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Kenmore 158: Recalibrated Optoisolator Drive

Because the motor will draw more current during pulsed operation, the ET227 needs more base drive. The existing circuit topped out around 2.5 A, so I reduced the current sampling resistor by a bit:

Optoisolator Driver

Optoisolator Driver

If you care about the exact current, you’d use a 1% resistor, but if you care about the current, you’ll be doing closed-loop feedback to compensate for the transistor gain variations. Compared to those, the resistor doesn’t matter.

Running the MCP4725 DAC through its range produces a nice graph:

Current Calibrate - DAC - 270k Hall 2.7k opto

Current Calibrate – DAC – 270k Hall 2.7k opto

The X axis comes from the Tek Hall-effect current probe, so the numbers don’t depend on the ferrite toroid & differential amp calibration. They do, of course, require a bit of eyeballometric calibration to extract the flat top from the waveform, as shown by this old waveform:

Motor current - ADC sample timing

Motor current – ADC sample timing

Ya gotta start somewhere.

The linear fit to those dots gives the DAC value required to produce the observed current, at least for these particular transistors at whatever temperature they’re at in a rather chilly Basement Laboratory.

Of course, the observed current tops out at 1.2 A: the motor’s peak current during normal linear operation. The line looks so pretty that I’ll assume it continues upward to the maximum 12-bit DAC value of 4095 and the corresponding ET227 current. Working backwards, that will be 3.1 A and should suffice for all but the highest peaks at high line voltage.

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