Need to Amplify Current of Analog Output

[JonathanLindsey]

I am using a NI USB-6218. I need to output an analog voltage to control a needle valve with a range of 1 to 5 volts. Unfortunately the DAQ can only supply 2.4 mA Overdrive current acording to its datasheet while the needle valve uses something like 20 or 30 mA signal. The current from the DAQ is just not enough. What is the best way to Amplify the current from the DAQ? I can use a seperate power supply which has enough current.

I called NI about this and they suggested a current operational amplifier but I have not been able to figure out how to use it to boost the current. I have also tried using a transistor without succes.

Another requirement is that I need the output to the valve to be 5 volts if the DAQ is disconnected from the computer and until it reconnects to the program after being plugged back in. I have been able to do that with a relay.

Thank you for your time!

[crossrulz]

There are plenty of linear amplifier chips out there.  Do a search on Digikey and you can limit it based on your specs.

[Henrik_Volkers]

If you run your valve in a control loop and some slow drift and a sligth offset doesn't hurt: A simple Transistor as a emitter voltage follower.

https://en.wikipedia.org/wiki/Buffer_amplifier

[JonathanLindsey]

This is what I was looking for although I may come back to crossrulz's suggestion if this does not work.

I am not sure exactly what circuit Henrik was referring to, however. Could you post a picture of the circuit? I have tried the following circuit but I only get 2.52 volts at the valve when it is set for 5 volts by the DAQ. Is this because I am using a 2222A transistor? If it is, what would be a more suitable transistor?

Sorry for my limited understanding of transistors and such.

[SnowMule]

I've used a voltage follower to drive a transistor before...

[JonathanLindsey]

This is what ended up working. We used a different transister so it is possible the other was bad.

Thanks!

[johnsold]

That cannot possibly work as you have it drawn. It might turn the valve on and it might damage the DAQ AO line but it will not give control of the valve.  The circuit  you posted in message 4 of this thread is more appropriate.

Lynn