DAQ Input - Holds Relay Energized

[Steve Miket]

Hi,

I have a fault relay, where one side is tied directly to 24VDC. 

The other side goes to a switch, ultimately to ground.

When the switch closes, the relay energizes and stays energized via a latching circuit.

The switch on the first line is a fault sensor which is normally open.

The switch on the second line is the latch, which is normally closed.

The third line is the DAQ input

24VDC --- RELAY --- SWITCH  -------------- GROUND    

               |--- CONTACT --- SWITCH --- GROUND

               |--- DAQ     --- 2.4K   --- GROUND

My problem is that I want to monitor the fault bus.  I've tied the switched side of the fault relay to a 6514 DAQ input via an SCB-100.  This works, and when the line goes low, the DAQ senses the fault.  However, the internal circuit of the DAQ input has a 2.4K resistor which is a low enough value to keep my relay energized.  I can't reset my fault.

Incidentally, the circuit I am monitoring is on a machine which has run for years.  It's the introduction of my connection that is affecting this.  Without the DAQ connection (remove third line from above circuit), I can reset the line and everything works fine.

Can anyone tell me how I can put something in between the external circuit and the DAQ input, such that I can sense the signal but not affect it?

Thanks,

Steve

[Buechsenschuetz]

You could try with a PhotoMOS relay, such as AQV212 from Matsushita.

These devices need a few mA of currents (up to 10mA or so) and a low voltage only (around 1V) to switch the output. The output is a FET transistor and can be tied to any circuit.

If your relay just needs up to 10mA of current you can insert the opto relay directly in series with the relay. Very probably your 24V relay will work properly with 23V. If your relay coil needs more current you will have to use some extra circuitry (some resistors and perhaps a zener diode) to energize the PhotoMOS relay.