USB-6008 digital out Amps

[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?  Is there something to be done in Labview or is this a hardware limitation?

Raymond

Message Edited by RL on 11-24-2006 05:31 PM

[Brian_Beal]

I have no problem driving OPTO22 relays with the USB digital outputs.  The current draw on those is about 15mA.  There must be something else going on with your application.  Are you able to test the outputs using MAX?

[RL]

Thanks,

First, we tried with a program written by a Labview Developer and MAX then I tried with MAX on another desktop computer.  Results were the same.  We measured the draw and we measured the output of each Digital Line (peak output was .77 mAmps using only one line at a time).  The USB is not the limitation because the +5V pin will power all four solenoids at one which comes to 280 mAmps. 

It appears that some kind of artificial limitation was put on the output amperage.  My other more expensive boards have no problems driving these devices.

Raymond

[Brian_Beal]

I think you are correct in that the better boards can drive more.  I use the USB M-series.  Still, less then 1 mA can't be within spec.

[Dennis_Knutson]

The spec says 8.5ma. That's much higher than what you are measuring but still not high enough for the 9ma you need. It's possible the device is damaged. You could use something like a ULN200x relay driver to provide the needed current and also give you isolation and protection.

[Cyro [RPF-BR]]

I use MAX 232 or JFETs to boost up the current.
For solenoids is better to use FETs to excite a small relay and then use the relay to drive the solenoid.
I had an application once a time that used a 150mA FET to drive a small relay to drive a big relay to drive a motor!
BF245 is a good FET to do it.
For currents up to 14A I suggest IFRP450N (take care to use a reverse diode against the GND in the source of FET to do not kill it).

[bmunden]

Crazy.  I'm trying to control 4 solenoids (and two pumps) with a DAQ USB-6008.  I'm just using an external DC power supply with a couple of transistors to act as a buffer drive circuit that'll activate the heavier solid-state relays.  The transistors are great, because they only draw milliamps, but can give out four or five amps without a sweat.

[AnalogKid2DigitalMan]

Make sure you put supression diodes or snubbers across the valve coils if they are DC powered. The back EMF created when the valves are turned off can eventually destroy the transistors and the DAQ card depending on the coil specs.

EDIT:

And there is also the ULN2003:

http://focus.ti.com/lit/ds/symlink/uln2003a.pdf

Message Edited by AnalogKid2DigitalMan on 04-13-2007 11:46 AM

[RL]

Thanks.

I am using Clare CPC1004N OptoMos Relays.  It is a small board I call the DAQ Booster which incorporates a 1N4004 diode (as suggested on this forum) for protection.  It works very well and I am able to control pneumatic relays with the USB-6008

I believe that there is a schematic that shows the placement of the diode in the hardware forum

Raymond

Message Edited by RL on 04-13-2007 01:57 PM

[RL]

I'm not certain if you succeeded or not.  If not you might have a grounding problem.  My DAQ Booster controls 5 Vdc pneumatic valves therefore I use the USB-6008 ground and 5 Vdc output therefore everything ties together easily.

There is another thread in the Hardware Forum if I remember correctly.  I also received some information on sizing the resistors for the Clare CPC1004N OptoMOS relay which should come out in a Knowledge Base search.

Raymond