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Simple DC motor control

Hi guys, further check measured voltage across trim pots are 5.6v so they seems ok, voltage at r43 was 3.8v but at r 44 measured only 0.14v and of course this is the output voltage that I see at motor. I am scratching my head .
 
Ok , i was able to link some images. Please see nex post. I did a thumb nail pic -terrible can't see a thing.
 
img5199h.jpg

a better picture i hope.
 
Can you restate your problem? Assume I don't know what R43 and R44 are, but I can reference the schematic on P1 of this thread.

And quite often the voltages at each terminal of a transistor can be enlightening. And in this case it would be useful to also know what is going on at the op-amp. Why not print the schematic, measure voltages all over the place, write them on, and post an image of that?

Paul
 
Hi Paul, my trouble is i am not getting the desired voltage at the motor output currently measured 0.14V only. The schematic looks simple enough with min parts but for some reason i can't get it to work . The desired voltage output would be 3.7v and 4.2V for 33.3 and 45 rpm respectively. Here is a picture with some measurements with the values of resistors used. Hopefully it is not too confusing.
Quan
img5283y.jpg
 
Hi Martin, r45 measured 12k not 22k as per picture,my bad. I wrote 22k instead of 12k(from changing Rs 1.5R to1R) as per excel work sheet. I just remeasured to ensure it is 12k.
Quan
 
Have you tried a real motor? A 5R resistor is a good emulation of a stalled motor. You are expecting the circuit to deliver 0.7A as configured. Which it won't do...

An ideal motor will spin at an rpm linearly related to supply voltage, and take current in proportion to the power it is delivering. Without a load it looks a lot like an open circuit. A real motor has an internal impedance due to the coils and as you take power from it this impedance effectively steals some of the supply voltage, which causes the motor to slow. Martin's clever circuit compensates for this.

Paul
 
Have you killed the transistor?

Check there is 0.6-0.7v between base and emitter. You could update your diagram with the voltages around the opamp, the two inputs and the output. These should be easy to get to.

Paul
 
Hi Paul, check the transistor the BE junction measured 0.62V so it is sweet. I also discovered just now that the circuit works without the load ie when no motor attached then i can get the desired output of voltages but when connect with my motor then the measured voltage drop down to 0.007V. So i presumed the circuit is unstable with my load. What do you think?.
Quan
 
With any reasonable choice for the pass transistor (hfe >50, Ic 1A or more) it'll supply 5-700mA or more easily.

- and if you used something like TIP142 instead (cheapo darlington) - a lot more, limited by the PSU / voltage regulator.
 
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