USB 6009 - Current Issue

[Sandeep_k]

Hi Everyone,

I am a novice at using LabVIEW and its devices. I am playing around with a project and I need to switch a 24VDC supply ON/OFF using an optocoupler with input 5VDC. I will be sending the control signals using the DAQ.

My problem is that the optocoupler requires a input forward current of  somewhere between 7mA to 12 mA to run but I cannot source this much current from the USB 6009. I ran a current test from the internal voltage supply of the DAQ which according to the datasheet is 5V, max 200mA. Upopn running the current test I only found that the internal supply is only sourncing .36mA which is very very low. Am I doing something wrong, is this data correct ?

I have attached the optocoupler datasheet incase anyone wants to have a look.

I read somewhere in this forum that the digital outputs source about 8mA from each port. Is that true ? If it is, is there a specific way to source it somehow, because that would make my work really simple.

Please help !!!!!!

[Sandeep_k]

Hi again,

Forgot to mention another test that I did: - 

I played around with the internal power spply and attached a 470 ohm resistor across it and measured the voltage drop, which was negligible.

I did the same with one of the digital output ports which I assume also produce the same voltage of around 5 V but this time I got a massive voltage drop of .45 V. Why was this observed with the digital output and not with the internal voltage supply ?

Cheers.

[Sandeep_k]

anyone ???

[Mark_SAI]

Hello,

The Optoisolator is rated at an average of 7-12mA input current, with a maximum of 20mA. The USB-6009 is rated to 8.5mA so you may have issues.

But there is something to try, please look at page 22 of the USB-6009 datasheet for guidance on using an external pull-up resistor.

I hope this helps!

[Mark_SAI]

In fact, your 0.3mA reading makes sense:

The optoisolator internal input circuitry probably drops about 3.5V accross the input. The internal USB-6009 pull-up is 4.7k, so (5-3.5)/4700 = 0.32mA.

Message Edited by macaba on 08-10-2009 04:10 AM

[Sandeep_k]

Hi Mark,

Thanks for responding. So if I follow the steps mentioned in pg 22, I should be able to source about 8.5mA from any of the DIO ports and that should be able to light up the LED in the optocoupler ?

Technically if the LED lights up then I can even use it as a switch to turn the 24VDC supply ON/OFF ??

Cheers,

Sandeep

[Mark_SAI]

Hello,

If you follow Pg 22 steps, then it should source 8.5mA from the pin that has a pullup on it. If you put pullups on all the DIO pins, then they can all source 8.5mA.

The optoisolator should be able to act as a 24V switch, provided its rated for 24V and the current you want to draw.

[Sandeep_k]

Thanks Mark.

I ll give it a shot tomorrow and if it works I ll post a ckt. diagram for future reference for others as well as mark the thread as "solution".

Oh yes, and just to confirm, the load in the diagram from Pg.22 is nothing but my optocoupler along with a pull-up resistor (or without) ?? ?

[Sandeep_k]

Hi,

I am sorry I have made a mistake with the value of what the DAQ Digital output is sourcing out. I earlier mentioned that it was 0.36 mA but actually its 0.036mA.

(5 - 3.5) V / 4700 ohm = 0.036mA