How can I use a DAQ device to control the current fed into a coil?

Thread Starter

siamak80

Joined Mar 2, 2016
3
I have a coil with 1 ohm resistance that requires about 600 mA current input to give the magnetic field I want. I already use a current supply for that, but I want to remove this current supply and use a DAQ device instead. I can control the voltage on the analog output channels of this device at -10 to +10 V, but the max current that this device can resist is only 80 mA. I don't want to blow up the device by exceeding the current. Does anyone have any idea how can I feed the required current on the coil by tuning the voltage on the AO channel of the device?
 

WBahn

Joined Mar 31, 2012
25,104
Convert the voltage output to a current using a transconductance amplifier and then convert the current in the magnet to a voltage signal using a transimpedance amplifier. Use the signal from the transimpedance amplifier to control the voltage going into the transconductance amplifier. Neither amplifier needs to be very sophisticated (the transimpedance amplifier could be as simple as a resistor), depending on what your specs are.
 

Thread Starter

siamak80

Joined Mar 2, 2016
3
Convert the voltage output to a current using a transconductance amplifier and then convert the current in the magnet to a voltage signal using a transimpedance amplifier. Use the signal from the transimpedance amplifier to control the voltage going into the transconductance amplifier. Neither amplifier needs to be very sophisticated (the transimpedance amplifier could be as simple as a resistor), depending on what your specs are.
Thanks!
Do I have to have the 2nd amplifier to convert the current on the coil back to voltage, given that I'm going to control the voltage going into the first amplifier with the computer on the AO channel of the DAQ device?

Thanks again.
 

wayneh

Joined Sep 9, 2010
16,162
The current has to come from somewhere. So far we know of only the current supply you want to replace. You may instead want to control it with the DAQ, not replace it.
 

WBahn

Joined Mar 31, 2012
25,104
Thanks!
Do I have to have the 2nd amplifier to convert the current on the coil back to voltage, given that I'm going to control the voltage going into the first amplifier with the computer on the AO channel of the DAQ device?

Thanks again.
If the transconductance amplifier is good enough and if you are willing to accept the uncertainty that results, you could run it "open loop". But what if you send it 1.2 V which is supposed to result in a current of 120 mA and, instead, it actually outputs 147 mA. You wouldn't know that this was happening and, if that would be an unacceptable error, then you want a way to monitor the actual current so that at least you can log it but, more likely, so that you can reduce the voltage being sent to the first amp until the actual output is 120 mA.
 

Thread Starter

siamak80

Joined Mar 2, 2016
3
If the transconductance amplifier is good enough and if you are willing to accept the uncertainty that results, you could run it "open loop". But what if you send it 1.2 V which is supposed to result in a current of 120 mA and, instead, it actually outputs 147 mA. You wouldn't know that this was happening and, if that would be an unacceptable error, then you want a way to monitor the actual current so that at least you can log it but, more likely, so that you can reduce the voltage being sent to the first amp until the actual output is 120 mA.
Got you!
Thank you so much.
 
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