AO 0 output 5V DC voltage reduced when connected to load

[umie]

Hi,

I am trying to generate a 5V DC voltage through the AO 0 pin from my PCI-6036E DAQ Card using DAQ Assistant. When I connected AO  0 and AO GND with a digital multimeter it shows 4.96 V but as I connected both pins to a 5V Brushless DC motor the voltage seems to reduced to 1.19V and my motor can't start not until I gave some starting torque to it. 

However, when I connected the DC motor to the +5V port from the DAQ card, which in this case pin 14, the motor runs smoothly at a higher speed, and the voltage through the multimeter with the connected load is about 4.7V.

Why is it that the motor seems to draw more current from AO port than the one from 5V? Any idea what should I do to maintain the voltage output from the generated through AO pin? 

Thanks.

Umi

[JB]

As mentionned in the help for your card, the max current of an AO is only 5mA ! Therefore you can't use it to drive your motor directly.

Without its "short-circuit to GND" protection, the AO would probably have been destroyed.

In the help you will also read that the +5V output of the card is able to source 1A. But you shouldn't use it to drive inductive loads like a motor.

The use of a power stage is unavoidable !

[umie]

Hi JB Oohh. Thank you for pointing that out! So does this mean that I need to do external circuitry before I can actually connect it to the motor?

[umie]

Hi JB Oohh. Thank you for pointing that out! So does this mean that I need to do external circuitry before I can actually connect it to the motor?

[JB]

Yes.

A power operational amplifier could be sufficient but, unless you are a HW specialist, the best would be to use a driver (eg maxon motor, Technosoft...).

[umie]

Well I'm not a hardware specialist  and my purpose now is just to make the motor run so I guess an op-amp is enough for my current situation now?

I might need a little help of what kind of op-amp circuit I need to build. Is it the inverting op-amp with gain of -R2/R1?

[johnsold]

Umi,

You probably want a non-inverting amplifier. Otherwise your motor will run at full speed when the AO is zero.  As JB said you need a POWER op amp.  Most standard op amps cannot source enough current to drive a motor. You will need to select an amplifier which can provide as much current as your motor requires.  Also consider that the starting current of the motor may be much higher than the running current.  The amplifier must be able to handle the starting current, not just the running current.

Lynn

[chuggins1434]

Thinking a little outside the box here, but could you send a PWM signal to a transistor that would control your motor?   There are several transistors available at RadioSmack that will handle the current you're playing with and considerably more.  I was playing with this idea (via an Arduino board) to control some electrically heated clothing for motorcycle riding in cold weather.   ...but I  don't' see why it wouldn't work with a brushless motor too.  Thoughts?

Chad

[johnsold]

Chad,

Brushless DC motors are actually AC motors with internal electronics to chop the DC input into a suitable AC pattern for the motor windings.  Chopping the DC might thoroughly confuse the internal electronics.  I would not recommend it unless the motor manufacturer specifically states that it will work.

If you add filtering and make a complete DC/DC converter, it would work, but then it is no longer a simple circuit.

Lynn

[chuggins1434]

Lynn,

Gotcha!  ...goes to show ya what I know about DC brushless motors!  🙂  Thanks for the lesson!  I'll do a little more research next time before posting up!

Thanks,

C