Need help specifying Relay (24V)

[hp1988]

I need help specifying a relay that will activate 24V power (ie close the circuit) coming out of the DAQ USB6501 output.

Basically, the way it works is when the virtual start button is pressed within the LabView program, it activates the output of the DAQ.  I measured the voltage from the DAQ and it comes out to 3.3V.

We can not find a relay that will activate the 24V to start the instrument.  I have attached a schematic.  Thanks in advanced for any help.

[RTSLVU]

Yeah 3.3 volt relay coils are not very common.

I believe 5 volts is available on the 6501, so I would recommend a relay with a 5 volt coil and a transistor to switch it on and off from the 3.3v DIO

[GerdW]

I would suggest to use SSRs when possible instead of coils.

Something like a Crydom MPDCD: this one switches at 3V with only 2mA driving current…

[hp1988]

Thank you for your response.  We originally tried this with the following result:

 3.3V comes out of the DAQ when the “virtual” button from LabView is pressed… \

Similarly, when I measure the resistance coming out of the relay (after the virtual start button was pressed), it went from 4MΩ+ resistance to ~311KOhm…so the relay “closed” but seems like the resistance wasn’t low enough to “short out” Hot and Return.

If you can suggest anything else, please do.

[RTSLVU]

Here's some ideas I posted years ago for someone who needed to control LED's that were powered from 24VDC using a DIO port. 

Simply replace the LED and resistor with your relay. Don't forget to put a snubber diode across your relay coil. 

[hp1988]

Hello everyone.  We are trying to specify a relay that will work with the 5V output.  Is there a software utility for 6501 from NI that I can download and it will be able to invoke output voltages?

(sorry, I'm not a LabView expert -- just an end user).

[Gregory]

The USB-6501 is a digital I/O device, meaning you don't have control of the voltage exactly, just high/low capability. According to this manual, you can expect 3.5 - 5.0 V for the digital outputs, and 4.0 - 5.5 V for the "5V" output. 

You can use the NI Masurement and Automation eXplorer (MAX for short) to configure the digital lines be input/output and if output you can select high/low. You need to find your device in MAX and open a "test panel"

I don't know of a good way to just download MAX if you don't have it. But I know it comes as part of the system configuration, so you can download that if you need it.

https://www.ni.com/en-us/support/downloads/drivers/download.system-configuration.html#371210

[RTSLVU]

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@hp1988 wrote:

(sorry, I'm not a LabView expert -- just an end user).

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Honestly this is NOT a LabVIEW issue, this is electronics 101.

According to the USB-6501 specifications in Open Collector mode the max DIO current is only 8.5mA.

Here's a selection of relay's that have a 5 volt coil and draw less than 8mA of coil current at Digi...

You will need to determine the proper contact rating for your application.