myDAQ, Increase Digital Output?

[SNorrell]

Hi y'all,

I'm currently working with a myDAQ trying to trigger three 3V relays through the digital outputs. My main problem is instead of giving me 5V when triggered, the digital outputs are only giving me ~2V since they can turn on an LED, but they don't switch my relays. Is there any way to increase the voltage coming from the digital outputs?

I've tried using the analog outputs instead (since they can put out 10V), but can't seem to get those to work the same as the digital outputs. I essentially just want to press a button and the output to send a steady voltage >3V and then turn off once I press the button again. I got this to work with the digital outputs, but the analog outputs throw me buffer errors and other input errors I can't seem to fix.

I'm a beginner to LabVIEW and myDAQ and would appreciate any help! Thanks!

[Dennis_Knutson]

The problem with driving relays has been discussed numerous times. Check your specs for the maximum current output. It's probably far less than what your relay requires. A quick and easy fix is to use an external relay driver such as a uln200x.

[SNorrell]

thanks for the quick reply!

I saw the uln200x as options discussed in other posts, but I don't think I fully understand it...

(datasheet: http://www.farnell.com/datasheets/1690348.pdf)

So essentially I would just use the program I have now that outputs ~1.5V to 2V through the digital outputs and wire it to one side of the uln200x and then wire my relay to the other side? The purpose of the uln200x is to increase the current/voltage? or do I need to wire another power supply in there somewhere?

[RavensFan]

No, your digital output puts out 5 V.  But when you connect it to something that is more load than it can handle (such as a relay coil), it is pulling the voltage down  to 1.5 V to 2V.

The ULN200x is a buffer that takes the digital output and drives a higher current source.

What are the specs for your relay coil?

[SNorrell]

the relays are  "Goodsky RW-SH-103D" (http://www.goodsky.co.uk/datafiles/RWRWH.pdf) I think from that sheet it's saying 5V at 10mA. The ones we have have "Coil: 3VDC" on top though, so I'm not sure if thats the same amps...

[0utlaw]

Hello SNorrell,

According to the myDAQ's specifications, each DI/O line can source a maximum of 4mA, which won't be enough to drive your coils unless you use multiple outputs to drive each.  I don't recommend attempting that. 

Depending on how many relays you need to use, however, it's likely that you can get a few to work without additional hardware by using the built-in 5V supply line to drive the lines and grounding them at the DO.  Invert the logic on your digital lines so that they go low to turn on the relays. You'll need to account for power-up conditions and ensure your hardware doesn't toggle on or off unexpectedly when powering on the DAQ or the PC. You'll also want to ensure you're not driving too much current to the digital pins, refer to the documentation for current and voltage limits.

Regards,

[RavensFan]

That 5V @ 10mA is "minimum switching current" which is related to the contacts.  The coil specs on page 2 show thaat it draws 73 mA at 5VDC.  That is a LOT more than the 4mA maximum the MyDAQ can put out of its digital output according to http://www.ni.com/pdf/manuals/373060e.pdf.

You definitely need to put a driver circuit in between.

[Dennis_Knutson]

And the 3V relays require 120 ma.

[SNorrell]

Tom I like this suggestion a lot, but I'm not getting my DO's to work as "grounds" when I power them on or off. Is there a special way to wire this so it'll act as a ground? I like this suggestion because I know the 5V constant output works with the relays when I connect the other end to the DGND

[RavensFan]

Check your voltage.  Dennis' post made me realize you have 3V coil relays based on the #103 in your part number.  Putting 5V into your 3V coils (167%) is too much.

http://www.goodsky.co.uk/datafiles/RWRWH.pdf

I think you'd be better off picking different relays, along with creating the driver circuit.