Melzi power MOSFETs and heat bed question

I'm nearing the end of completing my RepRapPro Huxley when I notice that both the Melzi electronics and the heat bed have a power MOSFET between them. The has brought me to a stand still since I've been following the instructions up until this point where it says there should be an empty spot for a MOSFET on the Melzi electronics, this is not the case.

Any advice on this issue would be helpful.

I suppose it's also worth noting that this is on a RepRap Melzi v2.0 board.

Edited 3 time(s). Last edit at 11/04/2012 08:29PM by Physics_Dude.

Update:

I just scrolled down to the very bottom of the wiring instructions page and read:

Quote

As you can see, "HOTBED" connector is not connected to anything, since we are using the bed mounted MOSFET to control the heatbed.

Ok, so I guess thats that then, but why can't I use the MOSFET on the main board? You'd think it would make the wiring look a bit neater, right?

A bed mounted MOSFET? O-o, what an idea. MOSFETs are expected to stay cool and how shall this work if the "heatsink" is 120°C already?

On how to wire a heated bed you can look up other electronics, like RAMPS or Gen7. MOSFET is MOSFET, as far as I can see all RepRap electronics use the same concept.

In case there is no MOSFET on the Melzi board and no even a spot for soldering one, you can use something like the SevenSwitch to deal with this.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

Traumflug Wrote:
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> In case there is no MOSFET on the Melzi board and
> no even a spot for soldering one, you can use
> something like the SevenSwitch to deal with this.

Yes, there is a MOSFET for the heated bed on the Melzi board because that is how my Mendel works.

I didn't reply to this thread before because I have no idea why Emaker/RRP didn't use it for Huxley. Good luck in getting info from them.

Andy

Edited 1 time(s). Last edit at 11/05/2012 07:59AM by ajayre.

Hi all

The MOSFET (IRLR3636) datasheet specifies an operating temperature range of -55°C to +175°C, so it stays cool (by its standards...). Its on resistance is a mere 7 mohm, so it dissipates no appreciable heat itself.

And the arrangement of having the MOSFET on the bed is neater for several reasons. It means that all the bed current (which is the highest in the machine) doesn't have to be routed via the control electronics; that just needs to send a logic signal to turn the bed on and off. This reduces noise on the control electronics power rails considerably. Also, the fat wires then just go straight from the power connector to the bed.

We don't do that on our Mendel yet, but we are considering using the same trick there too at the next upgrade as it works so neatly on our Huxley.

Edited 1 time(s). Last edit at 11/06/2012 01:16PM by Adrian Bowyer.

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best wishes

Adrian

[reprap.org]
[reprapltd.com]

@ Adrian & Other Helpful People


Hi, this maybe slightly off track, but I am having heated bed and printer disconnect problems with my RepRapPro Mendal and the Melzi board that I have recently finished building. In that if I set the bed to 85 Degrees via the Pronterface software, it instantly indicates that it 'can't talk to printer (is it disconnected?)'. Then 'Can't Write', Can't Read and so on. Sometimes if I swap out the USB port cable at the PC it will start responding again, I can then Connect and everything appears to be back normal (e.g carriage and axises move okay via command). Once Pronterface was back up, I set the bed heat to 45 Degrees and all stayed connected communicating. Monitoring printer BTW. However, if then set the bed to a higher temp for printing readiness, all stops communicating. Tried power cycling the printer in situ, logging out of connected PC and back in again. Restarting. Funny thing is that I'm it wasn't behaving this way after initial set up, all seemed fine. That tells me I something has changed, but I can't figure out what. Should I update firmware? If so, I'll research how to etc. Or is it something silly that causing the constant disconnects?

Somehow at one point I got bed temp to 85 and started printed, half through large filament usage print it stopped dead. Also I notice that the back of the Melzi board in the region near the USB connector felt very warmish to the touch. Didn't expect that, and can't say I noticed it before,

Will keep researching, try out a few things and post back.

Cheers

Sounds like the ground connection between the Melzi and the PSU is not good enough. When the bed current flows through it, it generates a small voltage drop. That appears across USB cable and upsets the comms.

I connect 12 black wires and 6 yellow wires from my ATX PSU to get the lowest voltage drop possible. When using 12V lighting PSUs I use 32/0.2 wire and keep it short.

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[www.hydraraptor.blogspot.com]