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Controlling Contact Closure Inputs (App Note)

 

Question: "I have a machine that turns on when two wires are shorted together or a toggle switch is used to short the wires. That is, the machine is looking for a contact closure signal. How can I control this machine with a LabJack?"

 

Some devices have contact closure inputs or dry contact inputs.  Those inputs detect whether they are open or shorted.  Here we discuss different options for using a LabJack to control those device inputs.

 

External Mechanical Relay

The surefire way to create a contact closure is by using a digital I/O to control a mechanical relay.  The output of a mechanical relay is a dry contact, so will work for sure.  Mechanical relays require high drive currents to control and thus most will require something extra between the LabJack and the mechanical relay.  See the Controlling Relays App Note.  The most plug-and-play solution would be an RB12 with some of the "Dry Contact Output" modules listed on the RB12 Datasheet.

 

External Solid State Relay

Using the LabJack to control an SSR is similar to the mechanical relay solution above in that the output side of the SSR is isolated from the input side of the SSR (and thus from the LabJack).  The output of an SSR is not identical to the output of a mechanical relay, but most contact closure inputs should recognize the off and on of an SSR, and the advantage is that an SSR is easier to control than a mechanical relay per the Controlling Relays App Note. 

 

Directly with Digital I/O

If the low side of the contact closure input on the device can be connected to LabJack ground, the contact closure can usually be simulated directly by connecting the high side of the device input to a DIO.  With the DIO configured as input the line is in a high-impedance condition, with about 100k from the DIO to 3.3V on the U3/U6/UE9/T7.  This 100k is generally too much impedance and will look like an open switch to the device input.  With the DIO line configured as output-low, the DIO is connected to GND with a small resistance in series (180 or 550 ohms on the U3/U6/UE9/T7).  This small impedance to ground is usually low enough to appear as a closed switch to the machine. 

If the 100k off impedance is too low, or the 180 ohm on impedance is too high, you can add the LJTick-RelayDriver.  It is the same in that you need to connect device GND to LabJack GND, but the off impedance is much higher and the on impedance is much lower.