How to input or output current?

[dragondriver]

Hi all,

  I have a question regarding the signal acquisition. How to output a current instead of voltage to one terminate? What will the current be different if it flows out/in from analogy or digital terminal? I don't have solid background on circuit 😞

[RavensFan]

If you want to output a current signal instead of a voltage signal, you need to use the appropriate DAQ module.  A current output module has an inherent feedback mechanism to be able to maintain the requested current no matter what the voltage drop is through the rest of the current loop.  (Within some limitations of course.)

I dont' understand your question out in/out and analog? vs. digital channels.

[dragondriver]

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@RavensFan wrote:

If you want to output a current signal instead of a voltage signal, you need to use the appropriate DAQ module.  A current output module has an inherent feedback mechanism to be able to maintain the requested current no matter what the voltage drop is through the rest of the current loop.  (Within some limitations of course.)

I dont' understand your question out in/out and analog? vs. digital channels.

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Thanks. I know it is kinda confusing. I don't know much about electronic circuit but as my understanding, a digital signal is a TTL signal with digital one is about 5vol. So in this term, is anything called digital current signal? i.e. is that any way to output a current signal via the DIO line? Also, if I output a digitial 1 to a digitial line, and have the line connecting to a load, should the actuall current flow into the load determined by the resistence and the 5volts?

[RavensFan]

There is no such thing as a "digital current signal".

A digital signal is either high or low, 5V or 0V like you said.  The amount of current that is drawn on that digital output does depend on the resistance of the load.  You want to make sure that you don't pull more current than the digital line is able to put out.  Typically the values are very low on the order of milliamps.  If you want to drive a heavier load such as a large relay, you may need to use a transistor circuit so the digital output can turn on or off the transistor that drives the relay coil.

[dragondriver]

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@RavensFan wrote:

There is no such thing as a "digital current signal".

A digital signal is either high or low, 5V or 0V like you said.  The amount of current that is drawn on that digital output does depend on the resistance of the load.  You want to make sure that you don't pull more current than the digital line is able to put out.  Typically the values are very low on the order of milliamps.  If you want to drive a heavier load such as a large relay, you may need to use a transistor circuit so the digital output can turn on or off the transistor that drives the relay coil.

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Thanks. So you mean the voltage of terminal line or AO channel are fixed to what fed to it but the current depends on the load, right? So how do I figure out the maximum output current so to figure out if it damage my load or not?

By the way, what happen if I want to output a constant voltage (AO or DIO) and constant current?

Finally, I just figure out something interesting. If I set any digitial line to "1" (5v) and even I terminate the VI code, the state of the line will stay as "1" until I reset it, why is that? Will it be the same case if I use AO channel instead?

[RavensFan]

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@dragondriver wrote:

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@RavensFan wrote:

There is no such thing as a "digital current signal".

A digital signal is either high or low, 5V or 0V like you said.  The amount of current that is drawn on that digital output does depend on the resistance of the load.  You want to make sure that you don't pull more current than the digital line is able to put out.  Typically the values are very low on the order of milliamps.  If you want to drive a heavier load such as a large relay, you may need to use a transistor circuit so the digital output can turn on or off the transistor that drives the relay coil.

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Thanks. So you mean the voltage of terminal line or AO channel are fixed to what fed to it but the current depends on the load, right? So how do I figure out the maximum output current so to figure out if it damage my load or not?  For a voltage analog output, the voltage at the terminal relative to common is fixed at whatever level you designate within your code.  You generally aren't going to damage a load.  What will happen is that if a load pulls too much current, you will damage the DAQ device.

By the way, what happen if I want to output a constant voltage (AO or DIO) and constant current?  You can't.  Voltage and current are directly related to each other by way of the resistance.  If you change the voltage, the current will change. Because the resistance isn't going to change.  V=IR

Finally, I just figure out something interesting. If I set any digitial line to "1" (5v) and even I terminate the VI code, the state of the line will stay as "1" until I reset it, why is that? Will it be the same case if I use AO channel instead?  The AO channels and DO channels will do the same thing.  If you end the VI, it is going to stay at the value you left it at.  Why?  Because you didn't tell the DAQ output to be anything different.  If you want the DO to return to zero, or the AO to return to zero, then you need to explicitly to a DAQmx Write to set it that way.

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[camerond]

If you really want to output a constant current, you can either buy something to do it (like http://sine.ni.com/nips/cds/view/p/lang/en/nid/10705 ), or build something to do it (like http://www.allaboutcircuits.com/vol_3/chpt_8/7.html ) and use an analog voltage output from your computer AO board in place of the 5V (variable) source.

There is a wealth of very readable information on basic electronics on the Web.

Cameron

[dragondriver]

Thanks all. It is pretty clear now and I did learn something here 🙂