Heat Bed Amperage Reduction

Hello all!

This is my first post, and I'm really happy to have found this forum, as it looks like a huge wealth of knowledge. I'm hoping you 3d veterans can share some wisdom with me.

I have a Megatronics V2 (upgrading to a Smoothieboard 5x in the near future). To date, I've only printed in PLA and TPE, but I'm ready to get into some more demanding materials. So, I picked up a silicone heater for my print bed, which is a 365mm diameter 5/8" aluminum plate. I built my printer as a spinoff of the mini Kossel, but it's quite a bit larger than the standard, hence the large plate. The heater itself is 300mm diameter, 12v, 300w. And that is where things get sketchy for me.

I've dug around my controller and its schematics and found a bunch of different (somewhat contradictory) information.

• The schematic states that the HB input runs through a 25A fuse, however the board appears to have a 12A fuse on the HB circuit.
• According to the pcb schematic, the HB connector is rated to 18A.
• The HB MOSFET datasheet rates it at 60v, 62a, with 135w of power dissipation
• The Megatronics datasheet rates the HB circuit for 15A at 12V.

Now, I understand that the 300w demand of the HB is going to exceed pretty much all of the specs for the board and its components. Therefore, what I'd like to know is can I somehow pull that power down to within a tolerable range without making any major modifications? I'd prefer not to use any external controllers if possible. My soldering skills are decent, but I would prefer not to modify the board itself to drastically. My calculations pit the resistance of the HB at 0.48Ω. If I wanted to get the amperage down to 15A (the alleged rating of the board), I'd need closer to 0.8Ω. If someone smarter than me can confirm that, how could I even attempt such a thing?

The second part, relating back to the info I found is: what is the actual limit of the board? If there's actually a 12A fuse in place, I'm assuming that's going to govern everything. But with the board datasheet claiming 15A, am I to trust it?

I appreciate any input as I'm just standing by until I know I won't start my gear on fire.

If you plan to migrate to Smoothieboard, one thought is to just look into adding a solid state relay to the board you have now and let that carry the high current for the heat bed. That way you have a solution you can use with the Smoothieboard, and that'll allow you to carry forward the experience you gain in working with it. Depending on the connector choice, the "big" Smoothieboard mosfets are only good to 13A or so.

One note - Smoothieware uses a fairly fast switching rate on the heat bed; I don't think you can use a mechanical relay with it, so I wouldn't go that route even though it might work with the board you have now.

Thanks @printbus. I considered using a relay, although I don't think there is room on the board, I would likely need to make a breakout and mount it near my controller. Could it be as simple as adding a beefier MOSFET and soldering the HB leads directly to the board?

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Originally Posted by metzp3369

Thanks @printbus. I considered using a relay, although I don't think there is room on the board, I would likely need to make a breakout and mount it near my controller. Could it be as simple as adding a beefier MOSFET and soldering the HB leads directly to the board?

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I'm not familiar with the physical constraints you have, but yeah, relays and solid state relay modules are usually mounted separate from the electronics board. Technically, swapping out the MOSFET that's there now might work, but there's a number of things that could go wrong if not done properly, such as burning up circuit traces carrying more current than they were intended to. Soldering power and HB connections directly to the MOSFET terminals helps with that, but 300W at 12V is 25 amps - that takes some pretty hefty wire and soldering it to the likely small pads on the circuit board could be tricky. You may also find that you have to add a heatsink to the MOSFET in order to keep it at an acceptable temperature. If you go that route, figure out what package style MOSFET you have now and look for one that is the highest possible current rating in the same pinout/package style. That will have less "on" resistance than a MOSFET simply rated for your needed current; the main advantage of this is minimizing power lost to heat in the MOSFET.

FWIW, here are the solid state relay options uberclock has for the Smoothieboard - http://shop.uberclock.com/products/s...relay-dc-to-dc. They're basically a MOSFET packaged in a block with screw terminals. For completeness, here's a screw-terminal mechanical relay that MakerFarm ships with their larger printers with 300-350W heat beds - http://www.makerfarm.com/index.php/h...bed-relay.html. Those printers have heat bed currents similar to what you're dealing with (25 to 30A). Multiple MakerFarm users have had wires or relay terminals melt on long prints - feeding 30amps around can be non-trivial.

Is there a warranty issue if you hack up the board you have? That could be another consideration on trying to replace the MOSFET with a beefier one.

Will your power supply give you that current at 12 volts?

Thanks again! I think getting an SSR set up and mounted to my printer permanently would be an intelligent move. Then, when I swap my controller, it should be plug 'n play with a Smoothieboard or anything else.

I got my Megatronics V2 on eBay, so as far as warranty goes, I don't think it's going to matter what I do. I only thought that replacing the MOSFET might be the cleanest approach. I like your suggestion, though. I think ultimately SSR is the way to go.

I've honestly never seen another person's 3d printer in person to be able to see how it's done. So I've spent a lot of time digging through forums and YouTube trying to find answers to this. I appreciate your suggestions and plan to go that route.

One other question, since I'm new to this forum, is there a Kudos system or anything in place? I want to make sure I give Karma where I can, and you've earned it.

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Originally Posted by Mjolinor

Will your power supply give you that current at 12 volts?

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@Mjolinor, yes, my PSU can do ~50A @ 12V. I regret not getting a 24V heat bed, though...