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MELZI Signals query

Posted by Supermec 
MELZI Signals query
March 24, 2017 07:25AM
Hi all, hope I have picked the correct section.

I have an Anet A8 with the original control board, which is a variation on the circuit of the Melzi 2 board. The picture attached shows the control circuitry for the print cooling fan, which is identical to that for the hotbed and hotend. What I don't understand is that the control signal for the fan is shown as FAN (NOT_FAN). Similarly the respective signals for the bed and extruder are shown as HOTBED and HOTEND. The signals are also marked in the same manner on the respective pins of the ATMega1284P.

The mosfets shown are N-channel enhanced mode, which requires that the gate voltage is higher than the source voltage before the mosfet will turn on. One of the output connections remains constantly at Vmot (12V). The other output connection is at Vmot when the mosfet is turned OFF and at 0V (GND) when the mosfet is turned ON. Thus the mosfet must be turned ON before there is a potential difference applied at the output.

A gate voltage of +5V will turn the mosfet, and therefore the Fan, Bed or Extruder on, so why is the signal described on the circuit diagram as FAN (NOT_FAN).
open | download - Fan Circuit.bmp (405.8 KB)
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