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General Mach Discussion / Re: PWM Spindle Control using Mach3 - How I did it
« on: March 25, 2015, 09:41:34 PM »
Just a note to tell you why I did not use Chris's circuit directly. I did not need an external supply, because there was one directly available in the spindle motor drive inverter, with its own analogue earth, for a potentiometer input. To get the maximum speed the voltage has to range right up to (usually) 10V. Hence I needed a low resistance drive and a low resistance in the filter. In my case the potentiometer input resistance was 20K, so that was the load on the filter. As it turned out the internal power supply was 10.2V, giving me some head room, and it worked out fine - I could get the full speed from the spindle.
So I was then happy to further load the logic chip with the extra filter to drive the forward pin. I was also happy to use the internal power supply analogue earth for the common, where I should have used the digital earth for this digital signal. I could get away with this because it is a steady load on the speed signal for the full time of when the spindle is spinning. If any other digital input loaded the circuit it could alter the speed slightly when active.
And I required optoisolation.
A further note. There are many thousands of transistors, all with similar specifications. So it generally does not matter what transistor is used. Voltage and current rating are important, but for small signal transistors your use will be well below the limits. FETs have different abilities than bipolar, and there are npn, pnp, n-type and p-type specs that you must get right.
Similarly resistors have voltage and power maximum ratings and capacitors have maximum voltage ratings. But again you are likely using them well below their maxima. The same is true for logic gates. We could have used any NOR, NAND or NOT gates available, because we are using the chip only as an inverter. Subject to their maximum voltage rating as already noted.
Krypton
So I was then happy to further load the logic chip with the extra filter to drive the forward pin. I was also happy to use the internal power supply analogue earth for the common, where I should have used the digital earth for this digital signal. I could get away with this because it is a steady load on the speed signal for the full time of when the spindle is spinning. If any other digital input loaded the circuit it could alter the speed slightly when active.
And I required optoisolation.
A further note. There are many thousands of transistors, all with similar specifications. So it generally does not matter what transistor is used. Voltage and current rating are important, but for small signal transistors your use will be well below the limits. FETs have different abilities than bipolar, and there are npn, pnp, n-type and p-type specs that you must get right.
Similarly resistors have voltage and power maximum ratings and capacitors have maximum voltage ratings. But again you are likely using them well below their maxima. The same is true for logic gates. We could have used any NOR, NAND or NOT gates available, because we are using the chip only as an inverter. Subject to their maximum voltage rating as already noted.
Krypton