In the need of controlling a 7A 20Volt DC Motor i designed a speed controller with some cheap components. The circuit is a digital circuit and works with Pulse Width Modulation (PWM) which is one of the best ways to control a dc motor. This circuit can drive up to 33A 10V DC motor with a big heat sink placed on the switching mosfet.
Summarily i used in this circuit, the AVR ATtiny13 to control the PWM , a n-mosfet IRF540N for switching the motor and a Rotary Encoder to adjust the PWM Duty Cycle.
Note. The same circuit can be used as a light dimmer for LEDs.
The circuit consists of:
The Power Supply.
It is a basic LM7805 power supply which adjusts the voltage at 5V for powering the ATtiny13 and the LEDs. Capacitors before the LM7805 C10, C7 and after C8, C11 have been used for noise filtering. Diode D1 1N4001 is there to protect the circuit from inverting voltages.
The main circuit.
The main circuit can be separated in 4 sub circuits to be more understandable.
3 Leds have been used for indicating PWM Duty Cycle value: Minimum (LED3), Maximum (LED2) and the Current Value (LED1). LED2 and LED3 are connected directly to the ATtiny13 I/O. LED1 which indicates the current PWM Duty Cycle value is driven by the transistor T1 BC337 triggered by the PWM output. R1 R3 and R4 are current limiting resistors for LEDs. R2 Limits the current draw to the BC337 base.
The Rotary Encoder
This is the Rotary Encoder connected with the appropriate components to work properly. R6 and R5 are pull up resistors which "pulls" A and B pins to VCC. Pin C is directly connected to GND. C4 and C3 are decoupling capacitors which are appropriate for noise filtering. If you don't place them the noise will be not filtered and the rotary probably will not work.
The Motor Switcher
For Motor Switching i have used a n-mosfet IRF540N which can Continuous Drain Current, VGS @ 10V 23-33A depending on the temperature (25C-100C). Diode D2 has been placed for inductor inverting voltages. Capacitor C1 is there to filter the noise provided from the motor. If you don't place this capacitor you may have noise at the rotary encoder and it will not work properly.
Make sure you have placed a heat sink on the IRF540N because at high currents it is getting realy hot!
Here you can find photos of the test circuit i built.
The code and the Circuit Scheme can be found: