DC Motor Controller for Electric Bicycle

I designed this controller for my Crystalite Sparrow 48V electric bicycle hub motor. The core function of a DC motor controller is to periodically read the throttle setting and adjust the current being supplied to the motor. It does this with a technique called pulse-width modulation or PWM (more on this later). Other functions of the controller include: 1) low-voltage cutoff .. monitor the battery voltage and shut down the motor if the battery voltage is too low .. this protects the battery from over-discharge. 2) over-temperature cutoff .. monitor the temperature of the FET power transistors and shut down the motor if they become too hot .. this protects the FET power transistors. 3) over-current cutoff .. reduce the current to the motor if too much current is being supplied .. this protects both the motor and the the FET power transistors. 4) brake cutoff .. shut down the motor when the brake is applied .. this is a safety feature .. if the user applies brake and throttle, the brakes win.

Note1: This is a relatively advanced instructable. Don't attempt it if you don't have experience with power electronics. The voltages and currents used in this project can be dangerous and appropriate safety precautions must be used. This instructable outlines what I did to make this project, but it is not a substitute for proper safety training in power electronics. Check with your local community college for availability of classes in your area.

Note2: In addition to the 48V battery voltage, this controller requires a 12V power supply. If your battery pack consists of 12V cells, then you can just tap 12V from the pack. This was not possible for my battery pack, so I used a separate DC-to-DC converter to supply the 12V power. See my other instructable on constructing this DC-to-DC converter.

Note3: This controller is over-designed for this application. The IRFP4468 FETs are rated for a maximum of 195 Amps (each) at 100V. This application will typically use less than 10 Amps at 50V. I have been commuting (10 mile round trip) almost every day for the past 2 months using this controller and it has been trouble free (knock-on-wood :)

Step 1: Parts List

Here is the parts list (with Digikey part numbers) for all the electronic parts.

You will also need:
a) a prototype circuit board (the one I used is from a local electronics surplus store)
b) wire. I used 30AWG wire for the low current connections and 14, 12 and 10AWG wire for higher current connections.
c) 1/8" heat shrink tubing (about 2" in length)
d) two 6-32 x 1" screws
e) 4 x insulating pads for the FETs and power diodes (these can be salvaged from a broken PC power supply)
f) a heat sink for the power section. (this can be salvaged from a broken PC power supply)
g) an enclosure. (this can be salvaged from a broken PC power supply)

The following tools are required:
a) a programmer for the microcontroller. I used an AVR ISP programmer (check EBay)
b) a soldering iron (and solder of course)

The following tools are recommended for debugging:
a) a digital multimetter (DMM) for checking connections, etc.
b) an oscilloscope is handy for checking the PWM waveform, etc.

Step 2: Schematic Drawings

Here are the schematic drawings. Sheet 1 is the digital section and sheet 2 is the power section.

Step 3: Digital Section

This photo shows the digital section (page 1 of the schematic) wired up and connected the the Atmel programmer.

Step 4: Pulse-Width Modulation (PWM)

This photo shows the PWM signal (pin 15 of the ATMega8 micro) used to turn on/off the power FETs. When the signal is high, voltage is applied to the motor. This waveform is for about a 1/4 throttle setting.

Step 5: Power Section Construction Details

These photos show how the FETs and power diodes are attached to the heat sink. They must be kept electrically isolated from the heat sink. The screws are covered with 1/8" heat shrink tubing and the FET and power The heat sink that I used is re-purposed from an old broken PC power supply.