Heated bed solutions and modularity

*please don't move this post to electronics for a few days, no one looks at that list, except for me when I'm looking for my posts, and I help anyone there I can*

This forum, is one of the things that keeps me going through the frustrations of working on such a quirky and experimental project...

I hooked up my prusajr heated bed to the extruder.

I go it to heat the bed, then the mosfet smoke and bubbled... I have the techzone board BTW which supposidly has bigger mosfets.

Now my board has the LED for that MOSFET always lit, no matter what. firmware, reset, software nothing turns it off except no power to the board.

Has anyone experienced this to tell me if I'll be able to salvage this extruder board?

I'm thinking I need to build my own extruder board... super modular so I can rip off components like this, replace and keep moving. I don't want to replace the whole damn thing each time.

THIS is why I don;t care for the new "gen 6" electronics I saw someone post earlier this month.

Thanks in Advance,
-Anthong

The reprap design uses protected NIF5003 MOSFETs so they should shutdown when overloaded. What part are Techzone using?

Edited 1 time(s). Last edit at 10/18/2010 11:09AM by nophead.

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[www.hydraraptor.blogspot.com]

I wasn't even running the board for that long, I was just testing the heated bed PCB... also the LED next to the mosfet was lit yellow, I haven't seen that before.

here's what they say on the wiki page on the mosfets:
[reprap.org]:

I'll try to read what the actual components say when I get home tonight, hopefully that will give more information.

Quote

We also used higher amp mosfets in a Dpak configuration (almost all standard mosfets in a Dpak will work on this board, so you can select values which are correct for your use, if you are building your own.

Obviously they didn't appreciate the original design choice.

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[www.hydraraptor.blogspot.com]

Anthong,
Have you measured the resistance of your heater? And what supply voltage is the MOSFET switching? Most common is to use the 12V supply to run the bed heater. If your heater resistance is too low, then when you start out to warm it up to operating temp, the current will be too high and the MOSFET will over heat. Try connecting the heater directly to 12V through a multimeter set for 20 Amps or more to see what current is being drawn. Better yet is with a clamp on current meter that won't be damaged by excessive current.

The LED may be correct in always being lit if the heated bed is still below desired temp due to not having a MOSFET sending it current. I would check the current draw and the capacity of the MOSFET before worrying about the LED always being on. On the other hand, I don't have a working reprap or repstrap yet, so I have not ever operated my electronics!

Mike

attached a pic, notice how nothing is attached to the mosfet but the led is still on, you can see the melted bits around it as well

the info on it says
_______
|20 n6L |==>
<=| |
|on 747 |==>

I may try to disassemble enough on the board to replace it, I have a pretty crappy soldering Iron though, so upgrading that would be a prerequisite.

rather dissapointing, especially since I;m afraid of it happening again.

this is the heated bed I'm try to hook up, it has all the specs on the page:

[reprapsource.com]

If you have the tools I would take it off asap....
for next time you could use a solid state relay? 13-15 bucks for piece of mind.

I can't work out what the MOSFET is from those numbers but the heated bed takes 10A when cold, so I don't think you will be able to drive it from an extruder controller. The wires on the picture are not thick enough for 10A and I doubt the PCB tracks are either.

A solid state relay won't work on DC and as they drop 2V are not good for low voltage AC either. A normal relay will work but only for bang-bang control, not PWM and PID and it will wear out pretty quick.

I have one of those beds so when I get it working I will publish a circuit. I will use a high current MOSFET and probably a level changer to boost the gate drive voltage to 12V. I might also need a heatsink.

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[www.hydraraptor.blogspot.com]

nophead Wrote:
-------------------------------------------------------
> I can't work out what the MOSFET is from those
> numbers but the heated bed takes 10A when cold, so
> I don't think you will be able to drive it from an
> extruder controller. The wires on the picture are
> not thick enough for 10A and I doubt the PCB
> tracks are either.
>
> A solid state relay won't work on DC and as they
> drop 2V are not good for low voltage AC either. A
> normal relay will work but only for bang-bang
> control, not PWM and PID and it will wear out
> pretty quick.
>
> I have one of those beds so when I get it working
> I will publish a circuit. I will use a high
> current MOSFET and probably a level changer to
> boost the gate drive voltage to 12V. I might also
> need a heatsink.

McMaster 7456K61 is for DC. Although they don't ship outside the US I'm sure the parts aren't unique and similar ones could be sourced elsewhere.

wow thanks nophead! Glad I took the picture!

Hello fellow reprappers.......I wanted to share my solution to the heated bed.

I'm using a single 40x40mm TEC(thermoelectric modulde) attached to the bottom of the the aluminum table and I'm getting ~90oC temperature and its working great.

Heatup time is 5~10minutes.

To test the reliability I'm using them unregulated (no control), but will work nicely with a mosfet that can handle 6A startup
12708 will draw about 5.6A@12v
12706 will draw about 5A@12V

Whats nice about using a TEC its resistance goes up as it gets hot so it keeps itself under control, and from a safety point if it does fail it goes to open condition.

with 1/4"(6mm) aluminum table
Using a TEC model 12708 will get to ~max90oC
Using a TEC model 12706 will get to ~max75oC

So to summarize
-easy task
-Its cheap (you can get them for $5) ..I have lots of Marlow/fuxin model:12708 so send me a PM if you can find them on ebay or elsewhere.
-much safer.....sorry about the tech talk>>>> because its a series of 127 pn junctions if one shorts the resistance drops ever so slightly, and if it opens(most common failure mode for this type of components) it stops heating.
-simple to use, no need for extra electronics or sensors
-lots of size options
-Live parts are isolated by the TEC ceramic plate so no worries about electrical shorts.

Enjoy!

PS I'll post some pics on Monday



I also got a TEC cooled extruder in the works!....I'll share that and the design details for that next week.