USB-6008 Digital Amperage

[RL]

I am trying to control 4 solenoid valves by writing to four lines on the Digital Output of USB 6008. 

Though the +5V has sufficient capacity to activate all the valves (280 mAmps) each line can only send out .77 mAmps.  The Digital lines Out do not appear to have sufficient power to trigger a solid state relay which only requires 9 mAmps. 

Are the Digital Outputs only good for powering LEDs?  The specification sheet does not give any information about this and it is a bad weekend to get information from NI.  Has anyone figured what the problem is?

Raymond

[Ed W]

Hi Raymond,

The USB-6008 is not capable of sourcing that much current on its own.  You will need to add external circuitry in the form of a pull-up resistor to get current values of up to 8.5mA.

The following Knowledge Base article discusses this issue.

Why Does the USB-6008 Digital Output Line Voltage Drop Below 5V When I Connect to an External Device...

I hope this helps!

Ed W.

[merit]

Hi¡ I`m sending a digital signal using USB 6008 by the terminal PO0 and GND, actually i can see it in an external oscilloscope but i need to know the current value for calculate a resistance, that signal its the input to an external circuit (optocoupler) for activate a mercury relay i wouldn´t like to damage the USB6008 and my circuit. I don´t really know if that value its contained in the device documentation. Could you help me please?

[RL]

Thank you for the information about the limitation of the USB-6008 digital output. 

Merit:  Rather than buy a different DAQ board I am making a small companion board that has a Clare opto relay for each digital output I require.  Depending on the model the triggering requirement is from 2 to 5 mA therefore within the normal range of the 6008 digital outputs. 

I intend to use the +5Volt pin as the power source to trigger the pneumatic solenoid valves (77 mA).

That is the plan.

Raymond

[AnalogKid2DigitalMan]

Ray:

Make sure you put reverse biased protection diodes across the valve coils so you don't fry your DAQ or PC.

[RL]

AnalogKid2DigitalMan:
Make sure you put reverse biased protection diodes across the valve coils so you don't fry your DAQ or PC.

OK so we have established that I am on either a dangerous path or an expensive one therefore I am prepared to stop for advice from a source of knowledge.

My contention was that the Digital Outputs are very weak therefore there had to be some protection built in.  Regarding the +5Volt pin: I only intend to drive two pneumatic solenoids at any given time (154 mV) which is well within its capacity.

Now regarding your warning.  I hear you but do not understand what needs to be done.  I don't know what size of diode to install and I don't understand how the opto relay would feedback power into the Digital Output.  I would appreciate it if you could give me "Hook Up for Dummies" version.

Raymond

[AnalogKid2DigitalMan]

Ray:

It's not really about the opto feeding back power into the digital out line. Rather it is the use of the 5V from the DAQ card as a source of power for the coils. When you turn off the coils via the transistor side of the opto, the magnetic field in the coil collapses. This generates a high voltage spike of short duration (10's-100 V) of opposite polarity that needs to go somewhere- typically back into the power source. Once in the power source, it can go anywhere. Such spikes can damage 'low voltage' devices- DAQ, optotransistor, PC. So, I suggest the use of a standard 1N4004 diode across each coil with the banded end connected to the 5V side of the coil. The other lead of the diode to the opposite side of the coil. The diode will turn on and conduct when the reverse spike voltage reaches about 1V. The diode effectively becomes a short and absorbs the spike.

Some people never do this and get lucky, other's aren't so lucky. It really depends on the coil inductance, currents, lead lengths, power supply integrity, etc, etc.

Hope this helps, I was probably too verbose. Let me know if you need more info.

-AK2DM

[AnalogKid2DigitalMan]

Here's a schematic example attached

[RL]

AnalogKid2DigitalMan:
"Hope this helps, I was probably too verbose. "

I assure you that the "Hook Up for Dummies" version can never be too verbose.  Actually it made me quite comfortable plust I have learnt something without damaging it first.

Raymond

[RL]

Merit,

I have had some very informative and helpful discussions with Brian from NI Support.  There is now more information available about the choosing of the resistor and the results of that choice.  I hope that it is added to the information available when searching USB-6008.  I will also add the diode that AnalogKid2DigitalMan: described for protection.

Raymond