Guide: Installing a MOSFET Board (Maker Select v2)

After countless hours of choosing a 3D Printer, I fell in love with the Maker Select (a.k.a. Wanhao Duplicator i3). I read every review on Amazon, Reddit discussion and web article I could find before finally landing there. The one major drawback was a problem from the factory… they are ticking time bombs. Without making some changes, they will either burnout or worse, catch on fire.

Despite being an IT guy, my experience with wiring was on par with that of a 12 year old child. Nonetheless, I was convinced I could figure it out while sustaining minimal injuries. I placed the order for the printer, followed by an extended shopping spree to turn this budget printer into something of beauty.

Thanks to all those at Reddit 3D Printing that helped me figure out how to get this done!

Purchased Parts

MOSFET Board - $7.99
14 Gauge Silicone Wire - $6.88
14 Gauge Spade Terminals - $7.29

Steel Thread Star Drive Screws - $8.33
Velcro Pads - $6.98

Printed Parts

MOSFET Board Mount
MOSFET Mounting Bracket (Better Alternative – See Notes Below)

Note: When this guide was written, the MOSFET Board Mount was used. The MOSFET Mounting Bracket was designed several months later and offers a better alternative. There is no need to purchase the Velcro Pads or Steel Star Drive Screws when using this design, it mounts under the Melzi board and uses M3 8mm screws.


The problem with these printers is that the Melzi board (basically the motherboard) connectors are not rated for the power used by the 3D printer. In other words, the printer is trying to pull more electricity than the connectors can supply and eventually result in the aforementioned problems.

To fix this, the MOSFET board will handle the power to the heated bed, routing it to the power supply rather than the Melzi board and solving the issue. In doing so, we essentially have 3 steps overall that we will need to do.

  • Connect the Hot Bed to the MOSFET board
  • Connect the MOSFET board to the original Hot Bed connectors
  • Connect the MOSFET board to the power supply

I took the process slow to avoid making mistakes, but it can definitely be completed in less than 30 minutes depending on your experience level.


We will start by mounting the MOSFET board to the mount that we printed out. The screws mentioned in the parts list were the only ones I had on hand that fit the mount. You could probably use M2 x 5 or similar but they are considerably small so keep that in mind.

You can then attach one of the Velcro Pads to the bottom of the printed mount. The other half will later be stuck inside of the control box, allowing you to mount the board with ease.

Mosfet Board Wiring

We then proceed to install the 14 Gauge Silicone Wire that will go to the power supply. To ensure a good connection, I recommend using the 14 Gauge Spade Terminals on both ends, which offer a snug fit around the screws.

For this step, I purchased a cheap tool from Home Depot to handle stripping and crimping wire. You can find decent information online if you are inexperienced, however the general idea is to expose around 1″ of bare wire. You then slide this into the spade connector and use the crimping tool to seal it inside. I used the brief guide Working With Wire and had it done in 5 minutes.

Crimped Spade Connectors

Now to the scary part, actually working with the inside of the control box (the black box with the LCD screen). Make sure to to power off and unplug the cord from the 3D Printer that runs to the wall. Afterwards, there are 11 screws that need to be removed, 7 on the bottom and and 4 on the back. Once all of them are out, slide the top of the control box forward until it can be separated from the base.

It will be easiest to remove the Melzi board, taking out the 4 screws placed in each corner. You can leave it in place but it may make working with it more difficult. Locate the orange/green plug combo labeled “HOTBED” and unplug the orange connector from the green. In my case these were super glued together, where you may need to peel this off before doing so.

Melzi Board Overview

Once disconnected, we have two steps. First we will remove the wires currently inside of the orange connector and plug them into the MOSFET board. Afterwards, we install the Control Input wire (white one) from the MOSFET board in their place.

If you rotate the orange connector, you will see two flat head screws. Loosen these to free the red/black wire from the connector. These will have straight metal ends called bootlace furrels. You can either use the same spade connectors from earlier to cut these off and replace them, or simply leave them as they are. The spade connectors are a better option but in case I need to revert back, I just kept them as they are.

When connecting the Hot Bed wires to the MOSFET board, the red wire goes where it says “HOT” and the black wire goes where it says “BED”. Make sure they are securely connected and adequate metal contact is being made to avoid shorts and other issues later on.

Mosfet Hotbed Connectors

Now we  just need to run the white wire from the MOSFET board to the orange connector and plug it back in. Make sure the positive/negatives are correct, slide them into the openings and tighten the connector securely where they will not pull free.

Note: The end of the white cable did not expose enough bare wire to make a proper connection. I had to use the crimping tool (#20-22) to open these up a bit more.

Melzi Board Connectors

To finish it off, we just need to connect the MOSFET board to the Power Supply using the 14 Gauge Silicon wires. The red wire is connected to “+V” and the black wire is connected to “COM”, where the spade terminals will provide a simple and secure connection on this end as well.

Image Credit

Power Supply Connectors

Final Thoughts

From here, all we have left is putting everything back together. Stick the other half of the velcro pad somewhere within the Control Box near the Hot Bed connector on the Melzi board. You can then reconnect the Melzi board (if it was removed) and stick the bottom of your new MOSFET board to the velcro base nearby.

Slide the top of the Control Box back on to the base and take a few minutes to organize all of the wires inside, ensuring they are not bent in strange angles. Screw the case shut and plug it back in to find out if it works!

If everything was done correctly, it should look like the following…

Mosfet Board Installation

84 thoughts on “Guide: Installing a MOSFET Board (Maker Select v2)

  1. This is great! Thanks for the tutorial. Do you or anyone else know how similar this is to the Maker Select (Wanhao Duplicator i3) Plus?

    1. I’m not very familiar with the Plus so I can’t say with any certainty. The guide should be pretty close for the Plus as well though, where I can’t think of any major differences that would pose problems. I would take a few minutes to open up the control box and check your connectors to see what they are rated for. If they are under-powered, installing the MOSFET with this guide should be perfectly doable.

    2. Plus does not need this as it is 24V and not 12V.

    1. This is indeed correct! I know others have also used Ebay modules without any problem as well. The exact one from Reprap Champions and most Amazon sellers seems to be bulk ordered from Ali Express for about $5.00.

      I just happened to go with this one and link to exactly what I used, although there are definitely a lot of options.

  2. I have a Prusa i3 (JGaurora – and this MOSFET. To wire this up, I assume that I leave the existing termistor from the bed connected to its original input to the printer control board? Next I pull the big 12v wires that go from the controller to the hot bed out of the controller and connect them to the MOSFET output terminals (on the left side) ? Next should I run a small set of wires from the controller where the 12v big wires were connected down to the small control wires of the MOSFET ? Does the MOSFET handle that 12vdc input to switch on and off the hot bed ?

    1. I went ahead and wired it up, and it works great!. So.. for those that found the documentation a little vague in this area and are adding this MOSFET and the 200w Silicon heating pad to their Prusa i3 (JGAURORA A3).. hooking the Silicon heater’s Thermisor wires to the A14 connector on the printer’s controller (where the original bed wiring was hooked up (small wires)). Remove the original 4 (2 x Red & 2 x Black) original heater wires from the top connector (BED) on the controller. Get some small gauge wire (same as the control in cable with connector on the MOSFET module), and solder to the cable that comes with the MOSFET. Connect these wires to the top connector where you removed the original 4 wires (BED). Next take the Silicon heater Red wires and connect to the MOSFET output terminals. Next get some 12~14 gauge wire and connect from the main printer’s power supply 12dvc to the MOSFET module input terminals. Plugin the power, and test. Hope this helps someone in the future with this combo.

    2. Thank you for this. Got one of these running in 2021. Ordered a 12v/24v compatible mosfet board. Exactly this fix i needed to get some peace of mind when running this unit.

  3. Be careful using Velcro to attach this next to Melzi board. Once re assembled it is hanging upside down and it is hanging right over the PSU. I did exactly this and after about two weeks it fell down. No harm done but I didn’t like the possibility of anything making contact with the hotbed power connections. I’d suggest you place the board in front of the PSU so it isn’t hanging or drill one or two holes in the top of the case to properly secure it.
    We are doing all this to make a safer printer, I’d hate to see anyone cause more issues with the fix.

    1. I’ve not had any problem using the industrial strength Velcro, why I opted to go with that option as opposed to drilling. To further strengthen it, you could of course apply some Loctite Gel or another adhesive as a safe guard. You are right though, a more permanent solution such as drilling some holes or re-positioning it would be a better long term solution indeed.

  4. Quick question: Are we not also supposed to be doing the MOSFET mod for the hot-end heater?

    1. That isn’t necessary, redirecting the power draw of the heated bed is enough. The heated bed has the highest power consumption, where just the extruder (hotend) remaining won’t cause any problems.

      1. OK, that makes sense, I remember now that the hotend heater is ~30W, so only about 2.5A at 12V.

  5. So my board did not have the orange bed and green connector. It was just green. When i tried to take out the green connector, it broke off the metal connectors. I guess it was hardwired in and should not have just been pulled out… 🙁

    I guess I need to try to find another green connector and do my best at soldering.

    1. Anyone know where I can find one?

      One side says “CE 129-5.08 200V 20A” and the other side says “ZHONGB ZB229”

      1. Do you have a Maker Select/Wanhao Duplicator and is it the stock Melzi board? From the factory each of these has 2 connectors, the green one connected to the board and the orange one plugged in to the top. From your description, you only had a single green one?

        It sounds like they have either changed the connectors around a bit (perhaps you have a newer one) or something else but it sounds strange. Perhaps you can post a picture so I can get a better idea of what you are seeing? You will most likely need to solder new connectors on.

        1. I’m having the same issue as this person. I don’t have an orange plug for HOT BED. Here’s a photo: . You can see the second orange plug says HOT END then the next one that is green it says HOT BED on the board but its not a plug. I haven’t fiddled with it as I don’t want to break like the previous poster did. What should I do? Thanks for any help

          1. Did you ever figure out a solution when you have the green connector only?

          2. Pretty much from what I remember, was straight forward. You just unscrew the top of the green connector and the cables slide out. You just follow the guide and put the connectors in and then screw down on them.

  6. It’s difficult to tell in the last picture of what it should look like, but is the white wire with the blue stripe on it the positive wire (coming from the MOSFET)? Just want to make sure I have it correct before putting it all back together. Thanks for the detailed write up!

    1. If memory serves, I believe that is correct. The MOSFET Board says “Control In”, where the positive is under the “Control” side and common is under “In”. My picture has the white wire with the solid blue stripe going into the positive. The images are actually larger than displayed on here, where it resizes them to make it fit. You can right click on the image, select “copy image address” and paste all of it into your browser bar up until the “.jpg” (excluding the resize part) to see the larger version.

  7. Hi, so I have a TronXY printer which uses the same Melzi board though mine looks like a different rev. I also bought the exact same mosfet you are using from Amazon, but I’ve been holding off installing it because the control wire (just like yours) looks too small a gauge to hook up to the hotbed connector considering it used to power the hotbed. At first I thought the control connector on the mosfet was for the bed thermistor, but in everything I found online, it’s not.
    Have you had any issues with the gauge of that control wire? I think I’m a little paranoid about that.


    1. I haven’t heard of any issues reported since this guide was posted. Based on the total number of views, I have to assume at least several hundred people have performed the install using these exact parts and instructions by now.

      The only reports of trouble that I have seen anywhere is from those that put this off for too long and burned out their connectors. I cannot speak for the TronXY specifically but this is pretty much a must for the Maker Select/Wanhao Duplicator i3.

  8. So I just got my replacement Maker Select and did this upgrade by following the guide.
    A couple things I noted:
    – Don’t use the screws this guy suggests. they have a teeny tiny hex pattern on them and are probably too small for a truly secure hold into the printed board holder.
    – They changed the connector on my printer, (maybe its v2.1?), and it does not have the removable orange connector anymore. This is still fine for this set-up, but the connector does not need to be removed (i learned the hard way), just switch the wires as explained
    – The white wires on the mosfet board are not clearly marked. The “X” is the power in and the dotted line is the common/ground.

    Otherwise, thanks! best guide on the internet that I’ve found

    1. Thanks for the feedback Corey, very good information to have! As for those star screws, they hold well enough for the mount I used but they are small. I just used what I had available but if I find something more suitable, I will certainly update the list.

      I have a v2.1 as well so they have likely changed connectors within the last several months. They were delisted on Amazon for being a hazard where they have likely improved the connectors recently. I will see if I can get some modern pictures and make notes of variations to help those will more recent printers.

      1. Hey my setup is similar to the other guys. I took some pics for you for reference. They may have corrected the power issue in this iteration.

        I can provide some more if you need.

        1. If our boards look like the one in Tertius’ gallery, do we need to add the MOSFET, or should we add one anyway for safety?

          I picked up what was branded as a v2 on Prime Day but I don’t know of a certain way to verify v2 or v2.1.

          1. All revisions of the Maker Select including V2.1 must have a MOSFET Board. They have upgraded the connectors on later models but they still suffer from the same problem. The Maker Select Plus has a 24V Electrical System and does not require the MOSFET Board but all standard Maker Select models need it.

          2. Ok, ordering the parts now. I should like to not burn things to a crisp.

          3. I am working on finding some alternative screws for the parts list, the ones I recommended have since jumped to $19 on Amazon. I believe some M2x5mm would work with that printed mount and are much cheaper, but haven’t confirmed that myself as of yet.

            As soon as I can confirm a different screw that fits, I will update that. Just wanted to throw it out there to hopefully save you from buying $20 worth of screws.

          4. Skipped them already, didn’t need 100. Zipties for now.

          5. I use them for all of my Raspberry Pi cases so they come in handy, but definitely don’t need them for just the MOSFET. Zip ties sound like a good alternative!

  9. I just did this mod on my mom maker select v2.1. I did exactly what the directions were for this mod and it worked for a couple days and then today my heated bed stopped working and now it won’t heat up. I checked all the connections and none of them are loose or burnt. Has this happened to anyone else?

    1. I believe someone else mentioned this happening on Facebook and found they mistakenly reversed the white wire from the MOSFET Board. I’m on mobile right now and can’t find it but I would double check that.

      1. I assumed that the white wire and the wires connecting to the hot bed does not matter since it is resistive heating. The only part that mattered was the connection to the power supply.

  10. Thanks for the tutorial. Most helpful.

    Was the “girl” part really necessary in “Despite being an IT guy, my experience with wiring was on par with that of a 12 year old girl”?

    1. Not a problem, I am glad to hear it helped. I only wrote that sentence in jest, where my 12 year old daughter is the last person who would ever be interested in such things. I have however changed it to “child” instead, where that is more along the lines of what I intended.

  11. Installed the MOSFET using this website as a guide! Thank you. Only difference is they’ve changed the orange connector for the hot bed to a beefier green one but the control wires still clamped in OK.

    1. Then during my first print I smelled burning and freaked out and turned it off. I opened it up and everything looked good. Then I ran another print with the cover off to see if I could tell where the smell was coming from. Turns out my neighbour was burning food!

      1. I am glad to hear it wasn’t something with your printer! That is hilarious though 🙂

        I had a similar experience, completely replaced my entire extruder assembly with new parts. After I finished and turned it on, got real close to see if I could smell anything burning and noticed an odd odor. Was absolutely panicking and then I realized it smelled kind of sweet, was just the smell of PLA I had pushed through lol.

  12. Can I use 16 gauge wire instead of 14? I already have 16 on hand…

    1. I would probably advise picking up 14 gauge. While 16 gauge would likely work as a temporary solution, I would have some concerns about using it given the power draw of the heated bed. Best case, I would expect some temp fluctuations, worst case it won’t be able to get enough power to operate.

    1. No problem, congrats on getting it installed and finished! It feels much better 3D Printing once you know that issue is taken care of!

      1. Definitely! I’m happy to no longer worry about frying the motherboard.

  13. Finally got a few hours (late in the night) to get everything stripped, crimped and rewired. There was even an unused* V+ slot on the power supply. Put 3-4 screws back in to hold everything in place well-enough. Flip the switch… no pop, no smoke, hooray!

    Tell the bed to warm up, and the icon on the LCD changes, but the temp never increases. Take everything apart, check that the control wires were in the right slots (they were reversed, oops), and move the velcro and MOSFET further towards the LCD. Switch on, no pop, no smoke; bed still won’t warm. As it is nearing 3am, I tell myself that only sunlight can help here.

    Next morning, take it all apart again, and check continuity. Everything checks out. Decide to run the box open, see if anything is obviously wrong or sparking. The MOSFET board has an LED that turns on when I tell the bed to warm, but there is zero voltage on the input or output.

    That unused* V+ slot is also unpowered. Shuffle the + wire over to another spot and everything works! I hope this will help someone else not make the same assumption** I did.

    * Unpowered, too.

    ** Certainly made an ass of me, not much umption.

    1. Definitely lots of little ways to make mistakes when working with 3D printers, especially the electrical components. I am glad to hear that you managed to get it resolved, thanks for sharing in case anyone else runs into that. I answer a lot of MOSFET related questions on Facebook/Reddit and the feedback here has given me more info on problems some people run into while doing it.

  14. I have a monoprice plus that is having some major temperature overshoot problems. (40C range, 20C+/- set temp)

    I came across this fix awhile ago, but wrote it off as not needed due to the 24V design, but when researching these temp swings, It seems other users reported a fix centered around shunt wires to help with grounding design issues on the board. (

    Later, this added mosfet mod was described as the solution that got past the requirement for shunt wires.

    So, on one hand, I’m told the mosfet mod will solve the issue, but the other I am told it’s not required.

    Brett, wondering if you have any insights for me before I start soldering?

    1. While I don’t have the Plus model, I would just double check the obvious before doing any soldering. Make sure the heating block is well insulated, where the parts cooling fan (especially blower fans) can cause massive temperature fluctuations. I’ve never had a faulty thermistor (they usually just stop working completely) but you could either swap it out to test or check to make sure it is seated well inside the heating block.

      If nothing obvious stands out though, doing the MOSFET is probably your best bet. One of the upsides noted by most is that it does stabilize the temperatures. While most Plus models are fine without it, it certainly won’t hurt and if you have an older model, I do recall mention of them requiring the MOSFET for some period of time due to an electrical problem.

      Sorry I couldn’t be of more help, shouldn’t be too difficult to get resolved though with a bit of tinkering. Let me know when you figure it out though, I would love to know the culprit and others often benefit from the additional info shared in these comments as well.

  15. Love the info here, in the process of doing the update, but can’t get the super glued orange hotbed connector off. How did you pull off the glue?

    1. Hi Drew,

      I am glad the information has helped! It has been quite some time since I did the mod, but from what I recall the super glue was somewhat accessible from the outside. I do remember having to mess with it for a bit as the connectors were definitely not easy to get off. I just picked at the visible glue and then carefully worked the connector off with minimal force until it popped out.

  16. I used this guide to install my MOSFET and it was definitely helpful. I had a few little differences though so I’ll list them here. I purchased my Maker Select v2.1 about 2 months ago so maybe it is only in the newer ones:

    * There is no “COM” on the power supply, there should be an empty “V-” terminal, this is where the ground wire (from the step above) goes from the Melzi board.
    * At least on my MOSFET board there was a white wire with little blue “+”s on it and a white wire with a solid blue line. The wire with the + on it goes to the POS terminal on the Melzi board, solid line goes on the GRD terminal.
    * The orange and green connector is now just a green connector with 2 flathead screws on the top, just unscrew those and screw in the wires mentioned above.

  17. Mine started turning off randomly when preheating. Opened it up, and the heatbed connector had burnt a bit. I had some XT60 connectors laying around from FPV racing projects, so I installed those in place of the heatbed connection. I already ordered the MOSFET, and it will be delivered tomorrow. Think I still need it? I believe those XT60 connectors are rated for 60A.

    1. Before I wrote this article, XT60 Connectors were the preferred method of fixing the issue. It was much more difficult to reverse soldering than the MOSFET though, so I opted to go that route in case I needed to RMA the printer under warranty. If you’ve already replaced the connectors with XT60 though you should be fine.

      1. Got ya. Printed an 8 hour ABS model last night without issue, so I think my issue is officially resolved. Though I won’t be letting it print without me being nearby any time soon.

        Does your Maker Select print better with ABS? Mine produces excellent quality ABS prints, but I still haven’t gotten PLA dialed in yet.

        1. I’m glad to hear it seems to be working better for you. I’ve never printed ABS, I live in an apartment and all 6 of my printers are in the office with me. I’m trying to avoid killing myself, girlfriend and dogs off with toxic fumes so that is a no go.

          Mine handles PLA and PETG like a dream though, the Micro Swiss Hotend (with a Dii Cooler) has been one of the best upgrades in my opinion. Once I got things dialed in, I can print pretty much anything and the results never cease to amaze.

          1. And just now, my heatbed stopped heating. I used to be a fan of this printer, but this is getting a bit ridiculous with all the issues it’s had. I guess I will tear it apart again in the morning.

          2. I’m sorry to hear that, it’s possible you had more damage than just the connector. If that is the case, replacing the connectors may have just been a band-aid for a larger problem. While these are definitely great printers, there is a lot of room for error without preemptive action. Mine isn’t even printing right now, dropped a spool on the Z motor and ripped out the connector so there is always something, even though it is usually user error in my case.

          3. I forgot that when I replaced the first connector, it was actually the power input from the PSU to the Melzi board. This time, it was the hotbed connector, which makes sense. The power connector was getting a bit more current, so it should fail first in theory. Replaced that one with an XT60 as well, and now it’s back up and printing. The most disappointing part is that Monoprice didn’t even take the time to email me back regarding this fire risk printer they sold me. I’m just a month past the warranty period, but not even replying says a lot about the company, in my opinion.

            But I am back up and printing now, so oh well. I’m about to print Knick’s Prosthetic Finger from Thingiverse, and it requires using TPU/Ninjaflex. Any advice before printing with those materials? If you haven’t seen that Thing, check it out. Very cool and actually functional prosthetic finger.

  18. Connected it up to my maker select v2 and the bed heated up as soon as I powered on. Removed it and the bed worked normally again. Bad part? Triple checked all connections.

  19. Thanks for the great tutorial. The only difference on my maker select v2.1 (ordered Nov 2017) was the connector on my board being green instead of orange. Everything seems to be working fine. Now it’s time for an all-metal hotend. 🙂

    1. Thanks for the heads up, I had not realized that was even a remix of a different design. As a rule of thumb though, I generally only link parts and designs that I have actually used. As you mentioned you have improved past that of the remix, can you elaborate on what those changes were? If they are substantial enough, I can look at printing it out, installing it and possibly update the links to include it.

  20. I am looking to buy one of these printers as my first 3D printer so I have no way of printing this bracket before installing the mod. Is there any way I can print off the bracket before installing the mosfet without melting my printer?

    1. You should be fine. I printed several items on mine before installing the bracket/mosfet. If something did go wrong, and you bought the printer new, it would be covered by warranty.

    2. Hey Andrew,

      I think most new owners have this question, but the odds of the first print frying the board are astronomically low. My thoughts when I received mine were that worst case, I could at least return it (should it burn out before I got the MOSFET installed). I’ve seen the problem happen for some within days, others within months and some people have used this machine absent of a MOSFET for years without issue.

      My advice would be to go ahead and print the bracket as a first print and just get it installed as soon as possible. Let me know if you run into any problems or have questions though!

  21. Thanks for the great write up. I used this guide to order my parts after my hotbed stopped heating. After installing, everything powers up and works great, with one problem …. now the hotbed doesn’t stop heating. With preset at 60 degrees, it just keeps going.

    I can do board swaps and such, but I’m not a circuit level kind of repair guy. Any suggestions on what to check?

    1. Did you install the MOSFET? Or just replace the connectors with higher rated connectors?

    2. It sounds like the MOSFET on the control board has broke, leaving it stuck in the ON position. The external MOSFET won’t fix that issue because the board will continue to signal that it should keep heating. The easiest solution is to replace the board itself, where the other option is to replace the MOSFET on it… but that will require soldering.

  22. This mod made a world of difference to my prints. The print I made for the bracket came out at a much lower quality because of temperature fluctuations. The print I made immediately after for a Pi Zero W case is probably the best print I have made so far.

  23. Thanks again for the right up, it was a great help. As for my installation, I had the maker select v2 and all went well as dar as the installation and testing out the heatbed etc, however when I started heating the nozzle up, it sparked when I attempted to clean off some debris. The nozzle was just homed and was near a lead screw on the front left hand corner of the bed, so I believe I bridged the nozzle to the screw.

    Any advice would be appreciated.

    Ps. So far everyrhing else works fine, and on a side note I just installed marlin firmware 1.8.8.

  24. Help!! i installed this before i printed everything using your guide! ive come around the completely finishing my mods. The most recent thing was upgrading the firmware. But now my mosfet board is smoking! i turned it off in time but there was visible smoking coming from the board. What should i do?

    1. Hi Conner,

      Assuming everything was wired correctly, it sounds like you may have just received a defective MOSFET board (assuming the smoke was coming from it and not the main board). I would contact the seller and return it as defective, making them aware that the product was faulty. It is pretty rare to get a bad MOSFET but I have seen some reports of it on Amazon. It shouldn’t have caused any harm to your Melzi board, although I would wire it back up and bypass the MOSFET just to make sure nothing else starts smoking or the connectors look burnt.

  25. I noticed the blue LED is flashing. Anyone else had this happen? The bed stays heated, and no smoking. Everything seems to work fine so far.

  26. My MOSFET board came with a white wire with blue dots and a white wire with more blue dots (or blue dashes). The connector has been rotated 90 degrees from all the pictures. Anyone know which of the white wires is positive? Here’s the pic

  27. Great tutorial! I had the board from 2017 when the orange connector was not on the hotbed connector, but tutorial is still easy to follow.

  28. Heat bed has short circuit. On my board, resistance between the two, 24volt solder connections was
    about 2.4 ohms, which would provide about 60 Watts of heating power, enough to eventually raise the
    board temp to over 55C. I’ll try raising the power supply voltage by up to 1.5 volts (currently 12.4) which
    would raise power to the original 75 watts, assuming that the Melzi onboard regulator can deal with
    the higher voltage. Insulating the heatbed may also help with bed temperature. I’ll have to check for damage to the Melzi from the over-current, but have a MOSFET on order. I’ll post any new info.

  29. Heat bed option: If the Melzi can’t provide power to the bed, the inexpensive industrial temperature controllers (10.00 or so) could do the job. Leave the original thermistor in place, and tape a second
    one to the back of the board for the controller. Configure the controller for 100k thermistor. Means
    manual setting desired temp, but may work if mosfets not available.

  30. Hi, I do have a big problem. I have the same 12 V PSU and MOSFET HA210N06 type except for motherboard which is MKS V1.1 .But let me tell you my concern: the output of the MOSFET is same all the time with the input of it, regardless the voltage of the Control IN, as it is always open. I did a wiring connection using same pins on the PSU ( and also using different pins as you did, but the results where same. When is 0 voltage on the Control IN should not be the same on the Output, if I only simply connect the MOSFET to a power source ?
    What might be the cause? Shall I change it ? Thanks.

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