Step 2: The Motor Driver
So I thought I would just make an extremely simple dual h-bridge that could handle at least 25 amps at 24vdc continuous and handle surges of up to 100amps for a few seconds. Once I found out that you can parallel Mosfets and multiply their current carrying capacity accordingly, I thought I would come up with a simple design and slightly complicate it by adding more mosfets until I had enough to handle the current that I needed. Digikey has a good selection of Mosfets to choose from and good filters to narrow it down by what you need, so I spent a lot of time looking for Mosfets that were rated for around 50amp and could handle over 30 volts. Also, they have to be cheap because my plan is to use a bunch of them. I decided on the FQP47P06 p-channel and the FQP50N06L n-channel Mosfets from Fairchild Semiconductor, which I bought from Digikey.
If you are wondering what an H-bridge is, find out here: en.wikipedia.org/wiki/H-bridge and this will all make more sense to you.
The design is simple: 2 P-channel mosfets control the high-side switches and 2 N-channel mosfets for the low-side switches. But instead of using 1 mosfet for each switch, lets use 3. Now we have 12 mosfets per H-bridge (3 mosfets x 4 switches) and theoretically the ability to carry 150 amps (that is not accurate though). The board is as small as I could make it with nothing touching. Each set of 3 mosfets have heatsinks and are bolted together to help dissipate heat. Also, there is an 80mm cooling fan mounted directly above mosfets to further keep them cool. The mosfets are very good at handling sudden changes in direction and speed changes.
Since there are 24 mosfets in total (8 groups of 3) I dubbed it the Triple-8. It is running at the Arduino default PWM frequency of 1kHz (I plan on playing with that to get the frequency higher). The board has 4 inputs, 2 for each bridge. If you bring an input HIGH, that side of the bridge goes HIGH.
Ideally, you would control the board by holding 1 input LOW and applying a PWM signal to the other input. This allows for easy speed control. I have written into the code that if you bring digital pin 7 HIGH, the code switches to Relay mode and either turns the mosfets all the way ON or all the way OFF. This is far more difficult to control, but is useful sometimes.
If you are interested in building your own H-bridge you can download the eagle file to etch a pcb and the schematic to show where everything goes. You can get everything to make this dual h-bridge at Radio-shack (including the copper clad), except the Mosfets and a special resistor network I used to save space. I bought most of the parts from Digikey though because it was cheaper and arrives to my house in 2 days.
Here are the parts needed for this motor driver:
(12) FQP47P06 - P-channel mosfet 47a 60v - Digikey - $1.73 ea
(12) FQP50N06L - Logic level N-channel mosfet 52a 60v - Digikey - $1.04 ea
(4) 2n7000 - Logic level N-channel mosfet 200ma 60v - Digikey - $0.26 ea
(8) 4606X-1-470LF-ND - 47ohm bussed resistor network - Digikey - $0.25 ea
(6) ED1609-ND - 2 position screw terminal - Digikey or Radio Shack- $0.46 ea
(24) CF1/84.7KJRCT-ND - 4.7k 1/8w resistor - Digikey or Radio Shack - $1.78 (for 50pk)
(1) PC9-ND - 3"x4.5" 1-sided copper-clad .064" 2oz copper - Digikey or Radio Shack- $4.66
(4) P5575-ND - 1000uf Capacitor or similar - Digikey - $1.19 ea
(1) 330ohm - 1kohm resistor 1/4w - for power LED, doesn't have to be exact
(1) power LED any color you like, I use the 3mm size to save space
Maybe something smaller?
If you are going to use this for something smaller than a 100lb lawnmower, you can look up one of the many H-bridge circuits and build your own smaller motor controller with as few as 4 mosfets (or BJT transistors) or even use a packaged IC H-bridge like the l293d (dual 1 amp) or the l298n (dual 2 amp).
Or if anyone is interested, I will post a schematic and Eagle .brd file for a smaller version of this H-bridge that only requires 8 mosfets total (everything else is the same), and it can handle about 10amps at 24vdc.
I am not going to go into all the details of PCB etching, because there are already many excellent instructables on that topic. So once you download my .BRD file of my motor controller, all you need to do is print the .brd file onto some magazine paper using a laser printer, and iron that onto a piece of clean copper-clad. Then etch it with your favorite etchant solution (I use 2 parts Hydrogen Peroxide to 1 part Muriatic Acid and it works perfectly). And remove the toner with Acetone when done etching.
For ease of assembly I designed this board to be Single-sided and to use only through-hole components, no surface-mount stuff to mess with! Yay for you.
You can get the .brd files for the various h-bridges at www.rediculouslygoodlooking.com