Hotend heater control circuit December 28, 2018 09:57AM |
Registered: 7 years ago Posts: 13 |
Re: Hotend heater control circuit December 28, 2018 12:05PM |
Admin Registered: 16 years ago Posts: 13,889 |
Re: Hotend heater control circuit December 28, 2018 11:01PM |
Registered: 7 years ago Posts: 13 |
Re: Hotend heater control circuit December 29, 2018 04:39AM |
Admin Registered: 16 years ago Posts: 13,889 |
Re: Hotend heater control circuit January 03, 2019 11:09AM |
Registered: 6 years ago Posts: 1,007 |
Quote
VDX
... the bigger MOSFET needs more than the 3.3V from the Arduino -- so this is a "Darlington-style"-pair ...
Re: Hotend heater control circuit January 03, 2019 11:15AM |
Registered: 10 years ago Posts: 14,672 |
Quote
reprapprochina-liang
I want to know why need two MOSFET?
What do R78 and R23 do in this circuit? what will happen if I remove them?
Thank you.
Re: Hotend heater control circuit January 03, 2019 11:33AM |
Registered: 6 years ago Posts: 1,007 |
Quote
dc42
....
That design has a number of disadvantages, for example, slow turn-on time and excessively fast turn-off time of the main mosfet, and the heaters turn on if the 3.3V rail is shorted to ground. So I used a different design in the Duet WiFi.
Re: Hotend heater control circuit January 04, 2019 03:28AM |
Registered: 10 years ago Posts: 14,672 |
Re: Hotend heater control circuit January 04, 2019 10:23AM |
Registered: 6 years ago Posts: 1,007 |
Quote
dc42
The issue with fast turn off is that the inductance of the bed heater and associated wiring causes a voltage spike when the MOSFET turns off, which exceeds the drain voltage rating of the bed heater MOSFET, causing it to avalanche. The energy involved is small and almost certainly below the avalanche rating of the MOSFET, but it and the fast change in current generate EMI.
The Duet 085 added a resistor in series with the gate of the power MOSFET to control the turn-off time. The Duet WiFi uses a separate chip to level shift and buffer the gate drive signal, again with a gate series resistor.