USB-6008 DIO source current drive?

[ei0hoda]

Hi

I'am using a USB-6008 device and I'm having trouble with my digital outputs. I want to controll a relay, which controls a motor, and it says that at 5 VDC it requires a current of 2.6 mA. As I see it the USB device should be able to deal with this however I can't get it to work properly. When I try do set a digital port high the voltage drops to about 2V and the current is only 0,67 mA which means that my realy won't consider the signal as High anymore and nohthing will happend. I have cheched the ports and when I dont connect then to the relay the gives a output voltage of 5 VDC, which seems correct. I have also tried to use the 5V output from the USB device and this works, the relay swithces and the current is 2.5 mA from the USB device. Is there something I'm missing ? or am I just doing something that won't work with this device ?

Thanks for your answers !

regards
Daniel

[Tom_W_[DE]]

Hi Daniel-

According to the USB-6008/6009 User Guide, the output current for Open-Drain setting (the only setting available with the USB-6008> for an output high level is 0.6mA. This seems to reflect the performance you are seeing.

In short, the USB-6008 will not be able to supply the current that your relay requires.

Regards,

[ei0hoda]

Hi

Thanks for your good answer. Looked in the guide and found what you are pointing out and it seems to be the reason why I can't get the coupling to work. I also noticed that you can use a external pull-up resistor to get a source drive current of max. 8.5 mA.

I tried this and it seems to work, I drive the extra current I need from the 5 VDC output from the USB device and I get my 2.5 mA required for the relay. However I get a high frequency sound from the USB-device when I connect a digital out line via a pull-up resistor to the 5VDC. Anyone that can guess what the sound comes from ? I can't anyway but everything seems to work properly.

Thanks again for your answer!