How It Works
DC Controller with 1A Capability
An instruction on how to build a Controller for DC motored models.
DC Controller mounted in Case
This power supply is based on the uA723 integrated circuit. The IC acts as a negative feedback voltage regulator driving the 2N3055 power transistor. The reference (pin 4) is a zener regulated voltage of 6.8 volts. This voltage is available across the SPEED control and the 470 ohm resistor going to earth (pin 5). The control (pin 3) is driven from a tap on the SPEED control, and can be adjusted from 6.8 volts at maximum setting to 0.58 volts [ie 6.8 volts * 470 / (470 + 5000)] at minimum setting. The control voltage (pin 3) is compared to the feedback (pin 2) voltage, (and here's the important bit) the track voltage driven from the output (pin 6) is varied by the IC until the voltages at the control pin and the feedback pin are the same.
Because the feedback voltage is derived from a dividing tap on the output voltage, then the track voltage has to be a multiple of the feedback voltage, as determined by the 560 and 470 ohm divider resistors. In this case the track voltage is 1.839 times the feedback voltage [ie Vfeedback * (470 + 560) / 560]. If the track voltage is 1.839 times the feedback voltage, it must be also be 1.839 times the control pin voltage ie 1.839 * 6.8 volts = 12.507 volts at maximum, and 1.839 * 0.58 volts = 1.066 volts at minimum.
The 0.47 ohm resistor detects the current flowing in the track and applies a voltage across the protection terminals (pins 10 and 1) of the IC. When the voltage reaches 0.6 volts across the resistor, the IC rapidly switches the output voltage to minimum, but maintains the current. The maximum current Imax = 0.6 volts / Rprotection = 0.6/0.47 = 1.276 Amps. When a short circuit is applied to the track, fast transient response is guaranteed by the 100pF capacitor coupled to the Frequency Compensation input (pin 9). The IC voltage output (pin 6) is always 0.7 volts above the track voltage due to the base to emitter voltage of the 2N3055.
PLEASE NOTE: Two versions of the LM723 IC exist. There is a circular 10 pin and a 14 pin DIL version. They have a different geometry of pin layout. Be sure to check the pins for correct PCB layout. The components are available from Dick Smiths, Jaycar or Altronics. Try using a 1.5 Amp 8 volt DC plug pack instead of the transformer and rectifier. Use a double pole, double throw switch to effect locomotive reversal. DO NOT earth the negative rail of the supply if you are using multiple supplies to a layout.
I have used these controllers for years, and find they are eminently successful at powering anything from Lima to Athearn locomotives. The short circuit protection is a real plus, as is the regulation which means the track voltage is kept constant with varying load current demand. I have modified the power supply in the photographs by adding a second transistor (BD139) to drive the output transistor (MJE3055) to higher output current. You will not normally require this, as the uA723 will deliver 100mA from the output (pin 6) and a 2N3055 is able to easily boost this to the required 1.2 Amps.