GT2560 Rev A usb power BUG
Posted: Mon May 01, 2017 11:17 pm
Sometimes I wonder if it is so difficult to copy a schematic .... 
It's been a while since I was wondering why the 3-in-1 3D printer control box gives power to the usb peripherals when my pc is turned off.
Obviously the 3-in-1 3D printer control box is on and the pc off.
Today I have investigated a bit and I found that you do not even know how to copy.
I am referring to Q7 and IC6B of the GT2560 card.
The Arduino UNO scheme provides for U6 an LMV358 with RAIL to RAIL output. If you mount a normal LM358 as you did on my board the circuit can not work because the LM358 can not go beyond the 3.7V at the output !!
The MOSFET is always ON because its gate never reaches 5V. The Q7 Source is at 5V while the gate is at 3.7v then Vgs = -1.3V.
If you check the datasheet of the FDN340p you see that the Vgs (th) is typically -0.8V therefore the MOSFET is ON even if the output of the LM358 is high.
In the coming days I will replace the LM358 with the LMV358 ....
It's been a while since I was wondering why the 3-in-1 3D printer control box gives power to the usb peripherals when my pc is turned off.
Obviously the 3-in-1 3D printer control box is on and the pc off.
Today I have investigated a bit and I found that you do not even know how to copy.
I am referring to Q7 and IC6B of the GT2560 card.
The Arduino UNO scheme provides for U6 an LMV358 with RAIL to RAIL output. If you mount a normal LM358 as you did on my board the circuit can not work because the LM358 can not go beyond the 3.7V at the output !!
The MOSFET is always ON because its gate never reaches 5V. The Q7 Source is at 5V while the gate is at 3.7v then Vgs = -1.3V.
If you check the datasheet of the FDN340p you see that the Vgs (th) is typically -0.8V therefore the MOSFET is ON even if the output of the LM358 is high.
In the coming days I will replace the LM358 with the LMV358 ....