NI 6009 LED switches OFF when switching the power supply of the connected circui

[9831238195]

I have a NI 6009 to give base drive signal (5V) to a relay driver ( CL100, beta: 100) transistor through a 15 K base resistor. The relay is of 12 V (nt73_jqc-3fc) with a field coil resistence of 400 ohms(connected between collector of CL 100 and power supply), thus it needs 30mA current for the field coil to energise ( accordingly the resistor value of 15K is calculated). I have also used a flyback diode across the field coil with proper polarity. The emitter of the CL 100 is grounded.

The problem is that while NI 6009 is connected with the circuit ( NI 6009 and the circuit operates normally at any other times) and the power supply mains (+-12V regulated, extracted from 230 V ac, 50Hz mains) is switched off/on, the green LED of NI 6009 also switches off most of the times !!! This means that NI card is getting disconnected from the PC. I have to reconnect the USB again for proper operation of the circuit that is already given the power.

Why is this happening?

Is this harmfull for the DAQ card in the long run?

What can be done for remedy? 

[crossrulz]

I would recommend some digital buffer circuits.  It is typically not a good idea to have a DIO module drive a relay circuit directly.

[9831238195]

We have tried with buffer IC connected but the problem persists. In fact, even if there is a single connection between the DAQ card and the circuit this problem is seen. In fact, I have disconnected all the IO lines from the NI card and only tied the ground of the circuit with the ground of NI card and even then also this happens!!

I suspect this has something to do with the power supply section of our circuit (PCB). Just to note: this problem is not observed when i run my circuit with an agilent power supply.

There may be a problem with our PCB circuit but in order to solve that i need to know why does the NI card gets disconnected from PC at all !!.

[K C]

You connected the relay in a nice way. So that should not be any problem. I would recommend that add a pulldown resistor (100k) at the base of the transistor.

Maybe the problem is the level difference between the 0V of your USB (PC) and the 0V from the power supply. Can you measure this without connecting the power supply to your board?

Kees

[9831238195]

There is 0.4 V difference between gnd of USB DAQ and the circuit (Power supply is built in the PCB circuit), will this make a significant difference?

[9831238195]

when i connect a GW-instek standard power supply (model GPS-2303) to energise my circuit this does not happen. However with a standard dual power supply design this keeps on happening !! any ideas where is the difference ?? Some Help will be greatly appreciated !!

[AnalogKid2DigitalMan]

Well, you have no problems when using an Instek or Agilent supply, these are probably floating ouputs with no power mains ground reference.

Can you post the schematic of your own +/-12V supply and your 6009 to buffer to relay wiring? I suspect it may be a ground or regulation issue when the power is cycled.

-AK2DM

[apok]

i would connect your daq to a UNL2003(TTL compatible) as a "low side driver", which provides 500mA for your 12V relay. The fly back diode is good, but the ULN already has it incorporated internally(output clamp diode)...no need for it.

[apok]

hope this helps...

[johnsold]

If you connect the two grounds with a conductor having a resistance of 0.1 ohms, then you could have 4 amperes flowing and something will probably be destroyed. You already know it does not work whne connected.

You almost certainly have something forcing the power supply of the external circuit to a different potential than the computer/USB-6009 ground.

Lynn