I want to control two relays, but Labview says I have something wrong with my VI

[johnsold]

Yi,

That makes more sense.  The way you described it originally, it sounded like you were driving a 12 V coil directly from the USB-6008.

You may need to look at the external pull up resistor calculations in the 6008 User Guide. It is actually a 3.3 V logic device internally which is tolerant of 5 V logic.

Lynn

[godpaul]

Hi Mr Lynn, thank you for pointing out this. Before that, I just told the manufacturer that the input TTL is 5V and should be under 800mA, they added resister on the board to make sure the current is below 800mA, but I didnt tell them about the logic voltage...

Today we wired the relay to the usb6008, connect external 12V and external load and run. it was successful 🙂

So what do you mean the logic voltage? after I wrote the Vi, all I tested was to measure the voltage at P0 and ground, it shows 5.02V, but not 3.3V?

Should I contact manufatcurer to tell them modify the circuit ?

Thank u!

[johnsold]

Look at the USB-6008 User Guide. There is an internal pullup resistor of 4700 ohms to +5 V. The active pullup on a Digital Output is to 3.3 V.  Depending on how much current you need to source to activate the relay, you may need to put an external resistor in parallel with the internal pullup.  The manual describes how to calculate the values.

If it works, do not worry about it.   The only concern would be if the present values are very close to the threshold of operation, it might fail with temperature or voltage variations.

Lynn