Electronics > Beginners
Measure reversible voltage without PD drop?
12407622:
Hi,
I want to measure a voltage with the ADC of a micro-controller. It's easy, I just use a simple voltage divider to condition the 0-5V back EMF of a brushed DC electric motor, so I can measure and control it's speed.
Okay, so I want to measure this motor's back EMF voltage when it rotates in either direction. I guess I just rectify the original +-5V back EMF signal, but that leaves me with a huge gap in the lower end of the measurement, associated with the forward voltage drop of the rectifiers. I think I'm then measuring in the range of ~1.2-5V, which won't help me at all.
Any clues how I might condition the -5V to 5V back EMF voltage from the motor, so it can be accurately read by an ADC?
All links and suggestions appreciated.
12407622 :-+
ledtester:
You can use an op amp to translate a -V to +V range to 0-5V.
Here's a TI applications note on how to translate a -V to +V signal to 0-5V for use with an ADC:
http://www.ti.com/lit/ug/tidu015/tidu015.pdf
12407622:
Thanks ledtester,
Bit of study to do, but I think you've hit the nail squarely on the head.
Muchly appreciated.
12407622 :)
12407622:
Urgh! Looks like I'm not out of the woods just yet.
To measure the speed of my brushed DC motor, I'm to periodically, briefly disconnect it from the PWM driven H-Bridge outputs, then read the voltage generated by the still rotating motor before it slows. That leaves me no ground reference, so op-amps are (I think) out for now.
I suspect I'm back to the full-wave bridge rectifier and voltage divider option, but that still leaves the rectifier's forward voltage drop...the first 1.2V of the 5V total generated (the important bit in my low speed application).
Any clues on what type of diode I might use for the bridge, so that I can get the least forward voltage drop, and the least initially diminished voltage available for measurement with an ADC?
...still open to other options. My knowledge of what might be available is quite limited. Maybe op-amps are still my solution, but I don't quite get how to employ one gainfully? - Oh, I feel a cluck! :palm:
Thanks,
12407622. :-//
MosherIV:
Try looking at making the ADC differential. That means the ADC measures the voltage between 2 input pins.
Right now, the ADC is in absolute mode, referenced to the 0V of the circuit.
In differential mode,the ADC is not referenced to 0V, hence allowing measuring between 2 inputs.
Now, the ADC can measure +5V or -5V. The ADC will tell you the voltage but not the polarity.
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