5V digital output for controlling a relay

[SiaVahidi]

I am using a NI USB-6009 to control a relay which is used for controlling a 24V Solenoid Valve. The relay control voltage is 4-28V and therefore the 5V output should be enough for it. Once I energize the pins, I can get a differential voltage of 5V, but when I connect the relay to the same pins, the voltage drops to 3.3V which is not enough for the relay. I assume it has something to do with the relay requiring a current more than what the DAQ can provide. Is there any way to fix this? Like any suggestions for a relay that can be controlled by USB-6009 and yet be able to control a 24V valve? (I am using another power supply for the valve so I just need the DAQ to support the relay.)

[crossrulz]

Look in Digikey.  There are plenty of 5V buffers that can be used to increase your current capacity.

[Buechsenschuetz]

Most relays need significantly more than 10mA current and there are not so many controller boards which can handle this output current. 

You may try a PhotoMOS relay, they need only a few mA on the input side. You will need a series resistor to limit the current such that the voltage on the input pins is the forward voltage of the input diode (usually around 1.2...1.5V) at the given current. 

Also, the output drivers of some boards can be connected in "sink mode" and can handle more current this way. In "sink mode", the load is connected to +5VDC supply (the same as for the board if possible) with one terminal and the other one is connected to the board output. Output logic levels are inverted, i.e. when the output is set to LOW state (under normal conditions), the load will be energized. In any case, check whether this is possible with your specific driver board. 

[SiaVahidi]

Thanks for your reply Buechsenschuetz. I believe this part of your answer is really helpful but the problem is I am a civil engineering phd student and do not understand any of that! Can you please elaborate on that a little:

"You may try a PhotoMOS relay, they need only a few mA on the input side. You will need a series resistor to limit the current such that the voltage on the input pins is the forward voltage of the input diode (usually around 1.2...1.5V) at the given current."

Are you saying that the PhotoMOS relay is capable of controlling a 24V solenoid valve and could it be controlled by digital output provided by USB-6009?

I understand it is a lot to ask but if you can be more specific on the exact relay and resistor that I need to get it would ne a great help.

[Buechsenschuetz]

There is an example how to connect a load in "sink mode" on p. 20 of the user's guide

http://www.ni.com/pdf/manuals/371303n.pdf

There are also hints how to determine a proper series resistor to the load. You can also calculate the value with some understanding of Ohm's law, calculatiing values of resistors connected in parallel (there is an on-board resistor) and diode forward voltage. 

The data sheet of the AQV212 PhotoMOS relay can be found here:

http://www.digchip.com/datasheets/download_datasheet.php?id=4672796&part-number=AQV212&show=inline

It says that this IC needs 1mA of input current. 

Please note that are diodes on the output so the DC circuit of the solenoid must be connected in proper direction. 

[SnowMule]

Or use a relay driver, like a ULN2003...