Relay to switch Pump and Fan with PCI 6221DAQ Card

[Simon790]

Hello

I want to switch on and off a pump and a fan of a watercooling system. They need together 12V/0,6A.

Is it right that the digital output of the 6221 37 Pin DAQ-Card delivers 2.2V and 24mA in the high-position?

What kind of relay do i need to achieve this?

thx

Simon

[Matthieu Papaux]

Hi Simon,

You are right, the digital output can deliver 24mA but they give 5 Volts (TTL output).
So you can use all kind of relay, they don't use more than 24mA with 5 Volts (R > 210 Ohms).

Regards.

Matthieu Papaux

Applications Engineer
National Instruments Germany

[Buechsenschuetz]

"You are right, the digital output can deliver 24mA but they give 5 Volts (TTL output)."

I do not know the specs of this board in detail but I have doubts that this statement is true. A TTL level input usually requires approx 2.3V to detect a logic high level, and most TTL compatible outputs are designed to generate such a voltage level. I have never seen any output which was rated at 5V and 24mA source current.

Open-collector outputs (or outputs which can be used with a pull-up /load/ resistor) are a different thing. If you connect the load between the +5V supply and the output most acquisition board outputs I have seen can handle up to 24mA.

In any case, do NOT connect any relay to an output without a clamping diode connected in reverse direction across the relay to prevent damage by inducted backlash current.

To drive relays with higher voltage and/or current rating than the output of a board can supply my first choice would be an opto-electronic relay like the AQV212 by Matsushita. They need a simple series resistor at the input limiting the input current to 10mA approx. The output can drive a load of 30V and 300mA approx. Of course you need an external power supply of suitable voltage rating. And again, do NOT forget the clamping diode...