My Heatbed controller board. Opto-isolated 30A mosfet. I had all this stuff already so i thought i would try it. Maybe tomorrow i will end up having to get a SSR,. So far it is working.
That looks properly dangerous!
I don’t have an argument for that!
Just in the testing phase. I will button it up tomorrow.
If you want that much wattage, you need to go with a mains-powered heater pad and an SSR. That much amperage is just simply wasteful to be pumping through there at that low of a voltage.
I miss ordered, should of gotten a 250W
Oh, why are you going isolated anyways?
Home made version of an DC SSR.
And that is better than DC-coupled because… ?
Nicee…
Opto-isolators will provide up to 2500 Volts isolation between circuits. If something should fail on the high voltage/current side the low current/expensive bits won’t be damaged… Also useful where ground potentials may differ. If ground current is significant a potential difference may occur and with logic levels @ 1.7-4.5 volts that would cause errors as well.
@Wayne_Friedt , why not just actually hit the +1 button? 
Geez i am getting beat up here, did i violate a rule with my use of +1?
I’m interested in this too (I hate the idea of a mains voltage wire connected directly to a part of my printer!) Is that at 24v? What kind of heat can it hit?
I had typed a big comment then hit something and it was gone, so this is the second time.
The power supplies are 12V same as the heating pad.
The reason i am using 2 ATX power supplies is i miss ordered a 350W calling for 29 amps instead of a 200W or so.
I don’t really know what is the best wattage to get
That is another question for the forum later.
This thing will heat up to 90C almost instantly.Makes the hotend now seem slow.
Connecting 2 power supplies in parallel will give the same voltage but twice the current.
Connect the thermistor to the correct location on your main board.
Connect the power out for heating the heating pad on the main board to your mosfet or what ever you choose for a controller. Your supposed to change the settings in the firmware which i have’t done yet. It is working and regulating, how accurate it is i am not sure. I have a temp gauge that i have built displaying on a scrolling LED board that i will check it with at some point.
You can use the circuit i have made up if it will help you.
If i didn’t cover all that you needed to know or need more help please PM me and i will do what i can to help you.
Good Luck
That’s great, I’m going to get a 24v power supply and silicon heated from China. Hoping it’ll get to 130 much quicker. Just looking for a cheap UK source of glass and aluminum to the right size. Think 230x310 ish would be good!
Just to get this off my chest - you don’t need opto-isolated transistors for 24V and with the standard negative lead is switched setup there’s practically no way that the higher voltage could feed back into and damage your controller. Isolated switching makes sense when you’re dealing with AC or, like @William_Frick mentioned, different ground potential (which is quite rare as well, since you need to tie the ground rails together anyways for non-isolated switching to work at all).
Optocouples can cause their own set of issues and are better left out for the typical RepRap use cases.
@Thomas_Sanladerer is quite right in that you don’t need one … except when dealing with AC. There is a risk however when using multiple power supplies, split mains, and conductors not sized for the load that unexpected or unfriendly things may happen.