avaliable on the USB 6212 with external 5 VDC supply?

If I can't drive SSR's with this module its useless to me.

[johnsold]

Look at something like the ULN2003 buffer/driver. The 2 mA which each output can produce is enough to turn on the ULN2003 which in trun cna drive the SSRs.

Lynn

[Dcarson]

Thanks, I will check the ULN2003 buffer/driver out.  

Next time I will read the specifications carefully, another module would have suited my application better.  

My assumption was that the output current would be similar to whats used in the USB DAQ 16 ma per channel.

[cbelting]

I'm sorry johnsold (Lynn) I had not seen your response of 06-03-2013 - it seems well reasoned.

I admit I had not read the Overcurrent section of the manual previously and while it does suggest caution is necessary, it's difficult to imagine from an engineering perspective that enforced current limiting at a 2mA level on an individual output basis is very practical or cost effective. Possibly highly integrated driver ICs could implement 2mA current limiting easily, I've just never needed that level of protection and so have never looked for a part to provide that function. As note 1 (page 11) seems to indicate, a 64mA limit on the module is much more practical from an implementation perspective. It's hard for me to turn off the "engineers" perspective.

The suggested solution of using multiple outputs in parallel requires one to assume multiple outputs will switch states at the same instance. It's probably a reasonable assumption and when properly implemented, including the current limiting resistors, could work flawlessly. Of course the devil is in the details and if one of the "parallel" outputs is inadvertently driven in the opposite direction ... well everything would depend on the resistor values.

Sorry, I don't mean to be negative, I think the suggestion is very creative. For me personally, it's not that difficult to add a transistor and a few resistors at the SSR to achieve the current required.

I just always hate to add unnecessary complexity and find sometimes it's beneficial to ask why, rather than assume. Years ago Richard Quinnell wrote a wonderful article for EDN News Breakpoints called "Llama Alert" - well worth the read if you can find it.

Sincerely - thanks for the thoughts.

Chris

[johnsold]

Chris,

I agree that it is not clear how a 2 mA per channel limit would be imposed.  Perhaps it is inherent in the IC they use for the D/A converter.

The beauty of the ULN200x family is that you get 8 drivers in one package and do not need any resistors or antiparallel diodes to drive moderate inductive or resistive loads.  One IC plus wires!

Lynn

[Zack_N]

I realize this thread is old but I found it recently searching for the same information.  I ended up emailing tech support and got an official answer, which I will share below.  Basically the total current source/sink of the module is 64mA which is divided among all IO.  So a single line can source/sink 64mA but in that case the rest of the lines could not be used.  Another consequence of this is that using multiple IO lines to drive a single device does not increase current source/sink capability.  I definitely think the documentation could better explain this.

"This is Jeff from NI's Applications Engineering group.  With this device, the total output is shared between the channels.  The specs listed on the sheet assume you want to use all 32 channels with the maximum output and show the respective voltage values.  If you do not want to use all 32 channels, the maximum output is shared between the channels you are using.  The maximum output for a single channel is 64 mA.  However, using this output will limit you to a single channel.  To determine what the maximum value you can see with your application, we can divide the 64 mA by the amount of channels you intend to use."