Heated bed issue

On the schematics the voltage for the heated bed, extrude and motors is 12V. This is far too low and Velleman added a 15V power supply. This works well for PLA but is not enough for the heated bed when using ABS. With 15V the maximum heated bed temperature is about 55°C but ABS likes to have something about 100°C. Not having the heated bed enough hot, makes that the printed piece shrinks a lot, sticks no more to the bed and falls of during printing.

Increasing the voltage from 12V becomes critical for the 5V linear regulator of the controller board that will have to dissipate more heat. The linear regulator could be replaced by a pin compatible DCDC converter to get a 24V system that is able to heat up the heated bed to above 100°C. Considering that it seems the board got designed for 12V operating it with 24V would need further investigations if all parts including stepper motor drivers can handle it.

An other approach is using two power supplies, the Velleman 15V power supply and a second 24V power supply connected to the same ground and having its positive voltage connected to the heated bed. The power MOS FET transistor on the controller board can handle 24V and the required current. This way it is possible to get the heated beds temperature above 100°C. I run my printer more than a year with this solution.

An other solutions found on the Internet http://www.k8xxx-3dprinters.crimed.be/w/index.php?title=Installing_a_24V_power_supply_for_heated_bed is using the controllers MOS FET just to turn on a LED of an optocoupler and having the optocouplers output turning on a second MOS FET. This solution has also the advantage that the two power supplies can be galvanic separated from each other and that the heated bed current will not flow through the controller boards ground.

It is wise to determine the resistance over current and voltage measurements and not via ohm meter (too little current).

At 28.1V the measured current was 5.75A resulting in 162W or 4.89Ohm resistance.

For the heated bed power supply a device that can adjust its output voltage as from 19.2V to 29.5V is recommended this gives different options for the required power.

19.2V/4.89Ohm = 3.93A or 75W

24.0W/4.89Ohm = 4.91A or 118W

29.5V/4.89Ohm = 6.03A or 179W


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