Let's Make Robots!

Determining the right mosfet current for a h bridge.

Hie again. i am about to place an order for some n channel mosfets for my h bridge and there is a few things id like to verify.
i will be making a 4wd robot with skid steer configuration using 4 motors, so there will be 2 h bridges, one controlling 2 motors. 
Since my motors stall  at 42A each, will a 120A for each h bridge be enough? or is 80A enough since these motors are quite impossible to stall?

here is my chosen Fet
http://au.element14.com/jsp/search/productdetail.jsp?sku=2253787

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Ok, before you waste a lot of money or kill yourself then you better start again. Here is a list of information that you either didn't consider or forgot to mention thus making it impossible for us to help you.

  1. What type of batteries are you using and what voltage are they?
  2. How much heatsinking are you planning to use?
  3. Are you planning on electronic braking?
  4. What flyback diodes are you using?
  5. Where is your schematic?
  6. It would be handy to have full motor specifications.

The problem is not just the current rating. With the FETs you also need to look at the "on resistance" to determine how hot it's going to get. You need to look at the body diode to determine if it has the same current rating as the FET, many do not in which case a flyback diode is essential. If your flyback diode has a higer instantaneous voltage drop than the FET's body diode then it won't do anything until after the FET has been destroyed.

Next, it sounds like you plan to design the "H" bridge yourself but you only mentioned the Nch FET. Usually the low side FETs are Nch and the high side FETs are Pch.

You can make a "H" bridge with all Nch FETs but then you need to have much more complex circuitry to provide the correct gate voltage for the high side FETs. This can be simplified greatly using a "H" bridge pre-driver IC.

How are you planning to make your "H" bridge? Do you have a schematic?

 

At this point, judging by your question I would say you are better off buying a motor driver that is rated for the voltage and current of your motors. It will be cheaper and safer.

 

 

I had intended on following this tutorial:

http://hades.mech.northwestern.edu/index.php/Driving_a_high_current_DC_Motor_using_an_H-bridge

The fets will be switched on by an ir2019 gate driver, and it will be controlled by the arduino pins and yes i will be using an electronic brake.

The motor that i am controlling is of this specification:

6.6A peak efficiency.
42A Stall
Website:
http://banebots.com/pc/MOTOR-BRUSH/M5-RS540-12

Which motor controllers do you recommend i use? I have been advised against this:

http://www.ebay.com/itm/60A-Dual-Motor-Driver-Drive-Module-H-Bridge-PWM-Control-for-robot-projects-NEW-/230965079712

It is rated at 50A max continous, 70A peak. Thank you very much for your help so far

Ok, at least you have a plan.

If you have the ability and desire to make your own PCB then give it a go. Please note that the tutorial you are working from did not mention the fact that not all FETs have a body diode with the same rating as the FET.

You will still need to study the datasheet of the FETs. If the body diode does not have the full current rating of the FET then you will need to look for another FET or get some flyback diodes with suitable voltage, current and instantaneous forward voltage ratings.

looking at the motor driver you found on Ebay I agree it may not be suitable. Just from the photos of the PCBs it's obvious that the track and terminals are not rated for more than about 10A per motor. Then again I am not sure the stall current of those motors is accurate, 12V x 42A = 504W = vaporized motor windings.

If you use the ebay motor driver then use a 10A slow blow or 15A fast blow fuse in series with each motor.

Sometimes when people ask questions like this, I just keep my mouth shut.  But my conscience won't let me.  If you need to ask that kind of question you really should NOT be playing with 168 Amps.  Period.

As long as he takes video when he connects the power so we can all watch it explode then tell your concience to be quite :P

lol

Oh the videos I could post if I had a video camera on!