Author: Ed

Given that the motor current amounts to maybe 3 A, absolute maximum, with a locked rotor, those skinny wires on the slit ferrite toroid won’t pose a problem:

I really like how that 3D printed armor worked out, even if I still haven’t poured any goop around the windings to lock them down; they’re held in by raw faith and friction.

The current sense circuitry appears along the bottom of the AC Power Interface schematic:

The differential amplifier lives on the Low Voltage Interface board, forming the clump of parts just in front of the LM324 op amp on the left:

Which has the usual handful of components required to get anything done in the analog realm:

The power supplies come directly from the ATX connector. I’m ignoring the whole decoupling issue, because the supplies have essentially no load (and it’s all DC, anyway).

The trim pot sets the offset voltage to bring the Hall effect sensors’s V_CC/2 offset down close to zero; the 100 mV figure is nominal, not actual, but should be a bit over 0 V to allow for a wee bit o’ drift. This time around, I’ll measure and subtract the actual offset, rather than (try to) auto-zero it.

The voltage gain runs just under 3, set by 1% resistors from the heap. The overall gain works out to about 1.9 V/A or 525 mA/V, setting the high end at 5 V to a scant 2.6 A. Subject to actual calibration with more attention to detail, that’s close enough; we’re more interested in the around-an-amp range where the motor operates under nominal load.

The nose-to-tail Schottky diodes clamp the op amp output to avoid annoying the Arduino’s ADC input. It has protection circuitry, too, but there’s no point in stressing it.

Given that the GX270 case has a power pushbutton on the front panel, it seemed only reasonable to let it control the ATX power supply just like it used to. Most of the parts clumped in front of the panel’s ribbon cable handle that logic:

The pushbutton on the far left parallels the front-panel button so I don’t have to reach around the box just to turn it on.

The schematic shows the relevant bits:

The ATX +5 V Standby output remains turned on all the time, so I wired that to the power button’s yellow LED, to show that the plug is in the wall.

The pushbutton pulls the ATX Power_On line down, which turns on the supply outputs, which fires up the Arduino, which turns on the transistor, which holds the Power_On line down. D302 isolates the transistor from the button, so the code can sense the button’s on/off state with the power on. D303 isolates the Power_On line from the sense input to prevent the pullup on the Power_On line from back-powering the Arduino through its input protection diodes when the power is off.

The Arduino code starts by arranging the I/O states, turning the transistor on, pulsing the green power LED until until the button releases, then leaving the green LED on:

	pinMode(PIN_PWR_G,OUTPUT);
	digitalWrite(PIN_PWR_G,HIGH);				// visible on front panel

	pinMode(PIN_ENABLE_ATX,OUTPUT);				// hold ATX power supply on
	digitalWrite(PIN_ENABLE_ATX,HIGH);

	pinMode(PIN_ENABLE_AC,OUTPUT);				// turn on AC power
	digitalWrite(PIN_ENABLE_AC,HIGH);

	pinMode(PIN_BUTTON_SENSE,INPUT_PULLUP);		// wait for power button release
	while (LOW == digitalRead(PIN_BUTTON_SENSE)) {
		delay(50);
		TogglePin(PIN_PWR_G);					// show we have control
	}
	digitalWrite(PIN_PWR_G,HIGH);

Every time around the main loop, this chunk of code checks the button input:

	if (LOW == digitalRead(PIN_BUTTON_SENSE)) {
		printf("Shutting down!\r\n");
		digitalWrite(PIN_ENABLE_AC,LOW);
		digitalWrite(PIN_ENABLE_ATX,LOW);
		while(true) {
			delay(20);
			TogglePin(PIN_PWR_G);				// show we have shut down
		}
	}

The never-ending loop blinks the green LED until the power goes down, which happens when the button releases. That terminates the loop with extreme prejudice., which is the difference between embedded programming and high-falutin’ Webbish stuff.

And it Just Worked the first time I fired it up…