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[Reply #15 on:]

Isn't power= intesity x intesity x resistance?
So 10x10x0.077= 7.70W

I measured again. The motors actually need from 2.5 to 7.6 A each (peak is about 9A)
This means 4.44W with 7.6A and 0.077ohms
Is it too much? A normal heatsink with a pc fan (60mm x60) shuold work to keep the temperature to resonable levels...

[Reply #16 on:]

Actually you're right.  My apologies to yourself and dc42.

For some odd reason I had it in my mind that 0.077 was the V_SD and not the resistance.

Yes, I think that heatsink would be capable of keeping things cool enough.

[Reply #17 on:]

This means 4.44W with 7.6A and 0.077ohms
Is it too much? A normal heatsink with a pc fan (60mm x60) shuold work to keep the temperature to resonable levels...

Yes, if you are happy to use a heatsink (and a fan if necessary). But unless you already have the IRF540s, why not choose a mosfet that won't need a heatsink?

[Reply #18 on:]

Is a IRLZ44N mosfet a good solution? It's the best I found from a seller near by.
Rds is 0.022 ohm
Ids is 47A

What about the IRF series? Are they ok for driving a motor from 5V pwm? Some people say the only difference is the voltage applied on the gate.. 5V with IRL, 10V with IRF. Is that correct or is there a more substantial difference?

Let me know so I can purchase it.
Thanks!

[Reply #19 on:]

IRF can work here. They  have lower Rds but high Vgs(th) than the logic level mosfets. Since you are powering the motor from 12v, you can  easily drive the IRF from that, solving the problem.

[Reply #20 on:]

Yes, IRLZ44 is a good choice.

[Reply #21 on:]

I purchased 4x  IRL3103 and other IRF mosfets.
Irl3103 has only 12 mOhms Rds
I'll let you know how it goes.
Thanks!

[Reply #22 on:]

An IRL3103 should work.  The R_DSon increases to about 0.016V at 4.5V V_GS (close to typical at an MCU output).  That would mean about a watt of dissipation at 7.6 amps.  Given a typical TO220 I think it will get to about 60 degrees above surroundings at your maximum.  Okay for the FET, but you won't want to touch it or keep it cooped up.

I use little heatsinks when I push TO220 devices to a watt or so.  Kind of like these: http://www.aztecmcu.com/catalog/i99.html

They still get warm, but not so hot they could burn you or melt something.

[Reply #23 on:]

Okay,
Why not to follow this instructable:
http://www.instructables.com/id/Super-Easy-Reversible-Motor-Control-for-Arduino-/
I am using it successfully, see the bottom of this blog,you can also watch the video of its working
http://my-diysolarwind.blogspot.com/search?updated-max=2012-08-31T18:54:00%2B05:00&max-results=20

[Reply #24 on:]

It's the same thing I making. Same exact circuit, but I didn't post the photo with relay.. it uses a relay to swith positive and negative wires from the motor and change direction.

[Reply #25 on:]

It's the same thing I making. Same exact circuit, but I didn't post the photo with relay.. it uses a relay to swith positive and negative wires from the motor and change direction.

I am using two IRL540 with success: