BS2 and bridge rectifier

alnajjar1

I have an experiment for children at our Museum about wind energy. I use the Stamp and a ADC0831 to measure the voltage (0 - 0.15V) range from a small DC motor. This works very well. To make it easier for children to understand, I want them to see the voltage generated regardless of the direction of rotation.

To do that, I made a simple full wave bridge rectifier. This works but it generate some 0.2V noise (even when the motor is not spinning) that masks the weak signal from the motor. I want to avoid having two channel ADC, because I am using all remaining stamp pins for other parts of the activities.

Is there any clever way of measuring the voltage while the motor is spinning in either direction? why is there background voltage with the rectifier bridge even when I am only measuring voltage and without any input voltage?

Help will be most appreciated.

Al

RiJoRi

0 to 150 millivolts?? The first thing I'd do would be to amplify the signal, probably with an opamp. It's too easy to lose such a small signal in general noise.

--Rich

erco

How is noise being generated when the motor isn't turning? Noisy lines need shielding, perhaps?

BTW, your bridge rectifier is throwing away at least 0.6 volt, assuming you're using Schottky diodes. If you're using silicon diodes, it's closer to 1.4 volts for the two diodes conducting. That's fairly significant at low voltages.

I'd lose the bridge rectifier and clutch the fan/crank so it only rotates the motor in one direction, or add a mechanism that spins the motor forward no matter which direction the fan/crank is turned.

stamptrol

A low cost alternative is an older style bicycle dynamo. They were a rotating magnet device and produced a significant AC voltage no matter the direction.

If you must stick with the DC generator, use a lamp to indicate the relative voltage being generated. The lamp won't care about polarity. The light falling on a photovoltaic cell will give a signal the Stamp can use.

Your volatge level is extremely low. Can you gear things to get more output voltage?

Cheers,

Tracy Allen

The ADC00831 with its differential inputs and separate reference pin gives you some nice options. Attach the motor between the differential inputs and bias the (-) input to around Vdd/2 with a couple of resistors. Then the ADC will be able to measure both polarities without a rectifier. Choose a lower voltage for the reference pin so that the expected input voltage fills up more of the range.

Rectifiers are great at detecting extraneous signals from AC lines and nearby radio stations.

erco

Or simply output the voltage to a digital voltmeter. They'll see a number , plus or minus won't reallly matter.

alnajjar1

Thanks Tracy, I am a big fan of your website and use it regularly. Solution you provided would be perfect but not sure how to go about the implementation. Is there a sample circuit diagram out there to measure?

many thanks,

Tracy Allen

I was thinking toward something like the attached diagram. One side of the motor is attached to 2.5V at the junction of the two 1kΩ resistors, and the other side is attached to the (+) input to the ADC0801. Adjust the 10k pot so that there is 2.0V on the (-) input. Adjust the 1k pot so that the reference voltage is 1V. That makes the input span equal to 1 volt, and that has a 2 volt offset, so the full scale range is 2.0 to 3.0 volts. When the motor is stopped, that should be at 1/2 scale, a count of about 128, and go up for one direction of rotation and down for the other direction.

alnajjar1

Thanks! will try this over the weekend and report back. Happy Thanksgivings....