Gen 2 OnABoard

I used 4-pin connectors (gnd,sig,NC,5v) for my endstops for polarisation- if you did the same, you could use 3-pin connectors for the 12v and run far less risk of blowing things up.

Do 0.1" pin headers have a high enough current rating for the heater?

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Wooden Mendel
Teacup Firmware

The header and connector I am planning to use for all the 3-pin .100" connections has a limit of 4 A. This is enough for the extruder heater, but will fall short of what is needed for a heated bed (8A?).

While thinking about this I decided to review my trace widths to see if they can handle the current. I used an online calculator set with 1oz/ft2 thickness and a 20 C temperature rise.

I had to make the traces that lead to the PWM drivers thicker. They will now be able to deliver 4A each with a max for the three of 8A. This rules out connecting a heated bed directly to this board. The extruder heater typically draws 2A, so there is a bit of room for modification. Since I don't have enough room for a trace to handle more than 4A, I'm going to use the 3-pin .100" connectors.

The total current the board can handle is 10A. I'm a bit concerned though, because all of the devices are going to be running off from the single +12V wire on the ATX-20 connector. I doubt this will overheat the wire, but it might cause a voltage drop. I checked out a power supply by measuring the resistance from the 20 pin connector to a 4 pin connector that had the same wire length. The resistance was 0.4 ohms, so 0.2 ohms for the 20 pin connector alone. With a 10A current, the voltage drop would be 2V. I believe most power supplies have a 24 pin connector. This gives me a second wire to work with, which would cut the voltage drop down to 1V, which is still more than I would like.

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Darwin clone, Gen 2 electronics, Arduino Duemilanove w/ AtMega328, 5D Firmware, Pinchwheel extruder
[www.codeerrors.com]

I've updated the project file in the wiki and all the images to reflect my current design. I will likely be ordering the PCBs on Monday. rocket_scientist, I can send you two of them for $10 when I get them.

I added an ATX 12V connector. This is a four pin connector found on most power supplies that has two +12V wires on it. It should help reduce the voltage drop.

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Darwin clone, Gen 2 electronics, Arduino Duemilanove w/ AtMega328, 5D Firmware, Pinchwheel extruder
[www.codeerrors.com]

I also have an atx 12v connector for my board, due to not wanting to put all the current on one pin- it just opens up the possibility for problems such as contact noise and hotspots and things like that. great minds think alike

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Wooden Mendel
Teacup Firmware

mccoyn,

Do you want to add the opto-end stop switches to the BOM? Also, did you have any cabling in mind for them, or just solder the ribbon cable straight onto the optos?

Mike
(just ordered my parts)

Edited 1 time(s). Last edit at 03/29/2010 08:05PM by rocket_scientist.

I still need to add the cable connectors to the BOM. I also realized today that I left out the reset button, so I added that to the BOM.

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Darwin clone, Gen 2 electronics, Arduino Duemilanove w/ AtMega328, 5D Firmware, Pinchwheel extruder
[www.codeerrors.com]

I tried testing the Pololu stepper yesterday as a last step before ordering the boards. I was not happy with the results so far.

I ran it without any external cooling. The chip got quite hot as expected, but the pins I had hoped would help to move heat did not get hot. So it is unlikely that that will make a significant difference. I ran a fan near the chip and it seemed to make a big difference even without any heatsink. I didn't get a chance to experiment to find out just how much of a difference it made.

The thermal shutdown didn't function as I expected. Once it got hot, it would run for about 0.3 seconds and then shut down for 0.1 seconds while it cooled and then turned back on. I spent some time trying to debug the thermal shutdown before I realized what it was.

At some point the chip stopped working. When I measure the resistance between VDD and GND it is less than 1 Ohm. I'm not sure what the resistance was before I started. This grounds my 5V and shuts down the microprocessor. It would appear this board is fried. It was either the thermal abuse I was giving it during testing or ESD from mishandling.

I'm a bit disappointed. The two properties that attracted me to the Pololu stepper was the through-hole mounting and its self-protection features. I found the Stepper Driver v1.2 annoying since it could fry itself if you weren't careful. I'd like to avoid repeating that issue.

If anyone knows of another stepper driver, I'll investigate it. When I was looking before it seemed that everything was either surface mount or unprotected.

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Darwin clone, Gen 2 electronics, Arduino Duemilanove w/ AtMega328, 5D Firmware, Pinchwheel extruder
[www.codeerrors.com]

mccoyn,
I still have a full set of the SParkFun Easy Driver boards. It is based on the older A3967 with only 8 microsteps. The current open source version is V4.2. It mentions 10K pull ups or downs on all input pins, so it might be a little more ESD resistant. It also has a small pot to trim the current limiter. But it has no better cooling than the Polulu board.

I have ordered tape on heat sinks > plan to cut them in half and stick them on the backs of the stepper drivers. I think that with the heat sink glued on, it will run cool enough to not be a problem. I am also looking for long lengths of extruded aluminum finned heat sink that is flat on on side to span the entire board across the stepper driver SMDs with bolts in between to get good thermal contact and no risk of the tape letting go. That should make the smaller Polulu boards quite adequate. Although it would sure save me some money to have a different board version using the SparkFun Easy drivers!

Another comment. You have the MOSFETs to run the extruder heaters and bead heater laying down on the board. I would rather have them all standing up off the board and add bolt on heat sinks even if we don't really require them, especially after the stepper motor trouble. If we stick to TO-220 and variants parts, we will have a wide range of heat sinks we could use. There are even Insulated, logic gate level input, 20A MOSFETs that have electrically insulated thermal tabs so that all could be bolted onto a single piece of aluminum. Or, you could use some less expensive Logic gate level 35A MOSFET for $0.87 US in single unit quantities.

Mike

My concern with the EasyDriver is that it is only 750 mA. I know this is probably enough for a Mendel, but I'd like to design in plenty of extra room so that people can experiment with new ideas. The second problem is the board is almost twice as big and I don't have space for it since Seeed Studio will only do a 100x100 mm board.

My idea on the MOSFETs was to just provide a TO-220 footprint so that any n-channel MOSFET can be used. The one I specified has some nice protection features like thermal shutdown so that it doesn't damage itself. My thinking is that people are going to be wiring this thing by hand and possibly experimenting with new designs so having a high degree of protection built in will prevent a failure from slowing someone down.

The MOSFETs can be mounted standing up, which is what I plan to do. The MOSFET I specified should have no problem doing 4A without a heatsink. One concern I have is that the MOSFETs are very close and it will be hard to mount heatsinks on them. Those MOSFETs with electrically isolated heatsinks would be nice for this.

It wouldn't take me long to modify the board to use the Easy Driver if you want, but you'll have to make the board yourself.

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Darwin clone, Gen 2 electronics, Arduino Duemilanove w/ AtMega328, 5D Firmware, Pinchwheel extruder
[www.codeerrors.com]

I've updated all the files on the wiki. This revision is likely to be the one I send out to have made. I've included a postscript file for easy printing.

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Darwin clone, Gen 2 electronics, Arduino Duemilanove w/ AtMega328, 5D Firmware, Pinchwheel extruder
[www.codeerrors.com]

mccoyn,
I thought you were using the gEDA package for developing this board. I thought it did not have a size limitation. Or are you starting in one package and finishing in another? The postscript file looks VERY familiar!

I have already ordered a set of Polulu boards, so I will build at least one version exactly as you are. I should be able to edit the source files you have posted to do s seconds version to use up the Easy Driver boards. I will probably use both for RepStrap designs. I want to work on an MDF, plywood etc based design with the cheapest and easiest parts and tools needed to make the StarterBot, and this electronics board should be perfect for that.

Mike


mccoyn Wrote:
-------------------------------------------------------
> My concern with the EasyDriver is that it is only
> 750 mA. I know this is probably enough for a
> Mendel, but I'd like to design in plenty of extra
> room so that people can experiment with new ideas.
> The second problem is the board is almost twice
> as big and I don't have space for it since Seeed
> Studio will only do a 100x100 mm board.

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Team Open Air
Blog Team Open Air
rocket scientists think LIGHTYEARS outside the box!

The new limit on the board side comes from Seeed Studio, which is the website I am going to have make the board. I am using gEDA/PCB, which means I can design any size board I want, but I haven't figured out where to get it made from.

I basically worked out the general position of everything when I was using Eagle and used that when I redid it in gEDA/PCB.

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Darwin clone, Gen 2 electronics, Arduino Duemilanove w/ AtMega328, 5D Firmware, Pinchwheel extruder
[www.codeerrors.com]

OK, that makes sense. Let me know when you send out for a batch of boards

Mike

I fixed one last bug and updated all the files on the Wiki page. I just sent out for the boards 5 minutes ago, but I think the fabricator will be on holiday for a few days. It is supposed to take 4-6 days to make the board and then 10-30 days to ship them. I wasn't expecting the long shipping delay. So, it looks like April 21 will be the earliest date I can expect them.

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Darwin clone, Gen 2 electronics, Arduino Duemilanove w/ AtMega328, 5D Firmware, Pinchwheel extruder
[www.codeerrors.com]

Tonok has an interesting photo. [picasaweb.google.com]

He used an M8 nut as a heatsink and attached it to the pololu board with double-sided kapton tape.

Edited 1 time(s). Last edit at 04/17/2010 05:12PM by mccoyn.

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Darwin clone, Gen 2 electronics, Arduino Duemilanove w/ AtMega328, 5D Firmware, Pinchwheel extruder
[www.codeerrors.com]

I've received the PCBs today. Here are some pictures.

[objects.reprap.org]
[codeerrors.com]
[codeerrors.com]
[codeerrors.com]

If anyone can translate what is written on the masking tape, I would be interested to know. My guess is it indicates that those are the boards that were E-tested.

The order was for 10, but they shipped me 12. My understanding is fab houses usually do this to fill out a panel.

I'll send out boards tomorrow.

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Darwin clone, Gen 2 electronics, Arduino Duemilanove w/ AtMega328, 5D Firmware, Pinchwheel extruder
[www.codeerrors.com]

They look great mccoyn! I can hardly wait!

Mike

There are a couple mistakes on the board.

The ATX connector has the wrong spacing between the rows. It appears that you can bend the pins in and jam in through the holes.

The through-holes for the diodes are too small. These diodes are only needed if you connect an inductive load like a fan. My recommendation for installing these is to drill out the hole with a 1 mm bit. Insert the diodes and solder them on the back side. You also need to solder the diode lead that is closest to the ATMega chip on the top side to be sure that they are all connected to +12 V.

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Darwin clone, Gen 2 electronics, Arduino Duemilanove w/ AtMega328, 5D Firmware, Pinchwheel extruder
[www.codeerrors.com]

OK mccoyn, I am getting close to time build my first board. After reading elsewhere that the 5D code will not fit in an ATMega164, I will use my 664s for the board so that there will be plenty of room for software upgrades.

Mike

> There are a couple mistakes on the board.

Are there files somewhere with an updated version without the known bugs?