3d Printer extruder issues and some other doubts

[Athul]

Hi,

First of all I don't have much experience in electronics (yes I'm anelectrical and electronics engineering graduate(fresher) but I'm more interested in power system) and I like to do hobby electronic projects with my basic knowledge in electronics. I'm having some problems with my 3d printer board, Second extruder in it turns on automatically with out any command if USB disconnected from computer and power supply is on. If I reconnect USB back to USB port(Doesn't require connection with host) it turns off.and it'll turn on again if I disconnect USB. I checked the pins which are controlling the MOSFET gate, they are HIGH without USB and and LOW with USB.

In some boards USB s connecting to send serial data as well as powering the micro controller(for eg : Smoothieboard), so I checked the schematics and there's an inbuilt regulator and this issue has't happened to me before .So I checked the regulator output it's around 4.3 V without USB and above 4.9V with USB. So I think it's the regulator problem.

Could there be any other reason.??
Board = Printrboard Rev D with Extruder board
FW= Marlin

When I was checking the schematics of the board I noticed few things , Input to all the MOSFETs are marked with complement (Invert).

Below is the entire schematics and part of MOSFET circut





As you see Gate signal are marked like this
________ ________ ________
PWM-FAN HOTEND HOTBED

Is this mean they're active low???

I know in some micro-controllers RESET pins are marked like
______
RESET , that means it's active low and a LOW signal at this pin will cause reset of micro-controller. But that can't be here, since n-channel MOSFET requires a positive Gate pulse.;

So can anyone tell me why they are marked like
________ ________ _______
PWM-FAN HOTEND , HOTBED this instead of PWM-FAN , HOTEND , HOTBED

Thanks

 

[Alec_t]

But that can't be here, since n-channel MOSFET requires a positive Gate pulse

I agree. So it's presumably a programming quirk, unless the fan etc have inbuilt logic inversion..

 

[Athul]

I agree. So it's presumably a programming quirk, unless the fan etc have inbuilt logic inversion..

If there was any logic inversion, shouldn't it have shown in the schematics??

Also what do you think about automatic turn On of extruder?

 

[Alec_t]

If there was any logic inversion, shouldn't it have shown in the schematics??

The fan itself isn't in the schematic.

Also what do you think about automatic turn On of extruder?

I wouldn't have designed it that way .

 

[Athul]

The fan itself isn't in the schematic.

I wouldn't have designed it that way .

Do you have any idea why is the extruder turning ON when USB is disconnected?

Could this be the regulator issue??

 

[Alec_t]

The regulator has no obvious interaction with the USB section. USB disconnection would be detectable by the micro, but what the micro does in response is down to the programming.

 

[Athul]

The regulator has no obvious interaction with the USB section. USB disconnection would be detectable by the micro, but what the micro does in response is down to the programming.

I've using both boards for around 6 months. I never had the problem. Both etruders turn on only when command is give to turn them on , otherwise it was OFF with or without USB connection. Second extruder issue happened recenly. SO I think it's an issue with control board.