Variable output on ELVIS board

[sasa880]

Hi,

I need a little help from anyone experienced with ELVIS board. I’m new to LabView, and specially to DAQ tools.
I have vi that is designed to drive small DC motor using ELVIS board and Variable output.

it’s capable of driving small DC and changing speed of rotation, but I need to drive this motor in both directions.
Does anyone know is there any way to achieve this using Variable output (to change it’s polarization), or any other way to do this on ELVIS board.

Thanks in advance.

[johnsold]

I have not used ELVIS, but from looking at the specifications, it appears that the positive and negative variable supplies are separate.  It also appears that they may share a common ground.

So, you will need to use some kind of switching device to reverse the polarity of the motor.  4 transistors with suitable power ratings connected in an H-bridge configuration will work. You may need level shifting and other circuitry to drive the high-side transistors.  

Another option is a relay.

Lynn

[sasa880]

This propose with 4 transistors (H-Bridge) is good but then I can't use Variable supply to drive transistors, as I need two outputs which should be switchable. In other words they should be able to change polarity so oposite transistors can conduct. I could use digital outputs, drive them with PWM, but it's too complicated for me.
The best way maby would be to generate two PWM's (in quadrature) using Analog outputs (arbitary waveform generator) and H-bridge, but I don't know how to accomplish this 

[johnsold]

You can still use the variable supply to change the motor speed. You need only the positive supply. Only switch the H-Bridge when you want to change the direction.  Control the transistors with digital outputs.  Since you are not too worried about efficiency, I suggest a modified H-Bridge where the high side transistors are p-channel MOSFETs or PNP BJTs. Then they can be driven with ground referenced logic plus some simple level shifting. No PWM required.

Lynn

[sasa880]

This is good. I dodn't figured out that i can use variable output as power suply for H-bridge  Now i know how to implement hardware design.
Thank you very much for response.

[johnsold]

Please post back after you get it built and let us know how it worked.

Another note: If you will ever try to reverse the motor while it is running, you will get large transient currents in the H-bridge transistors. Consider adding appropriate protection diodes or other devices.

Lynn