Longboat Prusa heat bed wiring - please help!

I have successfully assembled my Longboat Prusa and the motors all work ok, the hot end is fine, I am just having problems with the heat bed wiring. I have already tried it once and it melted my 644P chip! I have a new chip and all seems ok.

The attached image shows exactly what I am doing and where I am not sure what to do. Could you please take a look at the attached image and see if you can help.

Thanks to all who have contributed and helped others on this forum!

Mark

Edited 1 time(s). Last edit at 03/11/2012 07:30PM by Emin8.

I believe this is the source of the Longboat info but it does not match what is on the Longboat wiki page.
Adrians_Prusa_Notes
I followed Adrians instructions but still could not make it work so I (like most people I suspect) just used the Sanguinololu onboard drivers.

So in your diagram at the bottom, +12v goes to A and 0v goes to B.
The thermistor leads attach to the bottom pins on the right of the board.
You can remove the mosfet from the bed.

If you wire it direct don't forget to remove the link on the Sanguinololu, otherwise you will fry the Atmega.

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[www.hydraraptor.blogspot.com]

Ok.
So if I forget about the MOSFET on the bed do I need to remove the link I made on the board? An put the Q2 back that I de soldered

Thanks for your help.

If you are not using the bed mosfet then you should not have the link on the board.
Not sure what you mean by put Q2 back, there is only the one mod mentioned i.e. the link.

The first couple of lines of text on my attached image says

'Because the heated bed has its own driver transistor mounted on it, you don't need the one on the Sanguinololu board (Q2 on the Sanguinololu circuit diagram). Remove Q2 (desolder or cut legs) and replace it with a link as shown in the image.'

Sorry, this is where I get Q2 from. I removed Q2 from the board and soldered a link. So I am now thinking I re solder the Q2 mosfet and remove the link.

OK, That instruction was added since I attempted to get the bed working. In fact looking at the Longboat wiki page there is a whole load of new info so maybe it will work with the new instructions.
Still I am going to stick with my set up which I know works.
To answer your question yes Q2 should be in place if you are going to run the bed from the board.

Yes you can either remove the link, put the MOSFET back and connect the sanguinololu directly to the bed, or you can leave it on the bed and connect gate to the sanguinololu output, source to ground and drain to the heat resistors.

Pros of on the bed are that any heat generated in the MOSFET is not wasted and you don't have big current flowing through the inadequate connectors on the sanguinololu. The downside is the MOSFET is heated by the resistors instead of being cooled. For PLA temperatures that is probably OK but for ABS I think you risk the junction temperature of the MOSFET being exceeded.

Note that MOSFETs are very static sensitive and can easily be destroyed by a spark due to a loose connection. Having long wires on the source and gate risks it self oscillating. That will cause it to get hot and make a high power RF transmitter. A small series resistor close to the gate terminal will mitigate that.


IMHO, the correct way to drive the bed is with a suitably rated low RDSon protected MOSFET on the control PCB with suitably rated connectors. Boosting the gate drive from 5V to 12V only costs pennies and reduces the heat in the MOSFET. Protected ones are more expensive but virtually indestructible, so are far more appropriate for this application. Unprotected ones are very fragile when connected to external circuits with no ESD or overcurrent protection.

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[www.hydraraptor.blogspot.com]

Oh cripes another alternative! ...

...or you can leave it on the bed and connect gate to the sanguinololu output, source to ground and drain to the heat resistors.

Gate to sanguinololu output? - I thought that the +12v output from Sanguinololu goes to the heat resistors? As per the circuit diagram in Adrians Prusa notes Adrians Prusa Notes and the Gate goes to the resisitor.

I do appreciate everyones help with this, but this is still confusing. Am I being a bit thick here? (I havent discounted this yet)

The heater resistors go to 12V, better direct to the PSU but can be from the two pins of the bed connector that are connected to 12V. The other two pins on the bed connector are the control signal if you have the link on the PCB. In which one of them needs connecting to the gate.

I recommend a small series resistor close to the gate. Adrian has a large value resistor to ground. Presumably that is to ensure the MOSFET is off if the firmware hasn't defined the pin to be an output.

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[www.hydraraptor.blogspot.com]

I couldn't follow the official way to wire the bed when I built mine so just ignored the mosfet on the bed and wired the conventional way, seems to work fine.

Also beware of that hot end. It will destroy itself with time if you dont narrow the end of the PTFE barrel so it can fit inside the PEEK block so thatthe brass part is properly supported not just hanging off the PTFE M8 thread.

[forums.reprap.org]

@nophead - Other 2 pins on control bed? I only have one connected to the gate as per the instructions. The other 2 are the +12v and 0v from power. Does it just sit there connected to the connector on the bed? Nothing else connects to it? What is this control signal?

Unfortunately I cant resolder the original MOSFET back on to the board.

I have successfully done everything else on this project with little fuss but I find this really contradictory. I still dont know if the instructions are incorrect.

I said " other two pins on the bed connector".

The bed connector on the Sanguinololu (how I hate typing that name!) has four pins. Two are +12V the other two are normally the drain of the on board MOSFET. There are two pins because of the current involved and even with two it is underrated, IMHO.

When you remove the MOSFET and put the link on you are bringing out the gate connection to the two pins that normally connect to the drain. So that has to go to the gate of your off board MOSFET. Adrian doesn't use a series resistor but I strongly recommend one 10-100R, He has a pull down, but there is already one on the board, so that is not needed as long as you keep the bed plugged in to the board. It does no harm though.

The source goes to ground, again probably best straight to the PSU to avoid heavy current through the Sang.... . The drain goes to the resistors and the end of the resistors to 12V.

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[www.hydraraptor.blogspot.com]

Ahhh...the mist is clearing!!
I now know what I am doing! I actually understand how the MOSFET works now and what that link on the board does. Many thanks nophead and thanks for your patience with me

I think I have broke my MOSFET by wiring it up wrongly and have ordered another one that will hopefully come today.

Hi, sorry but it's the good way to wire the Heat Bed Longboat Prusa on the Sanguinololu?



Edited 2 time(s). Last edit at 03/15/2012 04:16PM by Djamuka.

I am reluctant to give advice to anyone else seems as though mine isnt working 100% yet but I would say that is wrong.

Your +12v to one end of the heat resistors is correct. The other end of the heat resistors that links to the Source is incorrect I think. That should link to the Drain, not the Source. What you have drawn seems to be as it is in the instructions which based on all the information I have received and what I understand now is incorrect.

Your Source needs to go to Ground on the Sang. RI needs to go between Source and Gate.

Like this ?



Sorry for the Noob Wiring schematic ^^

Edited 1 time(s). Last edit at 03/15/2012 04:19PM by Djamuka.

Yes that is what I believe to be correct and how |I am going to do mine. I dont have it working though yet. Anyone else confirm?

Yes that is correct, but as I said before, I would recommend a small resistor in series with the gate, close to the MOSFET.

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[www.hydraraptor.blogspot.com]