digital IO with relays

[sunny dee]

Hi, I am trying to hook up some relays to my digital I/O of my M series DAQ card. The relay requires a 5V signal to switch, but has an internal resistance of 250 ohms. The maximum output current of the daq card i believe is 24mA, which would work, but if the resistance varies at all the relay current draw may exceed 24mA. What is the best way to do this?

Would this work:

If I supply 5V, using an external power supply, to the source of the relay and connect the sink of the relay to a digital output from my DAQ card. The digital output would be held at 5V until I needed it to be switched, at which point I would change the Digital out to zero and so all of the current would be supplied from my external power supply. My question with this method is: if I specify a digital channel to be used as output, what is the maximum current that can flow into that channel?

Thanks!

Sunny

[Sean_C.]

Hello Sunny,

You run into a similar problem with this configuration. The maximum current that a digital output can sink is also 24 mA. So if your relay impedance varies slightly, you run the risk of sending too much current into the DAQ device. To remedy this, you could put another resistor in series with the relay to make sure that no more than 24 mA is sent to the DO channel.

I hope this helps,
Sean C.

[AnalogKid2DigitalMan]

You could use transistors to boost the relay drive, this may invert your logic levels. Another option are buffer/relay driver IC's. A word of caution, use surge supression diodes across the relay coils to prevent transient voltage damage to semiconductors.