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mosfet / Sanguinololu

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http://reprap.org/wiki/sanguinololu
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thermals in high current path limit bed power, pose failure risk #4

Open triffid opened 12 years ago

triffid commented 12 years ago

There are a number of pads with thermals between the power input and the bed output. Specifically: the mosfet source pins, the +12v side of the output connector, all the pins of the power input connector.

There can be up to 12A flowing through this path. I hate to think how hot those little bits of copper will get with that much current flowing!

How much resistance do they contribute?

mosfet commented 12 years ago

Good thinking! I'm wondering, though, that due to the length of the thermals, the resistance (and therefore current limits) is minimal. I previously did not have thermals on these devices, and instead connected them fully to the plane, but perhaps its worth adding a trace to the pad to get a larger track.

On Sat, Nov 26, 2011 at 3:23 PM, triffid < reply@reply.github.com

wrote:

There are a number of pads with thermals between the power input and the bed output. Specifically: the mosfet source pins, the +12v side of the output connector, all the pins of the power input connector.

There can be up to 12A flowing through this path. I hate to think how hot those little bits of copper will get with that much current flowing!

How much resistance do they contribute?

Reply to this email directly or view it on GitHub: https://github.com/mosfet/Sanguinololu/issues/4

triffid commented 12 years ago

perhaps, but when the bed is only 1 ohm, even a couple dozen milliohms is significant!

I concur that adding thick traces approx. the width of the pad to really beef up the thermals would help hugely.

triffid commented 12 years ago

For those of you with existing boards that find this, I suggest the following alterations:

connect thick wires from the ground pins of the power connector to the mosfet source pins. On each mosfet this is the pin furthest from the power connector. The power connector ground pins are the ones closest to the edge of the board (minifit), or closest to the mosfets (screw terminal)

connect thick wires from the +12v pins of the power connector to the output +12v pins. For each output, these are the pins closest to the power connector. The power connector +12v pins are the ones furthest from the board edge (minifit) or furthest from the mosfets (screw terminal)

The track between the mosfet and output does not have thermals and should be fine. If you want to beef it up anyway, connect thick wires from the middle pin of each mosfet to the remaining pins of the outputs.