Driving a -1V to +1V stepper

[StyxF2]

I have a stepper device that works from -1V through to +1V device. Have tested it using the DC function of my function generator and all works well.

Im now trying to drive it from a 0-5V DAC and have hit the limit of my knowledge. Getting -1V seems pretty hard, so figured moving the ground up to centre on 3.3V would be the way to go (Therefore voltage could swing from 1.8V up to 4.8V, and used a zener diode to do this. When i tried it out however the zener voltage (My artificial ground) was at 2.7V, the the output from my DAC got pulled to less the 600mV Pk-Pk.

Now im thinking the DAC can't pull enough power, so might need a push pull transistor config, but really thinking i might be going down the wrong path. Anyone have any other ideas how i might achieve this?

[mikerj]

You have the lower transistor connected backwards, the emitter should be connected to the motor. The easiest solution to get your circuit working is to buffer the zener voltage using another two transistors connected the same way.  This will provide current amplification.

[pcprogrammer]

You can probably just use a motor driver (H bridge) and apply pulse width modulation to keep the voltage in range. Depends a bit on the motor windings and the carrier frequency if it will work.

Just search a bit with google for H-bridge.

[StyxF2]

A week of rain meant i got to move along with this project. The circuit shown in previous post turned out to perform poorly. The top end swing would clip at around 4.3V. Increasing the voltage to 9V allowed the top end to work but the Zener was clipping the 'negative' voltage swing. Not how I thought it would work but guess thats bench testing for you...

I had considered a H-bridge, but the circuit spends about a 3rd of its time at 0 volts, which with some jitter could cause the H-bridge to switch rapidly back and forth...

So figured this was going to require some proper centre tap. Found a +/- 5V 100mA DC-DC converter on element14: PEME1-S5-D15-S. This would provide me with a proper ground reference. With this range, the DAC could output its full 8-bit voltage range, and now all that was needed to to massage this output to get the required -1 to +1 voltage swing, which is a perfect task for one of the old LM358 OpAmps...

See attached but basically R3 and R4 voltage divider (from the -5V supply) and the DAC output are used with the OpAmp as a summing amp to add (subtract) -2.5V from the DAC value, this centres it around the 0V reference. R6 is inverting amp feedback from the push-pull amp, setting the gain at 0.66. This gives a 5V signal from the DAC an output voltage of 3V (or +1.5V).

Except for a bit of distortion around the 0V reference (Which i think might be issues with the breadboard setup), it seems to do as required. So have turned it into a prototype board and send it off to China for Fab. Hopefully it'll be here in a week or two...



Yellow is an input from function generator, 5V P-P centred offset 2.5V
Cyan is the output from the OpAmp (Pin 1)
Pink the the output from the Push-Pull amp.





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