I am at the part of constructing my K8200 where you put together the heat bed and I’m thinking about wrap the carboard isolator in an aluminium foil to force more heat up way. I am surprised I didnt find here something like this…
The cardboard isolator is an electrical insulator as well. If you add aluminum foil, be careful that you don’t short out the heater or the thermistor.
I think about this thing too. Maybe it will be a good idea to put there piece of some big plastic bag (in which parts were been) or piece of paper.
I exchanged the cardboard with corc.
That helped the temp to be more stable.
But the only way to reach heatbed temperatures above 65°c is to use a secondary 24V supply for the heatbed.
So I put piece of alumium foil under a piece of potato chips bag (it has nice aluminum foil look but it is some plastic) under the pcb and with mirror (3mm with nice matte black back) and now I can get some 73°C stable… I use a piece of carboard wrapped in aluminium foil puted on bed for faster heat up…
Now I’m looking at universal notebook 20V 90W power supply. I measured 4.7Ohms on the bed so at 20V the heat bed should need around 85W. I just need to - and there is the problem - as I read here a lot people use rellays and the other half uses mosfets so I can: read 20 pages on forum to fin out what is better or just chose one solution. I have some rellays at home so this is what I should chose if I decide just chose something…
If you have soitable relays at hand, simply use them.
It’s relatively easy to wire and you don’t need to by parts. (aside from a diode if you don’t have one handy)
The Mostfet solution has no mechanical parts.
Choose that if you want it the “electronics” way. 
It is also possible to connect the onboard Mosfet as a low side switch and
switch the load of the additional supply without any additional parts.
I have SOME (I just walk in shop and ask for some Si diode 
) with no marking on it. Will it work?
And rellays make noise…
I use a relay and it works fine. The bed heater output from the controller switches the relay coil, so power draw through the Velleman controller is reduced to sustainable levels. (No more brownouts during long print jobs!)
I used the components and circuit board from an inexpensive “touch switch relay” kit, and simply tapped into the relay coil input. Just make sure your relay is rated to carry at least 5 or 10 amps DC at voltage. I have tried old notebook power supplies from 16 - 19 volts and they have worked just fine, but I eventually bought a 24V supply and, naturally, it works even faster.
A relay is a mechanical solution, so you will hear a “click” whenever the heat turns on or off. Another consequence is that you wouldn’t want to enable “pwm” on the bed heater in your firmware if you are using a relay or you will create a buzzer to alert you that your heater is running. (This is not a default setting.)
Using a MOSFET is a better solution for faster switching and will be completely silent. Many MOSFET circuits are driven “active low” so you may need to reverse the logic output for the bed heater, which requires changing a configuration in your firmware.
The diode is only for protecting the on board mosfet from voltage spikes caused by the switching relay coil.
So any value that has a sufficent voltage rating should do.
Shure, relays make a very little “noise”, but the relay switches the bed on and off only a few times a minute.
So that should be no problem at all.
