How to control Solenoids using USB6501

[Jason0502]

I would like to control 8-12 solenoids (12Vdc, 2.5W @ 😧 100%, 25W @ 😧 10%) from my usb-6501. I have read many posts that explain similar operations and would like to just double check as I don't completely understand "open collector" and "active drive", or which would be better suited for my application.

Because of the low current drive of this DAQ it appears that my options are to use 1) a transistor to operate a mechanical relay to operate the solenoid (as per post: drive relay coil with transistor powered by usb-6009?), or 2) use a mosFET to operate a solenoid (as per post: usb 6210 for 24VDC solenoid valve). I do not require super fast switching capabilities, at this stage I just need to be able to send a digital signal from LabView to turn on or off a solenoid.

Which option ( MosFET or relay) is the better choice (i.e. Most reliable, more robust and safest in terms of the USB6501)? Or have I overlooked an alternative solution (probably likely)?

I understand this is probably a very simple application and appreciate any assistance, also I can provide more information if required.

[SnowMule]

Transistor driving a relay that controls the solenoid is usually what I do off the digital outputs of the daqs (we use both 6501's and 6008's, but the DIO's the same). 

Smaller 5V solenoids I'll drive right through the transistor and eliminate the relay.  Big 12V solenoids (like what you're working with) need a relay to separate the voltages and reduce the possibility of inductive kickback current getting into the DIO.

[Tom__Sedlack]

You can also use opto-isolators to control the relays or, if you can find one that will handle the current, control the solenoid directly. Get a Darlington opto-isolator (such as a 4N33) so the LED current requirement can be reduced.  Using optos also alows you to electrically seperate the two systems (6501 and the Solenoids) for noise immunity.

There are also relay driver ICs such as the ULN2003 type that are designed specifically for this purpose.

[Jason0502]

Your suggestions have been very helpful. Thank you.

[Buechsenschuetz]

Galvanic isolation is always a good idea when driving relays with a coil voltage higher than the supply voltage of the driver circuit. We are widely using the AQV212 photo-MOS relays for this purpose. Different from the darlington output of the 4N33 and similar, the output FET of these opto-relays have very low saturation voltage (or very low resistance). Also, you can use them as high-side or low-side driver. 

In any case, do not forget to connect a snubber diode in parallel with the relay coil. The catode of the diode must be connected to the (+) terminal. For low-power relays a 1N4148 will do.  

[christinetaylor]

What you have just said is true. I did it in one of my school projects few years back and it really works. It worth giving it a try.