Does this affect some parts on the board in kind of to low voltage (voltage converter)?
Here i read the Mosfet can handle 5 to 6 A without cooling.
The BUK6215-75C chip can handle more. Datasheet BUK6215-75C
drain current
25°C 57A
100°C 41A
Peak drain
25°C 229A
total power dissipation
128 WMy ATX Supply could deliver 48A @ 12 V= 576W (PCI Express).
Switching to 12V would rise the current from 2.4A to about 3A. Since i cool the electronics allready, more Ampere should not be the problem.
i tried with 12V, drain is 1.926A for the Heatbed. it takes very long (30min) to heat up but it reaches temperature.
The Hotend uses 1.314A.
the motors seems to run very well, i hear no difference.
Well - the idea behind using a different power supply is to increase the power available for the heated bed.
Ohm’s law tells us that if you decrease the voltage applied to a resistor, you also decrease the current that flows and hence the power (which is the produt of voltage and current).
You’ve measured the current that flows at 12V, which is roughly 2A. Thus the power that heats the bed is 24W.
The heated bed’s resistance is about 6.5Ω, with 15V you get a current of 2.3A and thus a power of about 34W.
By your lowering of the voltage from 15V to 12V you’ve decreased the heated bed’s power by 10W.
That’s not the goal. The goal is to increase the power.
To reach that goal, you have to increase the current that flows. That goal can be reached by two measures: decrease the resistance or increase the voltage.
Decreasing the resistance would mean changing the heated bed PCB. No easy feat.
The conclusion is to increase the voltage applied to the heated bed. As our calculations have shown, with 24V the power will reach 92W – about four times the power that is possible with 12V.
Hi
I added an old notebook powersupply for the heatbed. It now runs on 20V. Controlled by the original board with a relais. Connect the coil of the relais to the heatbeddriver of the controllerboard and the powersupply to the relaisswitch. Works fine. Temperature of 70 degres within 4 minutes. No heat to the original flatcable.
I didn´t checked the current, but i think its about 3-3,8 amps when bed will heat up. If you see there will be only a short on-phase once the bed is at print temperature, i do not think the bed could overheat an get damaged.
The average current will be at 1.5-2 amps over a minute. This should be ok.
I also added a mirror to the heatbed. The pcb is not flat. Now its fine. Bed leveling is now with springs. Much easyer.
PS: Sorry for bad english. I did not wrote a while in an english speaking forum and I’m a bit out of exercise
Just a relay? No suppression diode over the coil to protect the electronics of the controllerboard? Do not know the schematic of the board but just be carefull you don’t kill your driver fets with the high voltage spikes that appear when the relay is powered down.
I think the best solution is to use a Mosfet and an optocoupler like this topic :
https://forum.vellemanprojects.eu/t/additional-power-supply-for-heatbed/7459/1
Hi
The relais has a build in diode. so there is no problem. 
Ok, no problem then!
[quote=“edirol”]Acually your are right. So I build a prototype of a 24V MOSFET controller, but to completely decouple the extra voltage from the controller board, I changed the electronic design and use now a optocoupler. Now I can keep the bed at 60° even with 2 fans at 100% speed ;-). 3.9A@24V=94W
@edirol :Can you explain your modification on your schematic ?
thebuglife,
Alltough the prototype of the previous design worked well, the gate of the MOSFET was driven slightly over its limit of 20V. However, now we are on the save side 
(Due to less feedback in here I actualy didn’t think my mod is very useful so I left the change uncommented, sorry)
Did you keep the flap cable to power the heatbed ?
The original ribbon cable goes to the X Support. From there is use a 2x0.75mm2 cable along the cable chain.
Buttom of the X support
Connection to the heatbed
I try to see on picture and understand, you use flat cable util aluminium plat with Y stepper motor, and after you change whit 2x075mm² cable on cable chain
Is exact ?
Can you share you cable chain with your specific attaches ?
[quote=“thebuglife”]I try to see on picture and understand, you use flat cable util aluminium plat with Y stepper motor, and after you change whit 2x075mm² cable on cable chain
Is exact ?
Can you share you cable chain with your specific attaches ?[/quote]
Yes thats correct.
When of interest, I will publish my cable chain solution, but i will have to prepare it first.
Very beautiful solution this cable chain!
Yes I’m interesting.
Thanks in advance
I am also interested in your chain
I use a relay and an adjustable notebook power supply from 12v to max 24V it is now set to 18V reach temp bed fast and 60 or 65 degree no problem
Well, my cable chain:
Parts list:
To prevent the chain from hanging down and touch the frame, connect 2 or 3 chain links with cable ties, see picture (Y=max!):
IMPORTANT
After assembly, check if there is no collision with the cable chain, in particular with the extruder wiring or the fan at Y=200 Z=0!
Enjoy
I see people making nice things for theire 8200!!
Congratulations!
I sold my 8200 and now i have a prusa I3 with .35 nozzle, and, even if i have (much) better prints, and more options (standalone printing is marvelous) , but i miss the nice mechanical construction of this Velleman!
@edirol: thanks a lot for these moficiations. the external power supply is absolutly necessary if you want to heat quickly or higher temperature than normal behaviour.
About the cable chain, I have done an other one myself, I will post pictures as soon as possible.
