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  1. #1

    FET Bed Heater Board

    So I noticed that the MakerFarm Pegasus 10" ships with a Bed Heater relay board and after doing some reading/thinking found it would not be very helpful in the long run and should be replaced with something better.

    The design as I have it below has more then just one use and I can see it being used for many things. It is setup to allow any input voltage (change R3 as needed) 10mA device to control a much larger power device. The best Fets to use would have very low RDSon. The Surface Mounted Fet may not have enough cooling for +20A loads and the TO220 would need a heatsink of some kind.

    To facilitate this I have started work on a replacement board design that will allow some customization of the parts.
    Right now its set up to use 2 different Opto types and 2 different Fet packages. Slow Transistor type Opto's, Fast gate driver Opto's. Dpak(TO263) and TO220 Fets. I chose the packages for 2 reasons.
    1. There is a large number of same pinout devices on the market.
    2. They are off the shelf parts that are not hard to get.

    HeadBed PCB1.jpgHeadBed SCH1.JPG

    My Question for you guys is as follows.
    1. Should it be the same size as the existing Relay Board and if so what are the dimensions since I dont have my Pegasus yet.
    2. Should it be smaller?
    3. Should I make it more specialized(like remove the opto and expect whoever uses it to have there own way of driving it)?
    4. From my standpoint the standard slow optos should be more then enough, 1-100kHz switching frequency is more then enough to take care of our needs and it reduces losses due to switching(as long as the slow rise/fall does not increase on/off losses). Should I get ride of the Dip8 Gate Driver and just use the Dip4 Opto Transistors. Or should I do a limited run from Oshpark and pull out my trusty O-Scope and compare the Fast vs Slow opto's?
    5. Did I make some huge mistake in this design. (considering I have used both circuits at home and at work they should be fine combined, as long as you dont mix resistors/misplace the dip4)
    6. Should I replace the 2 large terminal blocks with screw terminals like these http://www.digikey.com/product-detai...798-ND/2171014

    Not sure if this should be in one of the larger forums, but for me its first and foremost for the Pegasus and later on it may be used for other things.

    Edit
    Just noticed I put C1 wrong, should go to pin8 on U2 and GND.
    I tried replacing the 2 terminal blocks with those screw terminals. They are kinda big lol. Might make good heatsinks.


    Update......

    HeadBed PCB8.jpgHeadBed PCB7.jpgHeadBed SCH2.JPG
    Last edited by Donziboy2; 02-01-2016 at 09:32 AM.

  2. #2
    Got ride of the Pos Supply Post and just added a small jumper point for a wire.
    Shank the board down to 2.77in square from 3.17in square. I can probably make it smaller but would like some feedback.....
    HeadBed PCB3.JPG
    I kinda like the screw posts, they are a little cheaper and they make good large secondary heat sinks.

  3. #3
    Senior Engineer
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    I don't see why it is so complex. If I needed to do this I would use a P channel and a TL431 to drive the gate of the MOSFET. Making sure -ve was common.

    Can't see a need for anything more complex than that other than a couple of resistors to limit currents.

  4. #4
    Fewer things go wrong that way. I would rather desolder a few parts from a sub board then replace a micro controller...

  5. #5
    Trying to isolate whatever your driving the load with from the higher voltages the load may have.

  6. #6
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    Why? There is no need to do that at all.

  7. #7
    Staff Engineer printbus's Avatar
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    Add printbus on Thingiverse
    On the relay board, I recall maybe a couple of other threads where, in addition to providing a MOSFET switch, the person was adding other useful stuff onto their custom board. For example, adding an ABL servo or a Raspberry Pi Octoprint controller can leave you starved for 5V, so people were adding 5V regulators on their boards. I remember suggesting that providing 12V distribution and a few lightweight MOSFETs for things like fans or lights would be a good idea.
    Last edited by printbus; 09-20-2016 at 11:37 AM. Reason: image and attachment issues have improved

  8. #8
    Quote Originally Posted by printbus View Post
    On the relay board, I recall maybe a couple of other threads where, in addition to providing a MOSFET switch, the person was adding other useful stuff onto their custom board. For example, adding an ABL servo or a Raspberry Pi Octoprint controller can leave you starved for 5V, so people were adding 5V regulators on their boards. I remember suggesting that providing 12V distribution and a few lightweight MOSFETs for things like fans or lights would be a good idea.
    Im actually thinking of following in TechMasterJoe footsteps and using an ATX power supply. For me atm its hard to guess what will and will not be useful, I just got the check today that will be going toward my 10" Pegasus, I have spent the last week or so just looking at things I plan to change from the start. The Mosfet Driver seems to be the easiest to tackle and I can see it being used for other things, I just ordered 3 boards and parts today. p.s. I can see some Chinese guy making and selling them for $3 in a few weeks time lol.

    I will say the ATX power supply's nowadays can give more then 1A in standby on the 5V line, this can power all the small stuff until the controller decides it needs more power and turns the supply on. I also looked at reworking the Ramps 1.4, but thats a long battle for later.

  9. #9
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    Good design is one that fulfils the requirements but does not overdo it. Opto isolators will be the fail point in your circuit, they always are. They do fail safe usually but without it the circuit will not fail at all for many years. You are driving a mostly fixed resisitive load so the strain on any components is well within the design spec.

    Belts and braces do not make trousers stay up any better.

  10. #10
    The only time I have seen Opto's fail is if to much current is drawn from them or to much voltage is pushed into them. We use FOD3182's at work to drive 3phase 3HP motors and when the IGBT's go they take everything between them and the FOD3182's out due to the 700V bus voltage.

    This is more a generalized design then a specific one case setup, I want to give the end user some leeway in what they put on the board. I am considering removing the dip8 Gate driver opto's since I see very few cases in which you would need to switch at the very high speeds they can produce. The on/off losses with the slower 4Dip opto's is fairly small compared to the overall switching losses themselves.

    I agree that the TL431 can be a simpler way in doing it (you could just drive the N Mosfet directly to) but I have preferences towards Opto's due to the protection they provide.

    edit..
    Added 10K pulldown on the fet gate to actually allow it to switch on and off....
    HeadBed PCB4.jpg

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