@edirol, please bare this greenhorn question now: in your schematic you have several components connected to ground. I am confused how I should do this in practice on the PCB. Do I use the 24v DC return line as ground or do I need to use a separate (third) line for ground?
Anyone else who knows the answer feel free 
koke
Every GND connection on the schematic has to be connected together on your PCB. This GND only belongs to the 24V part. It is my intention to have no electrical connection to the controller board, so you must not connect the grounds from the heatbed and the controller board.
edirol,
OK, I think I understand now. This concerns only the 24V part o the PCB (right hand side of the optocoupler). The grounded parts will be going to the same line for ground and this will be connected to the X2-2 (-) connector of the 24V power supply.
Powering the heatbed with 24 Volts works rather well. After 30 minutes, the FET has a temperature of about 40 degrees and this does not change further. Tested with the board of Kuraasu. As this FET can withstand very high temperatures according to the datasheet, it looks safe.
This is great, i can heat up the bed (with mirror on top) to 55oC in 3 1/2 minutes. That is a big improvement from the 20 minutes I had to wait
Edirol, thanks for the schematics, pictures and advice
Youre wellcome 
[quote=“edirol”][quote]Well: What could happen?
The bed is a resistor, designed to produce heat
[/quote]
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
Edit: The MOS-FET can also be the IRLU8743PbF (I-Pak version).[/quote]
Hi,
Can I use this n channel FET instead AP60T03GJ ? only a fee differences the “Threshold Voltage Vgs” is 1.0V instead of 1.9V (I don’t know what this is or does). The Rds(on) Test Voltage Vgs is lower, but, that is good right?
The IRLU8743PbF is not widely available in Denmark.
Best regards,
Christian
[quote=“chris101”]
Can I use this n channel FET instead AP60T03GJ ? only a fee differences the “Threshold Voltage Vgs” is 1.0V instead of 1.9V (I don’t know what this is or does). The Rds(on) Test Voltage Vgs is lower, but, that is good right?
The IRLU8743PbF is not widely available in Denmark.[/quote]
Christian
Vgs is not the critical point here. The AP60T03GJ has a similar RdsOn like the controllers on board FET and people in here reporting that its works ok. If you use a heatsink im pretty sure it will work fine.
Distrelec may be also a source for the IRLU8743PbF.
How did you guys wire up those upgraded beds? I just got 100W bed I want to install, but I strongly suspect the original flatcable isnt going to cut it. Im just curious if you used thicker wires and if so, how you routed them. One of those printable “cable chains” sure looks like a good idea now (I wish I had printed that before I took everything apart lol).
BTW, in preparation for this bed, I powered my K8200 with a ATX 12V power supply, and it seems to work just fine. I didnt even need to recalibrate the pololu’s, Im still measuring the same 5V I did with the 15V Velleman PSU. Since I already changed the hotend to a E3D one, which comes with a stronger heater intended for 12V, this is actually the easiest solution for me. THough Ive ordered the mosfet and optocoupler so I can later run the bed at 24V / 200W if I feel the need.
Hi P4man,
how the ribbon cable holds up depends mainly on the current it’s supposed to handle. What kind of heatbed do you intend to install? Mk2(a/b)? What resistance does the heatbed have?
By the way: doubling the voltage on a given resistance won’t double the power, but quadruple it. That’s why the dual-power heatbeds have two connection variants with different resistance. As for the Pololus, that’s how it should be, the coil current is kept constant as long as the supply voltage is above some minimum level.
Cheers,
kuraasu
[quote=“kuraasu”]Hi P4man,
how the ribbon cable holds up depends mainly on the current it’s supposed to handle. What kind of heatbed do you intend to install? Mk2(a/b)? What resistance does the heatbed have?[/quote]
Its a Mk2 (a?). I measured the resistance at 1.3-1.4 Ohm. So ~9A @12V. Thats more than I trust the ribbon for. Ive not measured the cable thickness yet, but I remember them being very thin.
Ahm… yeah of course, brainfart. 
24v might be a bit much
after reading some horror stories like this one: forums.reprap.org/read.php?158,139579
I decided pulling 10A from the controller board for the heatbed alone is pushing my luck.
The mosfet for edirols schematic still hasnt arrived, so I wondered if I cant use a simple relay instead? I picked up this one from a local store:
be.mouser.com/ProductDetail/Omro … PU8r3GY%3d
Both the bed and the printer will be powered by the same 12V ATX PSU, but separately with the relay switching on power to the bed whenever there is 12V on the controllers heatbed port.
Im an electricity/electronics noob, so can anyone think of a reason this is a bad idea?
Hi P4man,
[quote=“P4man”]The mosfet for edirols schematic still hasnt arrived, so I wondered if I cant use a simple relay instead? I picked up this one from a local store:
be.mouser.com/ProductDetail/Omro … PU8r3GY%3d[/quote]
this is a “normal” relais with a coil and mechanical contacts, so you’ll have several issues:
In bang-bang mode (which is the default setting for the heatbed) the switching frequency is quite low, so it could work in principle. I.e., the second point is not that important, however, any solid-state relais would be much better suited for this.
Cheers,
kuraasu
[quote=“kuraasu”]Hi P4man,
[quote=“P4man”]The mosfet for edirols schematic still hasnt arrived, so I wondered if I cant use a simple relay instead? I picked up this one from a local store:
be.mouser.com/ProductDetail/Omro … PU8r3GY%3d[/quote]
this is a “normal” relais with a coil and mechanical contacts, so you’ll have several issues:
I believe it already has a one. I cant capture it on on photo, but this is the schematic on the relays:
No idea what those arrows are for, but that other thing I recognize as a diode.
[quote]- the number of switching operations may be limited,
Yeah, but if the relay fails, thats not exactly the end of the world. As long as I dont put anything else at risk, Im fine with that as a temp solution. Im sure that mosfet will arrive one day.
Hi P4man,
[quote=“P4man”][flyback diode]
I believe it already has a one.[/quote]
well - it has a diode, but not the right one, at least as it seems. The part No. you indicated in your link is “G2R-1-SN-DC12S”, which is noted on the catalog page as having an LED, but no flyback diode. That’s also, what the diode symbol with the two arrows means: “light emitting” …
The flyback diode, if present, is connected in reverse direction.
Yeah, but if the relay fails, thats not exactly the end of the world. As long as I dont put anything else at risk, Im fine with that as a temp solution. Im sure that mosfet will arrive one day.[/quote]
One point of failure for such relais is fusing of the relais contacts. I.e. your bed may simply switch on and not switch off again. If you want to take that risk for a short time, that’s your choice, but I would at least recommend having a smoke detector nearby, or, even better, don’t run the printer unattanded with this configuration.
Cheers,
kuraasu
As usual, it seems you are correct, and it does have a LED. Nice catch.
Well, luck would have it that the components arrived, so I wont have to test my luck.
For the record however, I rarely let the printer run unattended and there is indeed a smoke alarm right next to it. Though Im more worried about the extruder than the hotbed, if that ever gets hot enough to be a fire hazard, I will be impressed :).
So, i want to share my experience with you. I made this upgrade (exactly according to edirols instructions) months before, after it was discussed intensively in the german section of the RepRap forum… (search k8200)
Now: powering the whole thing using a external Supply with 24V / 4,8 A and:
Neither the cable nor the mosfet are getting hot. The operating temperature of 60 degrees is reached within 3 minutes.
The maximum temperature that the bed has reached was 97 degrees and everything works still perfekt. I can only say that this upgrade was really worth to build it.
Edirol, great work!
Don
