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Controlling Relays With U3, Lap Top and LJTick-RelayDriver


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#1 u3cool

u3cool
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Posted 27 May 2014 - 12:35 AM

I'm trying to control some 12 VDC relays with a U3, a lap top and a LJTick-RelayDriver.  The relays draw 125 ma.  When I set the DO to high the relay does not turn on.  If I connect the cable that connects to the DO to the 12 VDC negative terminal the relay turns on.  The 12 VDC power supply and the lap top are plugged into 120 VAC.  I thought the 12 VDC power supply and U3 grounds would connect through the VAC, but it appears they are not.  Should I connect the 12 VDC power supply negative to a U3 GRD through a 10 ohm resistor?  If this is the solution, would a 39 ohm resistor also work, which I have on hand.  I also have a flow meter coming into the U3 that is powered by the U3 VS.  Would connecting the U3 GRD to the 12 VDC power supply interfere any way with powering the flow meter with the U3?



#2 LabJack Support

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Posted 27 May 2014 - 04:22 AM

"I thought the 12 VDC power supply and U3 grounds would connect through the VAC, but it appears they are not."

You are right. A lot of AC to DC converters will isolate the two sides from each other.

 

The necessary wiring can be a little counter-intuitive. The datasheet has a couple schematics to get you going: http://labjack.com/s...river/datasheet




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